Hello I'm very confused with the difference between a readonly field and a private getter property. I've seen people use both in their code but I simply don't understand what makes them different.
private readonly int num = 0;
// or //
private int Num
{
get { return num; }
}
So is there a difference between using either one of these, or is it by someone's preference?
I'm still beginner in C# & I have some experience with python, so apologies if this is a dumb question. :-)
Getters are usually used to encapsulate access to a field so that implementation logic is hidden from users, making the class more of a black box.
A private getter would only be accesibile to a person implmenting the class (or someone using reflection to access this field) so it wouldn't be useful for encapsulation so much as for implmenting logic in a more convenient way.
A readonly field can only be set in the constructor or field initialiser (which are in both performed in the constructor in the underlying IL), whereas the variable behind a private getter could be set at any time.
A readonly value type is immutable, which means the data held by it in memory will never change once the constructor has executed.
As an example of the difference:
private readonly immutableNum = 0; // this value will be always 0 (unless asigned something else in the constructor
private int num = 0; // this value can be changed anytime by internal logic in the class
private int Num
{
get { Console.WriteLine("Accessed num"); return num; } // additional logic implemented in the getter that can't be done in a readonly field,
//num can be set/reset anytime, so its value can change
}
private immutableGetterNum => 6; //this value will never change
So, do you want to encapsulate access logic? Then you need to use a getter. Do you need to ensure that the value assigned to a field is not changed? Then use a readonly field (or a getter with no underlying field).
Related
Is there any difference between the following?
class C
{
// One:
public static readonly int ValueAsAMember = 42;
// Two:
public static int ValueAsAProperty { get { return 42; } }
}
I'm used to writing constants the first way (unless they're private/internal, in which case I use the const keyword), but I recently saw the second form.
Is there any advantage one way over the other in terms of readability, convention, performance, or anything else?
You have three choices:
public static readonly int Value = 42;
public static int Value { get { return 42; } }
public const int Value = 42;
Choose static readonly if the value will not change at runtime but might change in future versions of your code.
Choose a property if the value might change at runtime. Of course it won't change if you use the given code.
Choose const if the value is really a constant that will not even change in future versions (something like Math.PI or int.MinValue). And of course the use of const is limited by the type of the value.
The difference between const and static readonly is that the const value will be replaced on the call site. If you change the value of a const in a future version then all assemblies that rely on your class need to be recompiled using the new value.
The property requires a method call (calling a getter is a method call). So if the value is constant at runtime there is no need for that.
Yes, there is an advantage:
If the value gets changeable at any point in the future (e.g. in a future version of your code), in a way that it is, for example, time-dependent, you can support that in the read-only property without changing the public interface of your class.
If you have to replace a readonly field with a property, you will have to recompile any other assemblies that use your class.
There are two major differences:
The first is that fields cannot be on interfaces, whereas properties can. So if you want to use this in an interface, you have to use the property.
The second, more interesting, is that readonly fields CAN be modified, while the object is being constructed. Take the following code:
public class MyTestClass
{
public readonly int MyInt = 1;
public MyTestClass()
{
MyInt = 2;
}
}
If a caller does
new MyTestClass().MyInt
they will get 2. The same goes for static constructors for a static readonly field.
The way I see it, using the first way describes the intention of the value better - which is that it is immutable. When a person is looking at the class' interface, he will see that the value is read-only, and won't have to wonder whether it can be changed later (since in the second case he can't see the property's implementation).
An important thing to note about const declarations (I don't believe it's true for readonly) is that changing the field's value constitutes an API change, even if you're just changing the value from 42 to 41. The reason is that for consts, the value is determined during compile time, which means that if I compile a module that uses your constant, and you later change it, I will still be using the old value until I recompile my module with your new version.
readonly is nice to use on things that can only be changed in your constructor. Examples of this is typical services as interfaces when you are following the TDD pattern.
In your example const is best, it's a constant after all.
readonly
const
Cheers
I think the first line making something constant or rather readonly using readonly keyword.
and the second line is making use of a property to implement readonly. Both do the same but if you compare with the IL the property would add few extra lines of code to the dll.
The main advantage for me is with readonly you are allowed to declare it anywhere in your code. But, you will get a chance to set it only once. With the setter, you declare and set in one stroke.
Yes, there's a difference between the two.
A readonly field can only be set in the constructor.
A {get; private set;} can be set at anytime from within the class.
Example:
public class Car
{
public readonly string Name;
public string color {get; private set;}
public Car()
{
Name = "Car";
Color = "Red";
}
// will fail compilation
public void ModifyName()
{
Name = "Subaru"
}
// perfectly ok
public void ModifyColor()
{
Color = "Green"
}
}
A Property is just syntactic sugar around a field, a property without a setter is simply declared a readonly field so the compiler will allow you to set it at runtime in the constructor, because to the compiler you are referencing a readonly field. There is a larger discussion around what to use a field or property, which is not within the scope of the question. And yes its this syntactic sugar that you have to do the recompiling referenced by #SOreadytohelp. Just to be clear a property is a field with a get and set method created for it, C# will allow you to reference it like a field rather than doing an annoying call to the getter or setter everytime.
I have an Account class that contain a readonly variable balance.
Constructor will assign default value as 0.
But i want to update that value when I call CashIn method.
I tried this:
class Account
{
private string name;
private readonly double balance;
public Account(string n, double b)
{
name = n;
balance = b;
}
public void CashIn(double amount)
{
balance = balance+amount;
}
}
But as I know, readonly variable could be assigned through constructor or field.
Is it possible to update balance when i call CashIn method?
If yes then Please describe me how?
There is a big difference between the readonly modifier and the concept of a "read only" property.
The readonly modifier means your field can only be set in the constructor. You will not be able to set that variable to anything else outside of the constructor. Given the context of your code (a bank account simulator) it seems weird that you would have a balance that never, ever changes. Which leads me to believe you want a read only property.
A property is the preferred way of storing simple data that has get/set methods. For a bank account simulator it would be really bad to just have the balance exposed with a public setter right? So I think in that context it makes more sense to have a property with a public read, and a private set:
public decimal Balance { get; private set; }
This allows it to be read publicly but its only allowed to be modified inside the class (i.e from a custom method you write like CashIn())
public void CashIn(decimal amount)
{
Balance += amount;
}
Please note how I use decimal instead of double. Monetary values should always be represented as decimal, this is why it exists
The entire point of readonly is that you cannot do this, except in the constructor. Since readonly is a language level feature, not a strict runtime guarantee: you can cheat (via reflection or unsafe or various other evils), but it seems to me that the most pragmatic thing is to: remove the readonly modifier. Since you don't seem to want it to be read-only.
A simple solution is to remove a read-only tag for a variable. you can not change a value of read-only variables once it's initialized.
you can initialize it while creating the object and thereafter it will be treated as a constant for the lifetime of that object.
Create a private variable, and a Public Readonly Property that return its value, so you can edit the variable from the same class, but it is read only to other classes.
private double balance_;
public double Balance
{
get
{
return balance_;
}
}
Or you can use private set
public double Balance { get; private set; }
Check this link for more info: understanding private setters
Is there any difference between the following?
class C
{
// One:
public static readonly int ValueAsAMember = 42;
// Two:
public static int ValueAsAProperty { get { return 42; } }
}
I'm used to writing constants the first way (unless they're private/internal, in which case I use the const keyword), but I recently saw the second form.
Is there any advantage one way over the other in terms of readability, convention, performance, or anything else?
You have three choices:
public static readonly int Value = 42;
public static int Value { get { return 42; } }
public const int Value = 42;
Choose static readonly if the value will not change at runtime but might change in future versions of your code.
Choose a property if the value might change at runtime. Of course it won't change if you use the given code.
Choose const if the value is really a constant that will not even change in future versions (something like Math.PI or int.MinValue). And of course the use of const is limited by the type of the value.
The difference between const and static readonly is that the const value will be replaced on the call site. If you change the value of a const in a future version then all assemblies that rely on your class need to be recompiled using the new value.
The property requires a method call (calling a getter is a method call). So if the value is constant at runtime there is no need for that.
Yes, there is an advantage:
If the value gets changeable at any point in the future (e.g. in a future version of your code), in a way that it is, for example, time-dependent, you can support that in the read-only property without changing the public interface of your class.
If you have to replace a readonly field with a property, you will have to recompile any other assemblies that use your class.
There are two major differences:
The first is that fields cannot be on interfaces, whereas properties can. So if you want to use this in an interface, you have to use the property.
The second, more interesting, is that readonly fields CAN be modified, while the object is being constructed. Take the following code:
public class MyTestClass
{
public readonly int MyInt = 1;
public MyTestClass()
{
MyInt = 2;
}
}
If a caller does
new MyTestClass().MyInt
they will get 2. The same goes for static constructors for a static readonly field.
The way I see it, using the first way describes the intention of the value better - which is that it is immutable. When a person is looking at the class' interface, he will see that the value is read-only, and won't have to wonder whether it can be changed later (since in the second case he can't see the property's implementation).
An important thing to note about const declarations (I don't believe it's true for readonly) is that changing the field's value constitutes an API change, even if you're just changing the value from 42 to 41. The reason is that for consts, the value is determined during compile time, which means that if I compile a module that uses your constant, and you later change it, I will still be using the old value until I recompile my module with your new version.
readonly is nice to use on things that can only be changed in your constructor. Examples of this is typical services as interfaces when you are following the TDD pattern.
In your example const is best, it's a constant after all.
readonly
const
Cheers
I think the first line making something constant or rather readonly using readonly keyword.
and the second line is making use of a property to implement readonly. Both do the same but if you compare with the IL the property would add few extra lines of code to the dll.
The main advantage for me is with readonly you are allowed to declare it anywhere in your code. But, you will get a chance to set it only once. With the setter, you declare and set in one stroke.
Yes, there's a difference between the two.
A readonly field can only be set in the constructor.
A {get; private set;} can be set at anytime from within the class.
Example:
public class Car
{
public readonly string Name;
public string color {get; private set;}
public Car()
{
Name = "Car";
Color = "Red";
}
// will fail compilation
public void ModifyName()
{
Name = "Subaru"
}
// perfectly ok
public void ModifyColor()
{
Color = "Green"
}
}
A Property is just syntactic sugar around a field, a property without a setter is simply declared a readonly field so the compiler will allow you to set it at runtime in the constructor, because to the compiler you are referencing a readonly field. There is a larger discussion around what to use a field or property, which is not within the scope of the question. And yes its this syntactic sugar that you have to do the recompiling referenced by #SOreadytohelp. Just to be clear a property is a field with a get and set method created for it, C# will allow you to reference it like a field rather than doing an annoying call to the getter or setter everytime.
I need to implement a read only property on my type. Moreover the value of this property is going to be set in the constructor and it is not going to be changed (I am writing a class that exposes custom routed UI commands for WPF but it does not matter).
I see two ways to do it:
class MyClass
{
public readonly object MyProperty = new object();
}
class MyClass
{
private readonly object my_property = new object();
public object MyProperty { get { return my_property; } }
}
With all these FxCop errors saying that I should not have public member variables, it seems that the second one is the right way to do it. Is this correct?
Is there any difference between a get only property and a read only member in this case?
The second way is the preferred option.
private readonly int MyVal = 5;
public int MyProp { get { return MyVal;} }
This will ensure that MyVal can only be assigned at initialization (it can also be set in a constructor).
As you had noted - this way you are not exposing an internal member, allowing you to change the internal implementation in the future.
C# 6.0 adds readonly auto properties
public object MyProperty { get; }
So when you don't need to support older compilers you can have a truly readonly property with code that's just as concise as a readonly field.
Versioning:
I think it doesn't make much difference if you are only interested in source compatibility.
Using a property is better for binary compatibility since you can replace it by a property which has a setter without breaking compiled code depending on your library.
Convention:
You are following the convention. In cases like this where the differences between the two possibilities are relatively minor following the convention is better. One case where it might come back to bite you is reflection based code. It might only accept properties and not fields, for example a property editor/viewer.
Serialization
Changing from field to property will probably break a lot of serializers. And AFAIK XmlSerializer does only serialize public properties and not public fields.
Using an Autoproperty
Another common Variation is using an autoproperty with a private setter. While this is short and a property it doesn't enforce the readonlyness. So I prefer the other ones.
Readonly field is selfdocumenting
There is one advantage of the field though:
It makes it clear at a glance at the public interface that it's actually immutable (barring reflection). Whereas in case of a property you can only see that you cannot change it, so you'd have to refer to the documentation or implementation.
But to be honest I use the first one quite often in application code since I'm lazy. In libraries I'm typically more thorough and follow the convention.
With the introduction of C# 6 (in VS 2015), you can now have get-only automatic properties, in which the implicit backing field is readonly (i.e. values can be assigned in the constructor but not elsewhere):
public string Name { get; }
public Customer(string name) // Constructor
{
Name = name;
}
private void SomeFunction()
{
Name = "Something Else"; // Compile-time error
}
And you can now also initialise properties (with or without a setter) inline:
public string Name { get; } = "Boris";
Referring back to the question, this gives you the advantages of option 2 (public member is a property, not a field) with the brevity of option 1.
Unfortunately, it doesn't provide a guarantee of immutability at the level of the public interface (as in #CodesInChaos's point about self-documentation), because to a consumer of the class, having no setter is indistinguishable from having a private setter.
In C# 9, Microsoft introduced a new way to have properties set only on initialization using the init accessor, like so:
public class Person
{
public string FirstName { get; init; }
public string LastName { get; init; }
}
This way, you can assign values when initializing a new object:
var person = new Person
{
Firstname = "John",
LastName = "Doe"
}
But later on, you cannot change it:
person.LastName = "Denver"; // throws a compiler error
You can do this:
public int Property { get { ... } private set { ... } }
I agree that the second way is preferable. The only real reason for that preference is the general preference that .NET classes not have public fields. However, if that field is readonly, I can't see how there would be any real objections other than a lack of consistency with other properties. The real difference between a readonly field and get-only property is that the readonly field provides a guarantee that its value will not change over the life of the object and a get-only property does not.
yet another way (my favorite), starting with C# 6
private readonly int MyVal = 5;
public int MyProp => MyVal;
https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/properties#expression-body-definitions
The second method is preferred because of the encapsulation. You can certainly have the readonly field be public, but that goes against C# idioms in which you have data access occur through properties and not fields.
The reasoning behind this is that the property defines a public interface and if the backing implementation to that property changes, you don't end up breaking the rest of the code because the implementation is hidden behind an interface.
I'm currently implementing a poor-man's version of the RSA Algorithm and I wanted the prime numbers d, e, m, and n to be read-only as they will be automatically generated within ithe constructor body. However, I get two different results when I type:
class RSA
{
public RSA()
{
n = 4;
}
private long n { get; private set; }
}
or
class RSA
{
public RSA()
{
n = 4;
}
private long n { get; }
}
Reading the book Accelarated C#, I got the impression that a private set function can be implemented with auto-implemented properties. Turns out I can do it in the constructor itself too, but only for the first version.
Reading the C# 3.0 standard it says:
A property that has both a get accessor and a set accessor is a read-write property, a property that has only a get accessor is a read-only property, and a property that has only a set accessor is a write-only property.
Yet they don't behave equally.
Simple question: Why can I initialize the value in my constructor when I explicitly declare private set, but not if I do it implicitly? What are the differences here?
In the first case the setter is private. This means that you can set the value of this property everywhere from inside this class not only in the constructor. In the second example there's no setter won't be able to set the value. Using readonly field is probably most correct semantically in your case as it will allow you to set its value either in the constructor or directly when declaring the field (if this field shouldn't be visible from outside of this class):
class RSA
{
public RSA()
{
n = 4;
}
private readonly long n;
}
There's no such thing as an implicit autogenerated private setter. If you omit set;, you cannot assign it a value, which will not compile because it can never be assigned.
Additionally, by making n private you cannot access it from outside that class. If that is your intention, there's no point in using a property at all. You can simply declare it as a field: private readonly long n;
If you really want a property with a readonly value, you could use a Property with the explicit backing field IE:
public class RSA {
private readonly long _n;
public long n {
get { return _n; }
}
public RSA()
{
_n = 4;
}
}