Suggestion: allow use of non-default constructors in generics

[msauper]

Have you considered allowing a constructor prototype to be defined in the where clause of a generic. This would further constrain the type to have a constructor of the given format.

T GetFoo< T >(int i) where T : new(int i) // or just new(int)
{
return new T(5);
}

[GrabYourPitchforks]

(Not speaking for the language team, just speaking as a partial owner of the System.Reflection namespace.)

This would require runtime support if we wanted to make it efficient. I have some ideas for how we could plumb this through the runtime and reflection APIs in a high-performance fashion, so in theory this should be doable. But I'm having trouble seeing how this would be viable if targeting a runtime that doesn't natively expose these efficient APIs.

[Unknown6656]

@GrabYourPitchforks Is there a possibility that this could be made available in .NET5?
I heard the rumours that .NET5 will be partially re-written from the ground-up. However, I'm not sure whether that concerns the libraries or the runtime. I'm furthermore unsure, how much of a breaking change this proposal will be for the runtime :-/

[Joe4evr]

Duplicate of #769.

[GrabYourPitchforks]

@Unknown6656 The API portion would be tracked by dotnet/runtime#31895 and dotnet/runtime#23716. In theory you'd be able to use the public APIs even without language support. They're just APIs, after all.

I'm not on the language team so can't speak for their plans in this area.

[Richiban]

This would be covered by the Shapes proposal (#164), where generic constraints should cover every type of member of a type, including constructors.

[YairHalberstadt]

Closing as duplicate of #769.