C# compilation error with LINQ and dynamic inheritance

Question

Consider the following code (for the sake of this test, it doesn't do anything of particular use - it's just to demonstrate the error that occurs)

Dictionary<string, dynamic> d = new Dictionary<string, dynamic>()
{
    { "a", 123 },
    { "b", Guid.NewGuid() },
    { "c", "Hello World" }
};
d.Where(o => o.Key.Contains("b")).ForEach(i => Console.WriteLine(i.Value));
//retuns the Guid value, as expected.

I want to wrap Dictionary<string, dynamic> using inheritance:

public class CustomDictionary : Dictionary<string, dynamic>
{
}

Here is the examle above using this derived class:

CustomDictionary d = new CustomDictionary()
{
    { "a", 123 },
    { "b", Guid.NewGuid() },
    { "c", "Hello World" }
};
d.Where(o => o.Key.Contains("b")).ForEach(i => Console.WriteLine(i.Value));

This happens...

Any ideas on what is causing the issue, or how to solve it?

Answer 1

I believe I have narrowed this down the binding to the Linq Where extension method.

This works:

   d.AsEnumerable()
    .Where(o => o.Key.Contains("b"))
    .ToList()
    .ForEach(i => Console.WriteLine(i.Value));

And this works (calling the extension method statically):

Enumerable.Where(d.AsEnumerable(),o => o.Key.Contains("b"))
          .ToList()
          .ForEach(i => Console.WriteLine(i.Value));

but this does not:

   d.Where(o => o.Key.Contains("b"))
    .ToList()
    .ForEach(i => Console.WriteLine(i.Value));

If I call the static extension method without AsEnumerable():

Enumerable.Where(d,o => o.Key.Contains("b"))
          .ToList()
          .ForEach(i => Console.WriteLine(i.Value));

I get the better compiler error:

Argument 1: cannot convert from 'UserQuery.CustomDictionary' to 'System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string,dynamic>>'

So for some reason, the compiler is not able to bind the inherited class to the extension method.

The following methods also work:

• explicitly casting d to IEnumerable<System.Collections.Generic.KeyValuePair<string,dynamic>>
• using object instead of dynamic