Do interface variables have value-type or reference-type semantics?

Question

Do interface variables have value-type or reference-type semantics?

Interfaces are implemented by types, and those types are either value types or reference types. Obvi

Answer 1

A variable or field whose type is IComparable is a reference-type variable or field, regardless of the type of the value assigned to that field. This means that x in the sample code is boxed.

A simple test will demonstrate that. The test is based on the fact that you can only unbox a value type to its original type (and the nullable version of that type):

    [TestMethod, ExpectedException(typeof(InvalidCastException))]
    public void BoxingTest()
    {
        IComparable i = 42;
        byte b = (byte)i;      //exception: not allowed to unbox an int to any type other than int
        Assert.AreEqual(42, b);
        Assert.Fail();
    }

EDIT

On top of that, the C# specification specifically defines reference-type as comprising class types, interface types, array types and delegate types.

EDIT 2

As Marc Gravell points out in his answer, a generic type with an interface constraint is a different case. This doesn't cause boxing.