Stop hiding function pointer delegate constructor

[ThadHouse]

Since we now have full support for Function Pointers in C#, we should stop hiding the function pointer taking delegate constructor that exists on all delegates. This would allow some interesting delegate to function pointer mapping scenarios.

[bernd5]

That would be really great!

I wrote this as a workaround:

 public static DelegateType CreateDelegate<DelegateType>(object target, IntPtr methodPtr)
      where DelegateType : Delegate
{
    var ctor = typeof(DelegateType).GetConstructor(new[] { typeof(object), typeof(IntPtr) })!;
    var result = ctor.Invoke(new[] { target, methodPtr });
    return (DelegateType)result;
}

[tannergooding]

For reference, every delegate type inherits from System.Delegate or System.MulticastDelegate (the latter for those defined in C#).

These types expose the following public methods:

• public MyDelegate(object @object, IntPtr method)
• public virtual void Invoke(<args>)
• public virtual System.IAsyncResult BeginInvoke(<args>, System.AsyncCallback callback, object @object);
• public virtual void EndInvoke(System.IAsyncResult result)

The first is a constructor that takes the Target property and a function pointer (currently exposed as an IntPtr). Given function pointers now exist in C#, it might be beneficial to no longer hide this constructor and to simply allow new MyDelegate(null, someFnptr).

[333fred]

If anyone would like to take the time to propose the rule changes necessary to https://github.com/dotnet/csharplang/blob/main/spec/expressions.md#delegate-creation-expressions, I'd be willing to champion the feature.

[sakno]

Possibly dup of #3680