I have a function that returns a modified copy of the object that was passed to this function. I often do something like this:
obj = obj.Foo(param);
Don't ask why, I simply have to. But sometimes, I (and others) forgot to assign the return value, doing this:
obj.Foo(param);
which repeatedly leads to time-consuming debugging.
Is there any way to show a warning or error every time when the function result is not assigned to a variable? Or any other suggestions on how to solve this issue?
You could use an out parameter, so the call would look like this:
obj.Foo(param, out obj);
You can use Resharper to assist with this issue; you need to decorate your method with the [Pure] attribute:
[Pure]
public static IList<T> RemoveItem<T>(this IEnumerable<T> thisList, T item)
{
var list = thisList.ToList();
list.Remove(item);
return list;
}
then when you call it without assigning the return value you will see:
The [Pure] attribute is defined in Resharpers Data Annotations: You need to copy the classes into your project so you can reference them (many very useful other annotations too)
It's totally legal and often desirable to not assign the return parameter so it would be wrong to have a warning for it. Henrik's answer to use an out parameter is what I'd recommend too to ensure the result is assigned everytime.
you can enable visual studio warings.
you can even customize the rule you want to apply.
you should see warnings in the case you don't assign the function to a variable
you can also decide to treat the waring as errors
Example:
public static class MyClass
{
public static string GetStr()
{
return "";
}
public static void Main()
{
GetStr();
}
}
I can't comment on answers, lacking stackoverflow credits. But I agree with Chris that it's totally legal and often desirable not to assign values returned from a method. It's also occasionally not desirable. e.g.
public static int Square(this int myValue)
{
return myValue * myValue;
}
It's clear that calling this method without assigning it is probably a bug. I think creating a code analysis rule that warned every time you didn't assign a value as Massimiliano suggested would be worse than not having the rule at all. In such cases it would be nice to be able to apply an attribute to the method...
[MustAssign]
public static int Square...
You could create a rule as Massimiliano suggested but only invoke the warning when the method is adorned with the attribute and the value returned from the method is not assigned. Not a trivial exercise though.
Related
There are several similar questions about generic methods with concrete overloads here on SO and most of them say essentially the same thing: Generic overload resolution is done at compile time so if you want to use a concrete overload, you may need to use dynamic in order to have the runtime decide which overload to use. OK, cool--I understand that. None of the questions I've found deal with an out parameter and I'm wondering if there's a better way to handle this than what I've done. Let's start with the most basic case (my code was all written and tested in LinqPad):
void Main()
{
string x = "";
var g = new GenericTest();
g.RunTest(x); //Ran from TryFoo<T>
g.RunTestWithDynamic(x);//Ran from TryFoo(string)
g.Foo(x); //Ran from TryFoo(string)
}
public class GenericTest
{
//public void RunTest(string withInput) => Foo(withInput); <-- This would fix it
public void RunTest<T>(T withInput) => Foo(withInput);
public void RunTestWithDynamic<T>(T withInput) => Foo((dynamic)withInput);
public void Foo<T>(T withInput) => Console.WriteLine("Ran from TryFoo<T>");
public void Foo(string withInput) => Console.WriteLine("Ran from TryFoo(string)");
}
Here are some things to note:
RunTest is a generic method that calls another generic method, Foo. When I call RunTest, it appears the compiler doesn't follow all the way from the call site to see that g.RunTeset is passing in x which is a string and link it all up so that the Foo(string) overload is called; instead, it just sees that RunTest<T> is calling Foo. It doesn't make different "paths" based on different input values of T. OK, that's fair and understandable.
If I call Foo directly from my Main method, the compiler is smart enough to see that we are calling Foo with a string directly and correctly selects the concrete overload.
As the linked SO posts describe, I can call RunTestWithDynamic which will change which overload is used at runtime based on value. It feels just a bit "hacky", but I'm good with this solution.
I've commented out a line: a concrete overload of RunTest. This would be essentially the same as calling Foo directly. If that were un-commented, it would fix everything. Alas, that is not an option for me in my case.
Now, what if the T is for an out parameter? Consider the pattern used by, say, int.TryParse where you return a bool to indicate if it succeeded or not, but the value you actually want is an out. Now you can't really do dynamic resolution because you can't cast an out parameter. I considered doing something where I make a default(T) and then casting that to dynamic, but if that ever works well anywhere else, there is the problem of reference types that default to null to deal with. Nope, that doesn't work, either.
In the end, the best I could come up with was this:
void Main()
{
string x;
var g = new GenericTest();
g.TryRunTest(out x); //Ran from TryFoo<T>
g.TryRunTestWithDynamic(out x); //Ran from TryFoo(string)
g.TryFoo(out x); //Ran from TryFoo(string)
}
public class GenericTest
{
//This would fix it, but in my case, not an option
//public bool TryRunTest(out string withOutput) => return TryFoo(out withOutput);
public bool TryRunTest<T>(out T withOutput)
{
return TryFoo(out withOutput);
}
public bool TryRunTestWithDynamic<T>(out T withOutput)
{
if(typeof(T) == typeof(string))
{
var retval = TryFoo(out string s);
withOutput = (T)(dynamic)s;
return retval;
}
return TryFoo(out withOutput);
}
public bool TryFoo<T>(out T withOutput)
{
withOutput = default(T);
Console.WriteLine("Ran from TryFoo<T>");
return true;
}
public bool TryFoo(out string withOutput)
{
withOutput = "Strings are special";
Console.WriteLine("Ran from TryFoo(string)");
return true;
}
}
You can see that TryRunTestWithDynamic has to look for the concrete string type specifically. I can't figure out a way to do it so that I can just add overloads and then use dynamic resolution to select the overload I want without having to spell it all out (and let's face it--spelling it all out kind of kills the whole point of having overloads in the first place).
In his post I linked to above (and here for good measure), Jon Skeet mentions using MethodInfo.MakeGenericMethod as an alternative. Here he talks about how to use it. I'm curious if that would help me here, but I can't figure out how to use it with out parameters.
So, here are my specific questions:
While, yes this DOES work, it is VERY clunky. Is there a better way to do this? Most importantly, is there a way I can consume overloads that wouldn't require me checking each type specifically?
Is there a way to use the MethodInfo.MakeGenericMethod route using out parameters and would that help me?
Let's say we could wave a magic wand and all of a sudden a language feature was added to C# that would give us the option to force dynamic resolution for generic methods (either for all cases, or only for the case where the generic parameter is for an out, like my main problem). Taking from the above examples, let's say we had something like this:
//The following are all invalid syntax but illustrate possible
// ways we may express that we want to force dynamic resolution
//For out parameters
public void TryFoo<T>(dynamic out T withOutput){...}
//"dynamic out" could be a special combination of keywords so that this only works
// with output variables. Not sure if that constraint buys us anything.
// Maybe we would want to allow it for regular (not out) generic parameters as well
public void Foo<T>(dynamic T withInput){...}
public void Foo<dynamic T>(T withInput){...}
public void dynamic Foo<T>(T withInput){...}
Let's not focus on the syntax itself; we could easily use a different notation--that isn't the point. The idea is simply that we are telling the runtime that when we get to TryFoo<T>... pause and check to see if there is a different, better fitting overload that we should switch to instead. I'm also not particular on whether we are checking for a better resolution on T specifically, or if it applies to the method in general.
Is there any reason why this might be a BAD thing? As he often does, Eric Lippert makes some interesting points about generic and concrete overloading resolution when inheritance is involved (Jon Skeet is also quoted in other answers, as well) and discusses why choosing the most specific overload isn't always the best choice. I'm wondering if having such a language construct might introduce similar problems.
OK, so I proposed the idea of adding support for opting-in to dynamic binding at the C# GitHub and was given this alternate solution which I like MUCH better than what I am currently doing: ((dynamic)this).TryFoo(out withOutput);
It would look like this:
public bool TryRunTestWithDynamic<T>(out T withOutput)
{
return ((dynamic)this).TryFoo(out withOutput);
}
I will reiterate the warning I was given: this only works for instance methods. In my case, that's fine, but if someone else had a method that was static, it wouldn't help, so the second part of my question (regarding MethodInfo.MakeGenericMethod) may still be relevant.
I have a xml parsing code where I am parsing multiple nodes and attributes from xml. Using short circuit, I am able to avoid if in my code because I want to continue processing only in positive case. But I am getting fxcop error CA1002 for parameter as out usages.
How can I remove the fxcorp error?
public bool parseNode()
{
bool success = false;
string val1;
string val2;
string val3
success = TryGetAttributeValue(attribName1, out val1) &&
TryGetAttributeValue(attribName2, out val2) &&
TryGetAttributeValyue(attribName3, out val3);
if(success)
{
// do work
}
}
public bool TryGetAttributeValue(string attribName, out string value)
{
}
Assuming you're talking about CA1021 (Avoid out parameters) and not CA1002 (Do not expose generic lists), FxCop is complaining about the out parameter of your TryGetAttributeValue() method.
You can refactor that method so it returns the attribute value instead of storing it in an out parameter, and have it return nullif the attribute does not exist. From there, you can use the null-coalescing operator ?? to keep the same control flow:
public string TryGetAttributeValue(string attribName)
{
// Return attribute value, or null.
}
public bool ParseNode()
{
if ((TryGetAttributeValue(attribName1)
?? TryGetAttributeValue(attribName2)
?? TryGetAttributeValue(attribName3)) != null) {
// Do work.
}
}
Are you sure it's CA1002? Because that one is the Do not expose generic lists rule according to google. Anyway, I know that FxCop has rule which warns about using out (and also ref) parameters as they are not considered best practice for OO (your are supposed to return a object which represents the result).
In order to get rid of the warning you would need to change your method TryGetAttributeValue to not use out parameters.
As a side note: Microsoft apparently violated this rule in the various TryGet and TryParse methods. So just because FxCop says so, it does not make it necessarily a bad choice.
Assuming you are actually talking about CA1021, which fits your description better: This is the MSDN article about this violation. You can change the method type to something other than public or protected (internal ?)
Otherwise:
To fix a violation of this rule that is caused by a value type, have
the method return the object as its return value. If the method must
return multiple values, redesign it to return a single instance of an
object that holds the values.
If you are not able/willing to change the protection type or change the code to simply return the string, then you will have to ignore this fxcop rule . Which, is not a horrible thing. You have to decide which rules seem pertinent and which do not.
Your code would have to be something like GetAttributeValue, and use a check for null if you want to avoid this fxcop rule. Or, you could create a special class and use a Null object pattern, but that seems to be way too much overkill.
In the end, you are in control of your code, and not all rules are meant for everybody.
If you read the whole article, you can see at the end the following:
Methods that implement the Try pattern, such as
Int32.TryParse, do not raise this violation.
So as long as your methode return a bool and has a name like TryGetSomething, you are not violating the rule if you use the out parameter.
After reading on stackoverflow that in the case of checking the format of a DateTime you should use DateTime.TryParse. After trying some regex expressions they seem to get long and nasty looking to cover lots of the formatting.
But TryParse requires an "out" parameter and since I just want to do a validation format check I don't need the actual result.
So I am left with a variable that holds the "out" result and am to do nothing with it. Is there a way so I don't have to do a out parameter?
So I get rid of this warning and stop having a variable just flying around.
With C#7.0 (since August 2016) you can use the out var construct, and then just ignore the new var in subsequent code.
bool success = DateTime.TryParse(value, out var result);
If you truly do not care about the value of the result, use discards:
bool success = DateTime.TryParse(value, out _);
Nope. I'd wrap it in a method somewhere to keep the noise out of the main flow:
bool IsValidDate(string value)
{
DateTime result;
return DateTime.TryParse(value, out result); //result is stored, but you only care about the return value of TryParse()
}
I'm not suggesting you actually do this, but you could use a single helper class to make this easy for all out parameters:
public static class OutHelper<T>
{
[ThreadStatic]
public static T Ignored;
}
Then you can call:
if (DateTime.TryParse(text, out OutHelper<DateTime>.Ignored))
It's horrible, uses a public mutable field, and if your application is also executing with some malicious code, it gives that code access to the last value you've parsed... but it should work :)
No. You can't get rid of the variable but you shouldn't get a compiler warning either.
Passing a variable as out is "using" the variable. The compiler will not issue a warning because of that.
If you are using .NET 3 and above, you could always create an Extension method?
public static bool IsValidDate(this string value)
{
DateTime date = DateTime.Null;
return DateTime.TryParse(value, out date);
}
[Edited to rename the method name to a more appropriate one]
TryParse is a better option. Its just a variable that is wasted. Other options include using the Convert.ToDateTime() within a try-catch block. But again that would not be efficient because try-catch blocks are meant to be heavy. The next option is regex. This is a better solution. I guess this gives you the result instantly than compared to the others.
You can very well wrap the method like Kim Gräsman said...
I'm trying to do
myDic.TryGetValue("username", out user.Username);
but its not working.
is this not possible?
No, from the documentation:
"Properties are not variables and therefore cannot be passed as out parameters."
http://msdn.microsoft.com/en-us/library/t3c3bfhx.aspx
To continue John's answer, do this instead:
string username;
if (myDic.TryGetValue("username", out username))
{
user.Username = username;
}
You could do this in VB, but not C#.
VB will generate a temporary variable (on the stack), pass in it's address to the out value, and then do an assignment to the property after the method call.
Generally VB does this because it does lots of implicit stuff all over the place. That's just the way it works.
C#, on the other hand, tends to eschew implicitness as part of its philosophy. That's why, for example, you have to add "out" to the call site in order to get out parameters to work, and why it doesn't support "ref" parameters for the first argument to extension methods.
It would be possible to support properties here, using an explicit "out" syntax at the call site. However, I believe the reason they don't do this is because the trick VB uses does not behave exactly the same for properties as it does for fields. With a field, the assignment would take place immediately where it occurred inside the method. If there was other code in the method that read the field (by calling a method on the object), it would read the new field value assigned via the output parameter.
With properties, using the VB trick, the property doesn't get assigned until after the method returns. This means that any code that read the property directly after the out parameter assignment would read the old value.
Here's a simple example of what I mean:
class C
{
private int m_bar;
public int Bar { get { return m_bar; } set { m_bar = value; }}
void foo(out int x)
{
x = 2;
Console.WriteLine(Bar);
}
void DoStuff()
{
foo(out m_bar); //outputs 2
Bar = 0;
//pretend this works
foo(out Bar); //outputs 0
Console.WriteLine(Bar); // outputs 2
}
}
Inside DoStuff(), you would get different behavior for the first call to foo than you would from the second call to foo, although most folks would expect them to behave the same way.
Generally C# tries to avoid these types of things.
That's my guess as to why they don't support it (the spec just says that don't support, it doesn't really say why).
I've got a bit of a problem with a somewhat simple wrapper class I have.
It looks something like this:
public class Wrapper<T>
{
private T _value;
public Wrapper<T>(T value)
{
_value = value;
}
public static implicit operator Wrapper<T>(T value)
{
return new Wrapper<T>(value);
}
public static implicit operator T(Wrapper<T> value)
{
return value._value;
}
}
I've overriden the implicit converters from and to T, so it behaves almost like an instance of T itself.
e.g.
Wrapper<int> foo = 42;
However I've got a slight problem when assigning one instance of Wrapper to another, since I only want to assign the value of the second Wrapper class.
So right now, I have to do this:
Wrapper<int> foo = 42;
Wrapper<int> bar = (int)foo;
Or expose _value publicly through a property.
However since this is in a library, and I don't want the user to depend on remembering this, do you guys have any idea how I could mimic overridding the assignment operator ?
The problem in just changing the pointer (as it does when assigning a class instance to another), is that I've got a dictionary of pointers to these Wrapper objects, so I cannot have them changing all the time, since the dictionary would stop matching then.
I can see if this is somewhat confusing, so if I've left anything important out, please feel free to ask :-)
Since the assignment operator can't be overloaded, there isn't a real good solution. As somebody else pointed out, using a struct will give you the assignment semantics that you want, but then you're faced with value semantics--often not a good thing.
One option is to overload the constructor:
public Wrapper(Wrapper<T> w)
{
_value = w._value;
}
Which would result in this syntax:
Wrapper<int> foo = 42;
Wrapper<int> bar = new Wrapper<int>(foo);
Although more verbose than what you have, it reads better.
Or you could add a Clone method (not the ICloneable interface), so that you could write:
Wrapper<int> bar = foo.Clone();
You could get really creative and overload some operator, making it do essentially nothing. I wouldn't recommend that, though. Using operator overloading for those kinds of things typically makes code cryptic and often breaks.
You could make Wrapper<T> a struct. However I'm not sure if this would suit your application design or not.
If you look at Nullable<T>...which does a very similar thing to what you are doing here, it exposes the internal value using a .Value property.
The problem in just changing the pointer (as it does when assigning a class instance to another), is that I've got a dictionary of pointers to these Wrapper objects, so I cannot have them changing all the time, since the dictionary would stop matching then.
I'm not sure I follow this, what exactly are you storing in the dictionary? Because if you are storing references, the CLR will update them as necessary.
Don't implicitly cast your wrapper both ways.
public class DBValue<T>
{
public static implicit operator DBValue <T>(T value)
{
return new DBValue<T>(value);
}
public static explicit operator T(DBValue <T> dbValue)
{
return dbValue.Value;
}
private readonly T _value;
public T Value { get { this._value; } }
public DBValue(T value)
{
this._value = value;
}
}
Casting from DBValue<T> to T is a lossy conversion (as a minimum, you lose the fact that it's a value from the database), and by best-practice should be explicit. If you don't lose anything by casting from DBValue<T> to T, you might as well just use properties that return T.
Basically, you've already seen why you shouldn't be trying to do this: if a DBValue can be substituted for T and the other way around, how does the compiler (or developer) know which one to choose?
Requiring down-stream developers to write:
string value = MyProperty.Value
or
string value = (string)MyProperty
instead of
string value = MyProperty
...isn't all that onerous, and makes sure that everyone knows exactly what's going on.
EDIT:
To actually answer the question, you can't override reference assignment - or make it look like you have - but you shouldn't really need to.
This is what properties are for. They allow you to define what assignment means. You can't define it for a class or struct itself because they are already defined by the language to do necessary things. Just add a Value property to the class.
Alternatively, edit your question to give a broader description of your design and how this Wrapper fits into it, as someone may be able to suggest a simpler approach.
I just looked into it, making the class a struct is really not an option, since it has some logic in the parameterless constructor, plus it inherits an abstract class, which contains internal abstract functions.
I cannot use an interface, as that'd make those functions public, which would break the logic entirely.
I can post the entire class if that'd be helpful, but it's somewhat long (130 lines)
Or I could toss up on a seperate server, if that'd be better ? (though it hurts the integrity of this question, as I may delete it eventually from that server)
Also explaining the class is really difficult, without writing a complete essay :-/
Anyway I'll try to illustrate the problem I'm having.
Assume 2 table classes: CustomerTable and UserTable:
public class CustomerTable
{
Wrapper<string> Name;
}
public class UserTable
{
Wrapper<string> Name;
}
Now the problem is that some other developer, may use the above code as follows:
CustomerTable customer = new CustomerTable();
UserTable user = new UserTable();
user.Name = customer.Name; // This breaks my internal dictionary
What the developer should had done, in order for it to work, was:
user.Name = (string)customer.Name;
The problem is however, who in their right mind would think about that, when writing code ?
Even if I used a Value property, the developer would still have to remember to write
user.Name = customer.Name.Value; // or user.Name.Value = ....
And again the developer may forget this, and all of a sudden he gets exceptions, or worse: data which isn't persisted to the database.
So my issue is really, that I want the wrapper to be completely transparent (it should be usable as if it was in fact the class/primitive it's wrapping).
However when assigning from one wrapper to another, my internal logic breaks.
Phew a lot of writing, and a lot of code - let me know if I overdo the writing.
A J Lane I see what you mean, and I guess you're right - I just wanted to make it as simple as possible to use the library.
The reason for the implicit cast from DbValue to T, is to simply functions which expects T.
for example
literalSomething.Text = Server.HtmlEncode(SomeTable.SomeStringColumn);
rather than
literalSomething.Text = Server.HtmlEncode((string)SomeTable.SomeStringColumn);
This requires the cast to be implicit.
That being said I just read your comment whilst typing this, and I can see that's quite the issue.
I think I'll go back to exposing value through a property, it just requires the developer to type more, and kinda makes the code ugly I think.
Just imagine DbValue:
if (someValue.Value.HasValue) // someValue is DbValue<int?>
But then again it's probably better with "ugly" code, than code which behaves differently from what you'd expect by merely reading it.
I guess this question ends up as a "best practice" question really.
So to conclude, I'll create a Value property and use that instead of implicit casts, and the developer using the library will just have to live with that.
Thanks for your inputs :-)
This old post stills needs additional information to be complete. It's apparent that the original desired behavior cannot be accomplished since the = operator cannot be overloaded, and likewise C# cannot be "tricked" into casting an object to its own type... it will always boil down to a class reference assignment. But Steffen's further posts show the Wrapper class being used not just with local variables, but as a class field type. The desired semantics can be used AND the integrity of the internal dictionary maintained by using class properties instead of public fields.
Even keeping the original given Wrapper<T> class with both its implicit operators, here's code that would work:
public class CustomerTable
{
private Wrapper<string> _Name;
public Wrapper<string> Name {
get { return _Name; }
set { _Name = (string)value; }
}
}
public class UserTable
{
private Wrapper<string> _Name;
public Wrapper<string> Name {
get { return _Name; }
set { _Name = (string)value; }
}
}
If this change were made, it would not break existing code since it still allows various modes of setting the property:
CustomerTable customer = new CustomerTable();
UserTable user = new UserTable();
user.Name = customer.Name; //*** No longer breaks internal data structures
user.Name = "string literal"; // Works as expected with implicit cast operator
user.Name = (string)customer.Name; // Still allowed with explicit/implicit cast operator
user.Name = customer.Name.Value; // Also works if Value property is still defined
Because this still doesn't answer the original question, use of the Wrapper class could still be problematic if its used outside the class property context, i.e. passed between object, etc. Perhaps the entire Wrapper class could be eliminated with the proper class design, including use of property set/get accessors.