Suggestion: Interface union types for C#

[RobBlob]

In C#, I frequently want to express a type as interface that implements a number of interfaces. I can derive an interface as so...

interface IDerived : IBase1, IBase2, ... {
}

and declare the signature of a method like so...

void MyMethod(IDerived arg) {
}

but that requires the argument to explicitly derive from IDerived (which is not possible for types I have not defined). What I would really want to express is simply that the type of this variable is the union of the types of the inherited interfaces.

For example, would like to be able to declare...

void MyMethod((IBase1 + IBase2) arg);

(Note - the syntax here is purely expository, not a serious suggestion. I suspect it may clash with other language constructs).

Alternatively, allow declaring types like IDerived...

union IDerived : IBase1, IBase2;

And allow objects that implement IBase1 and IBase2 to conform to the type of IDerived.

As a concrete example, it may be desirable to declare an argument as being a union of IList, INotifyCollectionChanged and INotifyPropertyChanged. Many existing types implement these interfaces, but it is impossible to express in a (statically) type safe manner that an argument must implement all these interfaces that may be satisfied by objects of existing classes.

[iam3yal]

I think that this is covered by #113 and you can also have a look here #399.

[quinmars]

No, #113 is something else, but #399 has the same intention with slightly different syntax.

[RobBlob]

Thanks, I was unaware of them. #399 in particular looks like the same area.