Question

I tried code below in .NET 3.5 but got exception at line shown in comment:

Cannot convert type 'TUnpaid' to 'ClassLibrary1.Unpaid'

How to fix this in .NET 3.5 ?

namespace ClassLibrary1
{
    public class EntityBase
    {
        public virtual void Save<T>(T dok) where T : EntityBase, new()
        {
        }
    }

    public class Unpaid : EntityBase
    {
        public override void Save<TUnpaid>(TUnpaid dok)
        {
            // Cannot convert type 'TUnpaid' to 'ClassLibrary1.Unpaid'  
            var kup = (Unpaid)dok;
        }
    }
}

Answer 1

You either should write an operator of type conversion, or write a method (or class) which establishes the connections between two types.

In your code sample you use the two different types (TUnpaid and Unpaid) with no connection between them.