Hi,
When I try to compile this class, I get two compilation errors:

Error    1    The best overloaded method match for
'PolymorphismTest.PolymorphismTest.DoSomethingElse(System.Collections.Generic.IList<object>)'
has some invalid arguments    C:\Documents and Settings\aw\My Documents\Visual
Studio 2005\Projects\Reporting\PolymorphismTest.cs    16    13    ReportsGenerator
Error    2    Argument '1': cannot convert from
'System.Collections.Generic.IList<PolymorphismTest.Special>' to
'System.Collections.Generic.IList<object>'    C:\Documents and Settings\aw\My
Documents\Visual Studio
2005\Projects\Reporting\PolymorphismTest.cs    16    29    ReportsGenerator


using System;
using System.Collections.Generic;

namespace PolymorphismTest
{
    public class PolymorphismTest
    {
        public void DoSomethingElse(IList<Object> objects)
        {

        }

        public void DoSomething()
        {
            IList<Special> specialObjects = new List<Special>();
            DoSomethingElse(specialObjects);
        }
    }

    public class Special : Object
    {

    }
}

As far as I can see this should work - I must be missing something?

can anyone help?

Thanks in advance :)

skywalker <skywal***@discussions.microsoft.com> wrote:

<snip>

Yes. IList<SomeType> isn't convertible to IList<Object>. Generic types
don't have covariance.

The reason for this is simple. Consider if DoSomethingElse did:

objects.Add (new object());

You've just passed in an IList<Special>... what should happen? A
runtime exception? One of the points of generics is to give compile-
time type safety, avoiding runtime exceptions due to type issues.

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