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Provide "static virtual interface members" with examples and description of static virtual methods #43130

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7629f32
Implement entities showing static virtual in interface
BartoszKlonowski Oct 19, 2024
8cfa247
Utilize the static virtual methods defining in TestDuplication example
BartoszKlonowski Oct 19, 2024
dd40445
Describe in details the static virtual methods of an interface
BartoszKlonowski Oct 19, 2024
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14 changes: 14 additions & 0 deletions docs/csharp/whats-new/tutorials/snippets/staticinterfaces/Duplicate.cs
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@@ -0,0 +1,14 @@
public struct Duplicate : IGetDuplicated<Duplicate>
{
public int _num;

public Duplicate(int num)
{
_num = num;
}

public static Duplicate operator ++(Duplicate other)
=> other with { _num = other._num + other._num };

public override string ToString() => _num.ToString();
}
4 changes: 4 additions & 0 deletions docs/csharp/whats-new/tutorials/snippets/staticinterfaces/GetDuplicated.cs
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public interface IGetDuplicated<T> where T : IGetDuplicated<T>
{
static virtual T operator ++(T other) => throw new NotImplementedException();
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I'd like to think through this and use a different example for a static virtual (not abstract) method. That's because in practice, if the only reasonable implementation in the interface declaration is to throw a NotImplementedException, that indicates that it should be abstract.

Maybe consider an interface that provides < and >, but assumes that T implements IComparable<T>. Something like:

public interface IArithmeticComparable<T> where T : IComparable<T>
{
    static virtual bool operator > (T left, T right) => left.CompareTo(right) > 0;

// etc.
}

I know that adds a bit more concepts to explain, so I'm open to any alternative example. That's just the first obvious one that came to my mind.

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That is valid point, @BillWagner, thanks! But it looks like my knowledge is missing something, because my question is: What would be the use case of such implementation? In other words how to call the static virtual defined in an interface without any definition in the type implementing that interface? I know I'm asking kinda for an implementation details, but can you point me some examples of such use cases or implementations?

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I agree it's difficult to have a good scenario here @BartoszKlonowski

That's why I suggested the < and > operators. That's the one scenario I could think of where this works. It's not great, but it's reasonable.

Or, since any static method could be used, something like this might work:

public interface ISingleton<T> where T : new()
{
    public static virtual T Instance
    {
        get
        {
            field ??= new T();
            return field;
        }
        private set
        {
            field = value;
        }
    }
}

I have concerns here as well, because it uses a preview feature. But maybe we can adapt it somehow to do something like Lazy<T>, but use a static virtual method.

}
6 changes: 6 additions & 0 deletions docs/csharp/whats-new/tutorials/snippets/staticinterfaces/Program.cs
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Expand Up @@ -9,6 +9,12 @@
Console.WriteLine(str++);
// </TestRepeat>

// <TestDuplication>
var num = new Duplicate(2);
num++;
Console.WriteLine(num.ToString());
// </TestDuplication>

// <TestAddition>
var pt = new Point<int>(3, 4);

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20 changes: 20 additions & 0 deletions docs/csharp/whats-new/tutorials/static-virtual-interface-members.md
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Expand Up @@ -66,6 +66,26 @@ AAAAAAAAAA

This small example demonstrates the motivation for this feature. You can use natural syntax for operators, constant values, and other static operations. You can explore these techniques when you create multiple types that rely on static members, including overloaded operators. Define the interfaces that match your types' capabilities and then declare those types' support for the new interface.

## Static virtual interface methods

Similar can be achieved using the static virtual methods of an interface. This is done with the syntax of `static` and `virtual` modifiers added to any member that should be implemented in the type implementing that interface. The following example defines the `IGetDuplicated<T>` interface, providing any type with a constraint that a type will implement the `operator ++`:

:::code language="csharp" source="./snippets/staticinterfaces/GetDuplicated.cs":::

Because virtual methods aren't abstract, they must declare their body. However, for now it's throwing the `NotImplementedException` exception by default, to highlight the lack of implementation.

The next snippet creates a structure that implements the `IGetDuplicated<T>` interface. This `Duplicate` structure uses the `++` operator implementation to duplicate the initial number given when creating the structure:

:::code language="csharp" source="./snippets/staticinterfaces/Duplicate.cs":::

You can use this `Duplicate` type by calling the `operator ++`:

:::code language="csharp" source="./snippets/staticinterfaces/Program.cs" id="TestDuplication":::

which will print the result of `4`.

Note that without having the `operator ++` implemented, calling `num++` would still not compile, leading to [CS0023](../../misc/cs0023.md). Reason for that is that even if we have the default implementation in the static virtual method of an interface, this is still only an interface and the type implementing that interface has no such operator implemented.

## Generic math

The motivating scenario for allowing static methods, including operators, in interfaces is to support [generic math](../../../standard/generics/math.md) algorithms. The .NET 7 base class library contains interface definitions for many arithmetic operators, and derived interfaces that combine many arithmetic operators in an `INumber<T>` interface. Let's apply those types to build a `Point<T>` record that can use any numeric type for `T`. The point can be moved by some `XOffset` and `YOffset` using the `+` operator.
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