Expand where T : new() constructor constraint to allow known parameter types

[thomhurst]

I don't see any reason this couldn't be done. The compiler will be able to look at the public constructors of a type and know if those are available.

This can be useful when wanting to instantiate generic unknown types but with well-known parameters.

A good example of this could be custom exceptions that follow the standard exception constructor patterns.

Syntax:

where TType : new(TParameterTypes..) where TParameterTypes can be as many types as you define, comma separated.

E.g.

where TType : new(TParameterType1, TParameterType2, TParameterType3)

Use case example:

public class GenericFailureHandler<TExceptionType>
    where TExceptionType : Exception
    where TExceptionType : new(string)
    where TExceptionType : new(string, Exception)
{
    public void Handle(Exception parentException)
    {
        ...
        throw new TExceptionType("My reason", parentException);
    }
}

If TExceptionType didn't have a constructor taking a string, and a constructor taking a string, Exception, then the compiler would give you an error, much like it does with where T : new() when there isn't a parameterless constructor.

This can avoid the Activator classes, and eliminate their need for reflection in cases where the parameters are known.

This also means it would help with Native AOT if the compiler knows about the constructor usage at compile time.

[colejohnson66]

Logically, yes, this makes sense. But the IL does not support it. Generic constrains must be one or more of the following:

• a covariant type
• a contravariant type
• class
• valuetype (struct in C#)
• .ctor (new() in C#)

See II.10.1.7 in ECMA-335 (6th edition)

[huoyaoyuan]

#769, #1330, #1574, #1626, #1633, #4532