IEnumerable doesn't seem to be covariant, what is the workaround?

I've been using Unity to learn C# and I believe the answer to this is quite explicit, but I need some confirmation before blaming C# instead of blaming myself.

According to the source of the IEnumerable, it doesn't seem to be covariant:

 using System;
 using System.Runtime.CompilerServices;
 namespace System.Collections.Generic
 {
     [TypeDependency ("System.SZArrayHelper")]
     public interface IEnumerable<T> : IEnumerable
     {
         //
         // Methods
         //
         IEnumerator<T> GetEnumerator ();
     }
 }

So if it isn't covariant, what are the usual workarounds into this issue?

Thank you!