Adding operator support to interfaces (Preview Feature in .NET 6)

Question

Just a warning, this question requires a preview of .NET 6 to be installed

I'm trying to create an interface in C# that can allow the + operator similar to how it is implemented in Microsoft's INumber<T>.

Interfaces.cs

using System;
using System.Runtime.Versioning;

namespace InterfaceTest;

[RequiresPreviewFeatures]
public interface IExtendedArray<T> : IAdditionOperators<IExtendedArray<T>, IExtendedArray<T>, IExtendedArray<T>>
    where T : INumber<T>
{
    Array Data { get; }
}

[RequiresPreviewFeatures]
public interface IExtendedFloatingPointArray<T> : IExtendedArray<T>
    where T : IFloatingPoint<T>
{

}

InterfaceTest.csproj

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <EnablePreviewFeatures>true</EnablePreviewFeatures>
        <TargetFramework>net6.0</TargetFramework>
        <ImplicitUsings>false</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

    <ItemGroup>
        <PackageReference Include="System.Runtime.Experimental" Version="6.0.0-preview.7.21377.19" />
    </ItemGroup>

</Project>

However, this code produces

error CS8920: The interface 'InterfaceTest.IExtendedArray' cannot be used as type parameter 'TSelf' in the generic type or method 'IAdditionOperators<TSelf, TOther, TResult>'. The constraint interface 'System.IAdditionOperators<InterfaceTest.IExtendedArray, InterfaceTest.IExtendedArray, InterfaceTest.IExtendedArray>' or its base interface has static abstract members.

Is there a way achieve this with custom types?

dotnet --list-sdks shows I have 6.0.100-rc.1.21458.32 installed. But I just had it installed through the Visual Studio 2022 Preview 4.

It is a [.NET 7 preview feature](https://devblogs.microsoft.com/dotnet/announcing-net-6-preview-7/), add `True` to the project file. — Hans Passant, Sep 18 '21 at 22:31
@GuruStron That's annoying. I updated the question to include more info about my specific dotnet version. It's also an RC, but don't know if it is the same as yours? — Dan, Sep 19 '21 at 11:25
@HansPassant Thanks for that. I had already included that, but forgot to put it in the question. I have updated my question to include the csproj — Dan, Sep 19 '21 at 11:26

Guru Stron · Accepted Answer · 2021-09-19T18:00:15.357

Finally I was able to reproduce your issue - needed to install VS 2022 preview (2019 version compiled code just fine but failed in the runtime) or use dotnet build from terminal.

If you want to do something similar with INumber you need to follow the same pattern with TSelf type reference (if I'm not mistaken it is called curiously recurring template pattern):

[RequiresPreviewFeatures]
public interface IExtendedArray<TSelf, T> : IAdditionOperators<TSelf, TSelf, TSelf> where TSelf : IExtendedArray<TSelf, T> where T : INumber<T>
{
    T[] Data { get; }
}


[RequiresPreviewFeatures]
public interface IExtendedFloatingPointArray<TSelf, T> : IExtendedArray<TSelf, T>
    where TSelf : IExtendedFloatingPointArray<TSelf, T>
    where T : IFloatingPoint<T>
{

}

Adding operator support to interfaces (Preview Feature in .NET 6)

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