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I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}
I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.
There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.
I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)
Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.
I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.
I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.
I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.
Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}
I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.
I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.
Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}
Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).
[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.
You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))
I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.
In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.
I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.
I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...
I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.
I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}
[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}
this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life
'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.
I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.
When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.
It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.
I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.
1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);
There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.
You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}
I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.
There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.
I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)
Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.
I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.
I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.
I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.
Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}
I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.
I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.
Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}
Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).
[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.
You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))
I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.
In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.
I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.
I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...
I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.
I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}
[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}
this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life
'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.
I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.
When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.
It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.
I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.
1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);
There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.
You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}
I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.
There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.
I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)
Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.
I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.
I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.
I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.
Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}
I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.
I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.
Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}
Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).
[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.
You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))
I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.
In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.
I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.
I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...
I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.
I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}
[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}
this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life
'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.
I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.
When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.
It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.
I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.
1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);
There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.
You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.
I enjoy and highly recommend Juval Lowy's - C# Coding Standard. Juval explicitly avoids rationale for each directive in order to keep the standard tight (see the preface). However, there are a few directives for which I find myself curious as to the rationale.
What is the specific rationale to the following directives from Lowy's C# standard?
Hopefully there are hard (non-subjective) answers to these.
1.13 Avoid fully qualified type names. Use the "using" statement instead.
Is this a performance issue? Sometimes I only need one instance of the fully qualified name and adding a using seems heavy.
1.26 Use empty parenthesis on parameterless-anonymous methods. Omit the parenthesis only if the anonymous method could have been used on any delegate.
Actually I am just confused by the second sentence. Explanation with example(s) would help, thank you.
2.19 Avoid defining custom exception classes
What are the considerations in minimizing their numbers? (He next gives guidelines if you do define them (in 2.20).)
2.29 Avoid using the ternary conditional operator
Too hard for the reader to digest, or other considerations?
2.31 Avoid function calls in Boolean conditional statements. Assign into local variables and check on them.
I don't think I do this, but I am curious...why not?
2.47 Avoid interfaces with one member.
Because it is always/usually more prefereable to do what? One method interfaces work when?
2.53 Prefer using explicit interface implementation
Why? Also, Jon Skeet disagrees here.
Thanks in advance!
Robert
2.29 Avoid using the ternary conditional operator I have no problems with "simple" uses of the ternary operator but have recommended against using it in a nested fashion:
// This is fine
x := (conditionA) ? true_resultA : false_resultA;
// This would probably be clearer using if-then-elseif
x := (conditionA) ?
((conditionA1) ? true_resultA1 : (condition2) ? true_result2 : false_result2) :
((conditionA2) ? true_resultA2 : false_resultA2);
Obviously, I'm not Juval, but I can take a stab at these
1.13 Avoid fully qualified type names. Use the "using" statement instead.
Performance can't be the issue here. I'm sure the issue is readability.
1.26 Use empty parenthesis on parameterless-anonymous methods. Omit the parenthesis only if the anonymous method could have been used on any delegate.
public delegate void Foo1();
public delegate void Foo2(int val);
public void Foo()
{
Foo1 first = delegate { Console.WriteLine("Hello world"); };
Foo2 second = delegate { Console.WriteLine("Hello world"); };
Foo1 third = delegate() { Console.WriteLine("Hello world"); };
Foo2 fourth = delegate() { Console.WriteLine("Hello world"); }; // does not compile
}
Without the parens, the anonymous delegate can be applied to any delegate. With the parens, you're being specific about the signature of the delegate. Prefer the second syntax unless you really need the flexibility.
2.19 Avoid defining custom exception classes
Again, readability is the issue here. The framework exception classes are rich and well-understood. Be wary when replacing them.
2.29 Avoid using the ternary conditional operator
It's a readability and expandability thing. I don't really agree, but it's a standard religious fight.
2.31 Avoid function calls in Boolean conditional statements. Assign into local variables and check on them.
Partially this is readability, and partially it's for ease of debugging. I've starting to assign almost everything to temporary variables just so that they're easily found in the debugger later on.
2.47 Avoid interfaces with one member.
"Avoid" is kinda like "prefer", he's just saying think twice before you do it. If you only have one member, is the interface really modeling something useful and complete in your design? It's pretty rare to have a class with just one member, think seriously about why your interface is any different.
2.53 Prefer using explicit interface implementation
This is similar to the idea of using the least-public accessor you can. If your class doesn't need to make the interface public, then it probably shouldn't. This is going to differ significantly based on your design, obviously, but given the fact that most people just make the interface implicit without really thinking about it, it's advice worth considering.
1.26 is about pre-lambda delegate { } syntax.
// #1 Empty parenthesis on parameterless-anonymous methods would be:
delegate() { }
// #2 ... anonymous method could have been used on any delegate, is:
delegate { }
Remember, the latter can be assigned to any delegate, regardless of its parameters. The delegate just ignores these using some compiler trickery.
If you define a delegate taking no parameters, explicitly say so using #1. Don't "leave the parenthesis out because your delegate doesn't take any parameters anyway".
Regarding 1.13 (Avoid fully qualified type names. Use the "using" statement instead):
It may be a bit more than readability. If you have too many usings at the beginning of the file, you have a class that is coupled with classes from too many namespaces.
The class is screaming out for refactoring. Using usings instead of fully-qualified class names lets you identify such tightly-coupled classes more easily.
A lot of these guidelines speak to the "quality attributes" of good software design (i.e. maintainability, reliability, reusability, testability, expandability, debugability, interoperability, and what other -ilities you can name).
Often people create code that works fine at the time but may not be the best choice when considering all the quality attributes (in the sense of "where can this software go in the future" or "someone else has to use this code, too").
For example:
2.29 Avoid using the ternary conditional operator
I have no problem with ternary expressions, per se, but by writing code such as: int result = CheckMethod() ? OnTrueDoThis() : OnFalseDoThat()... you are saying, "I have a conditional that, if true (or false), you can do one and only one thing." The whole construct discourages expandability. You have to recreate the construct (with an if..else statement).
Similarly...
2.31 Avoid function calls in Boolean conditional statements. Assign into
local variables and check on them.
You called a function and essentially "discarded" the results for later use. If that information is needed later, either the function would have to be called again or the structure of the code would have to be rewritten. It would also make checking or logging the results (for future debugging) more difficult.
This is my best stab at the questions you've listed. For the ones that I can't really say, I've omitted.
1.13 Avoid fully qualified type names. Use the "using" statement instead.
Readability. It's must harder to read code when you have to read fully qualified type names.
2.19 Avoid defining custom exception classes
The .NET framework comes with a good set of exceptions built into the system. Unless the exception you're modeling is business domain specific, you'll probably be able to use one of the existing exception classes.
2.29 Avoid using the ternary conditional operator
I think this is most likely because he thinks people may not understand the operator, but I disagree.
2.47 Avoid interfaces with one member.
He might be warning people of building interfaces that are too thin. However, I would actual say the converse, warning people of making interfaces too verbose. If you've ever had to deal with ASP.NET MembershipProvider, you know what I'm talking about.
2.31 Avoid function calls in Boolean conditional statements. Assign into local variables and check on them.
A couple of reasons I can think of here. Readability. It can make conditional statements hard to understand if you are making function calls in them. Also, it's harder to debug if you're not watching.
2.53 Prefer using explicit interface implementation
I believe his reasoning here is to for brevity. However, I don't actually agree with this assessment. I think Jon is correct, implicit interface should be used when you can, and explicit when appropriate.
Here are some of my reactions of which I dare answer them :)
1.13 Avoid fully qualified type names. Use the "using" statement instead.
I disagree. It's certainly not performance related. It can lead to improved readability to have var foo = new Foo() instead var foo = new MyCompany.MyNamespace.Helpers.Xml.Foo() but other than that - no.
2.19 Avoid defining custom exception classes
This is nonsense imho. You should avoid creating custom exceptions that derive from ApplicationException, but there is nothing wrong with custom exceptions (as long as you're not going to reinvent existing exceptions that is).
2.29 Avoid using the ternary conditional operator
I have no idea why that would be a guideline. I have read that not all people use it and may not recognize it, but that is not a valid reason to not use a useful operator.
2.31 Avoid function calls in Boolean conditional statements. Assign into local variables and check on them.
This is simply a readability issue in my opinion.
2.47 Avoid interfaces with one member.
I also disagree here. You should avoid 'marker' interfaces though - interfaces with no marker, but who just serve the purpose that something is '...ble'. But, one method on an interface seems fine to me.
2.29 Ternary operator
To start with, if you start to use the ternary operator, there should be a reason for the use of the ternary operator over a regular if-then-else. Observe :
if (x == 0) {...} else{...} //A set of statements demand a regular If-then-else
//A simple assignment can be handled by the ternary operator
y = (x == 0)? 1 : 0 //this is readable and how it should be used
(x==0)? callSomething() : callSomethingElse() //this is NOT how it should be used
The ternary statement is meant for returning one of two values depending upon the conditional it is evaluating. This is extremely handy when doing FP. For call statements that do not return a value, you should revert to if-then-else.
I'm trying to formalise the usage of the "out" keyword in c# for a project I'm on, particularly with respect to any public methods. I can't seem to find any best practices out there and would like to know what is good or bad.
Sometimes I'm seeing some methods signatures that look like this:
public decimal CalcSomething(Date start, Date end, out int someOtherNumber){}
At this point, it's just a feeling, this doesn't sit well with me. For some reason, I'd prefer to see:
public Result CalcSomething(Date start, Date end){}
where the result is a type that contains a decimal and the someOtherNumber. I think this makes it easier to read. It allows Result to be extended or have properties added without breaking code. It also means that the caller of this method doesn't have to declare a locally scoped "someOtherNumber" before calling. From usage expectations, not all callers are going to be interested in "someOtherNumber".
As a contrast, the only instances that I can think of right now within the .Net framework where "out" parameters make sense are in methods like TryParse(). These actually make the caller write simpler code, whereby the caller is primarily going to be interested in the out parameter.
int i;
if(int.TryParse("1", i)){
DoSomething(i);
}
I'm thinking that "out" should only be used if the return type is bool and the expected usages are where the "out" parameters will always be of interest to the caller, by design.
Thoughts?
There is a reason that one of the static code analysis (=FxCop) rules points at you when you use out parameters. I'd say: only use out when really needed in interop type scenarios. In all other cases, simply do not use out. But perhaps that's just me?
This is what the .NET Framework Developer's Guide has to say about out parameters:
Avoid using out or reference parameters.
Working with members
that define out or reference
parameters requires that the developer
understand pointers, subtle
differences between value types and
reference types, and initialization
differences between out and reference
parameters.
But if you do use them:
Do place all out parameters after all of the pass-by-value and ref
parameters (excluding parameter
arrays), even if this results in an
inconsistency in parameter ordering
between overloads.
This convention makes the method
signature easier to understand.
Your approach is better than out, because you can "chain" calls that way:
DoSomethingElse(DoThing(a,b).Result);
as opposed to
DoThing(a, out b);
DoSomethingElse(b);
The TryParse methods implemented with "out" was a mistake, IMO. Those would have been very convenient in chains.
There are only very few cases where I would use out. One of them is if your method returns two variables that from an OO point of view do not belong into an object together.
If for example, you want to get the most common word in a text string, and the 42nd word in the text, you could compute both in the same method (having to parse the text only once). But for your application, these informations have no relation to each other: You need the most common word for statistical purposes, but you only need the 42nd word because your customer is a geeky Douglas Adams fan.
Yes, that example is very contrived, but I haven't got a better one...
I just had to add that starting from C# 7, the use of the out keyword makes for very readable code in certain instances, when combined with inline variable declaration. While in general you should rather return a (named) tuple, control flow becomes very concise when a method has a boolean outcome, like:
if (int.TryParse(mightBeCount, out var count)
{
// Successfully parsed count
}
I should also mention, that defining a specific class for those cases where a tuple makes sense, more often than not, is more appropriate. It depends on how many return values there are and what you use them for. I'd say, when more than 3, stick them in a class anyway.
One advantage of out is that the compiler will verify that CalcSomething does in fact assign a value to someOtherNumber. It will not verify that the someOtherNumber field of Result has a value.
Stay away from out. It's there as a low-level convenience. But at a high level, it's an anti-technique.
int? i = Util.TryParseInt32("1");
if(i == null)
return;
DoSomething(i);
If you have even seen and worked with MS
namespace System.Web.Security
MembershipProvider
public abstract MembershipUser CreateUser(string username, string password, string email, string passwordQuestion, string passwordAnswer, bool isApproved, object providerUserKey, out MembershipCreateStatus status);
You will need a bucket. This is an example of a class breaking many design paradigms. Awful!
Just because the language has out parameters doesn't mean they should be used. eg goto
The use of out Looks more like the Dev was either Lazy to create a type or wanted to try a language feature.
Even the completely contrived MostCommonAnd42ndWord example above I would use
List or a new type contrivedresult with 2 properties.
The only good reasons i've seen in the explanations above was in interop scenarios when forced to. Assuming that is valid statement.
You could create a generic tuple class for the purpose of returning multiple values. This seems to be a decent solution but I can't help but feel that you lose a bit of readability by returning such a generic type (Result is no better in that regard).
One important point, though, that james curran also pointed out, is that the compiler enforces an assignment of the value. This is a general pattern I see in C#, that you must state certain things explicitly, for more readable code. Another example of this is the override keyword which you don't have in Java.
If your result is more complex than a single value, you should, if possible, create a result object. The reasons I have to say this?
The entire result is encapsulated. That is, you have a single package that informs the code of the complete result of CalcSomething. Instead of having external code interpret what the decimal return value means, you can name the properties for your previous return value, Your someOtherNumber value, etc.
You can include more complex success indicators. The function call you wrote might throw an exception if end comes before start, but exception throwing is the only way to report errors. Using a result object, you can include a boolean or enumerated "Success" value, with appropriate error reporting.
You can delay the execution of the result until you actually examine the "result" field. That is, the execution of any computing needn't be done until you use the values.