Question

Why does the following cause a compilation error?

interface IX {}
interface IY {}
class XY : IX, IY {}

void Foo<T>() where T : IX, IY
{
    T xy = new XY();
    …   // ^^^^^^^^
}       // error: "Implicit conversion of type 'XY' to 'T' is not possible."

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Note: The same error would occur if class XY : IX and where T : IX. However, I have chosen a more complex example because a simpler one might have provoked circumventive answers such as, “Just change the type of xy from T to IX“, which would not answer why this conversion fails.

Answer 1

Given class ABC : IX, IY { } and Foo<ABC>, would you expect to be able to use new XY() then? Because you shouldn’t have that expectation. The compiler will not, either.

T is not always going to be XY. T is going to be ABC, DEF, or anything else that could implement your two interfaces and therefore meet the constraints you have. XY is not convertible to ABC, DEF, or any of the infinite possibilities for T, and therefore you have your error message: the implicit conversion of XY to T is not possible.

What would be legal there is simply new T(), and this is true only if the method is constrained to support it.

void Foo<T>() where T : IX, IY, new()
{
    T obj = new T();
}