A method was marked as an override, but the compiler found no method to override. For more information, see override (C# Reference), and Knowing When to Use Override and New Keywords (C# Programming Guide).
The following sample generates CS0115. You can resolve CS0115 in one of two ways:
Remove the override keyword from the method in MyClass2.
Use MyClass1 as a base class for MyClass2.
// CS0115.cs namespace MyNamespace { abstract public class MyClass1 { public abstract int f(); } abstract public class MyClass2 { public override int f() // CS0115 { return 0; } public static void Main() { } } }