How to verify generic return type in BuildEligibility for async methods?

[joakimriedel]

New to metalama, so this might be obvious but cannot find any help through the docs.

I would like to check something like

builder.MustSatisfy(m => 
    m.ReturnType.Is(typeof(Task<ServiceResponse>)) || 
    m.ReturnType.Is(typeof(Task<ServiceResponse<>>)), 
    m => $"{m} must return Task<ServiceResponse> or Task<ServiceResponse<T>>");

but this obviously wont work since typeof(Task<ServiceResponse<>>) is not valid C#.

I tried the following, but it won't work due since runtime reflection cannot be used at compile time.

builder.MustSatisfy(m => 
    m.ReturnType.Is(typeof(Task<ServiceResponse>)) || 
    m.ReturnType.ToType().GenericTypeArguments[0].GetGenericTypeDefinition() == typeof(ServiceResponse<>), 
    m => $"{m} must return Task<ServiceResponse> or Task<ServiceResponse<T>>");

Resorted to the following solution, which works, but there must be a better way..

builder.MustSatisfy(m =>
    m.ReturnType.ToString()!.Contains("ServiceResponse"),
    m => $"{m} must return Task<ServiceResponse> or Task<ServiceResponse<T>>");

Please help me understand how I will properly test for a generic return type for async methods.

[svick]

You can use Is with ConversionKind.TypeDefinition for this:

builder.ReturnType().MustSatisfy(
    type => type.Is(typeof(Task<ServiceResponse>)) ||
        (type.Is(typeof(Task<>), ConversionKind.TypeDefinition) &&
        ((INamedType)type).TypeArguments.Single().Is(typeof(ServiceResponse<>), ConversionKind.TypeDefinition)),
    t => $"{t} must be Task<ServiceResponse> or Task<ServiceResponse<T>>");

(I also changed the code to use builder.ReturnType(), but that's not necessary.)

Another option is to use GetAsyncInfo(), which will work for any awaitable type (like ValueTask<T>):

builder.ReturnType().MustSatisfy(
    type =>
    {
        var asyncInfo = type.GetAsyncInfo();

        return asyncInfo.IsAwaitable &&
            (asyncInfo.ResultType.Is(typeof(ServiceResponse)) ||
            asyncInfo.ResultType.Is(typeof(ServiceResponse<>), ConversionKind.TypeDefinition));        
    }, t => $"{t} must be Task<ServiceResponse> or Task<ServiceResponse<T>>");

[joakimriedel]

Yes, this is the solution! Thanks 🙏

I actually did find, and try, using GetAsyncInfo() but I did not know about the ConversionKind.TypeDefinition flag so I tested for (asyncInfo.ResultType.Is(typeof(ServiceResponse)) || asyncInfo.ResultType.Is(typeof(ServiceResponse<>)) which would not work. This would make an excellent note in some docs for the future!