Passing an array

Question

In the sample code below, why does the call to ArrayMethod fail for a genric type when I don't include the class constraint

public interface IFoo { }
public interface IBar : IFoo { }

public class Test
{
    public void ClassConstraintTest<T>() where T : class, IFoo
    {
        T[] variable = new T[0];
        ArrayMethod(variable);
    }

    public void GenericTest<T>() where T : IFoo
    {
        T[] variable = new T[0];
        ArrayMethod(variable);  // Compilation error:  Can't convert T[] to IFoo[]
    }

    public void InheritanceTest()
    {
        IBar[] variable = new IBar[0];
        ArrayMethod(variable);
    }

    public void ArrayMethod(IFoo[] args) { }
}

Answer 1

It's because array covariance, i.e., the fact that MySubtype[] is a subtype of MyType[], only works for reference types. The class constraint ensures that T is a reference type.

(Note that, in retrospect, array covariance is considered to be a bad idea. Try to avoid it if you can, for example, by making ArrayMethod generic or by using IEnumerable<IFoo> instead.)