Question

[Edit: My apologies … the original question wording was ambiguous and I was not getting the responses I am looking for]

For any class X that inherits from class Y, new List<X>() is IEnumerable<Y> is true. However, this doesn’t hold for structs: new List<int>() is IEnumerable<ValueType> is false. My question is: Why?

Here is a sample program:

class Program
{
    class Y { }
    class X : Y { }
    struct Z { }

    static void Main(string[] args)
    {
        Test(new List<X>());
        Test(new List<string>());
        Test(new List<Z>());
        Test(new List<int>());
        Test("blah");
        Test(1);
        Console.ReadLine();
    }

    static void Test(object o)
    {
        if (o is IEnumerable<Y>)
        {
            Console.WriteLine(o + " is a list of Ys");
        }
        else if (o is IEnumerable<ValueType>)
        {
            Console.WriteLine(o + " is a list of ValueTypes");
        }
        else if (o is IEnumerable<object>)
        {
            Console.WriteLine(o + " is a list of objects");
        }
        else if (o is System.Collections.IEnumerable)
        {
            Console.WriteLine(o + " is most likely a list of ValueTypes or a string");
        }
        else
        {
            Console.WriteLine(o + " is not a list");
        }

    }
}

Output:

System.Collections.Generic.List`1[ConsoleApplication1.Program+X] is a list of Ys

System.Collections.Generic.List`1[System.String] is a list of objects

System.Collections.Generic.List`1[ConsoleApplication1.Program+Z] is most likely a list of ValueTypes or a string

System.Collections.Generic.List`1[System.Int32] is most likely a list of ValueTypes or a string

blah is most likely a list of ValueTypes or a string

1 is not a list

So why is new List<int> not a IEnumerable<ValueType>?

Answer 1

Covariance works only for reference types, not for value types. So a List<string> is assignable to an IEnumerable<object> because string is a reference type, but a List<int> is not assignable to an IEnumerable<ValueType>. See section 13.1.3.2 of the C# language specifications for details