C#•8mo ago

❔ How to call static virtual interface method without completely reimplementing in derived class

This mainly pertains to generic math in C# 11 / .NET 7, but say I have a minimum of something like this:

interface ITest
{
    static virtual void Test() => Console.WriteLine("Test");
}
class Testing : ITest
{
}

interface ITest
{
    static virtual void Test() => Console.WriteLine("Test");
}
class Testing : ITest
{
}

I cannot find a way to call the Test method from ITest without just redefining it entirely in Testing, even if said redefining is 100% identical to the interface's code (which seems terrible to me). Obviously I can't call ITest.Test() because it is an interface and not a class, and I can't call Testing.Test() because the method technically doesn't exist on Testing. The only workaround I've found is doing something like void RunTest<T>() where T : ITest => T.Test(); and calling it via RunTest<Testing>(); but that doesn't work for my purposes. I also can't define the method in Testing as public static void Test() => ITest.Test(); for the same reason as being unable to just call the last part due to it being on an interface.

5 Replies

reflectronic•8mo ago

RunTest<ITest>() also works other than that, yeah, this is one of the pain points with default interface members. there is no easy way to call them directly

CyberBotX•8mo ago

I see. I was just mentioning on another server that I could also do this:

class Testing : ITest
{
    static void TestInternal<T>() where T : ITest => T.Test();
    public static void Test() => Testing.TestInternal<ITest>();
}

class Testing : ITest
{
    static void TestInternal<T>() where T : ITest => T.Test();
    public static void Test() => Testing.TestInternal<ITest>();
}

Then I can call Testing.Test(); and it works, but it looks ugly to me. But it might be what I need to do because I want it so I call the method for the class, not from a method external of the class.

reflectronic•8mo ago

https://github.com/dotnet/csharplang/issues/2337

GitHub

Champion: base(T) phase two · Issue #2337 · dotnet/csharplang

Proposal added (below) Discussed in LDM https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-02-27.md Decision in LDM Finalized (done, rejected, inactive) Spec'ed #2910 On 20...

CyberBotX•8mo ago

That'll be nice if it gets implemented, for sure. I guess related to this, then, I'm trying to do this for one of the static virtual properties in IBinaryNumber and it isn't letting me do it. If I have a class that is like:

class Testing : IBinaryNumber<Testing>
{
    public static Testing AllBitsSet => Helpers.AllBitsSet<IBinaryNumber<Testing>>(); // Doesn't work, gives errors CS8920 and CS0311

    // Assume full implentation of the interface is here
}

class Testing : IBinaryNumber<Testing>
{
    public static Testing AllBitsSet => Helpers.AllBitsSet<IBinaryNumber<Testing>>(); // Doesn't work, gives errors CS8920 and CS0311

    // Assume full implentation of the interface is here
}

With Helpers being a static class containing:

internal static TSelf AllBitsSet<TSelf>() where TSelf : IBinaryNumber<TSelf> => TSelf.AllBitsSet;

internal static TSelf AllBitsSet<TSelf>() where TSelf : IBinaryNumber<TSelf> => TSelf.AllBitsSet;

The two errors say that it cannot be used as a type argument (trying to say there is no specific implementation for IsPow2, despite that the class for TSelf does have IsPow2 on it) and that it cannot be used as a type parameter for TSelf because it can't convert IBinaryNumber<Testing> into IBinaryNumber<IBinaryNumber<Testing>>. If I change the type parameter to just Testing, it compiles but gives me an obvious stack overflow. (I just really don't want to copy/paste the code from the interfaces into my code unless I absolutely have to, even though I can see that .NET itself has done that for some things, like for instance, int's CreateChecked is just a carbon copy of the INumber<T> code.)

Accord•8mo ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ How to call static virtual interface method without completely reimplementing in derived class