Looking for an interface/abstract but with methods returning different types

Question

I am looking for a design pattern for a base class that forces all derived classes to implement all base class methods, while the methods in the derived classes have different return types. The scenario is a generic MemoryObject class (interface/abstract), keeping and returning whatever kind of objects in/from memory. But the derived classes (static/sealed/singleton) should return the correct type.

So the requirements are - force all derived classes to implement all properties and methods of the base class - methods in derived classes must return correct type

Here's some sample code, not compilable, but for illustration:

    public interface MemoryObject
    {
         object FromMemory { get; }
         //... other properties and methods
    }

    public sealed class MemoryObjectA : MemoryObject
    {
        public static List<string> FromMemory 
        {
            get { return new List<string>(); }
        }

        //... other properties and methods
    }

    public sealed class MemoryObjectB : MemoryObject
    {
        public static DataTable FromMemory 
        {
            get { return new DataTable(); }
        }

        //... other properties and methods
    }

Thanks for your suggestions in advance!

Answer 1

It sounds like you just need to make it generic:

public interface MemoryObject<T>
{
     T FromMemory { get; }
     //... other properties and methods
}

public class MemoryObjectA : MemoryObject<List<string>> { ... }
public class MemoryObjectB : MemoryObject<DataTable> { ... }

Note that the implementation methods will have to be instance methods, not static methods. (You can make the implementations singletons though, should you wish.)