What are the good reasons to wish that .NET generics could inherit one of the generic parameter types?

Question

This post is in continuation of this one.

I am trying to understand if I am the only one who misses and needs the ability of a .NET generic type to inherit one of it

Answer 1

The basic rule of generics that prevents this is "the content of a generic type must be well-defined on the generic argument." Lets look at how this applies to the following code:

public abstract class AbstractBase
{
    public abstract string MyMethod();
}

public class SomeType : T
{
}

public class SomeUsage
{
    void Foo()
    {
        // SomeType does not implement AbstractBase.MyMethod
        SomeType b = new SomeType();
    }
}

So we try to implement MyMethod():

public class SomeType : T
{
    public override string MyMethod()
    {
        return "Some return value";
    }
}

public class SomeUsage
{
    void Foo()
    {
        // SomeType does not inherit a virtual method MyMethod()
        SomeType b = new SomeType();
    }
}

So lets make a requirement that T be derived from AbstractBase:

public abstract class DerivedAbstractBase : AbstractBase
{
    public abstract string AnotherMethod();
}

public class SomeType : T
    where T : AbstractBase
{
    public override string MyMethod()
    {
        return "Some return value";
    }
}

public class SomeUsage
{
    void Foo()
    {
        // SomeType does not implement DerivedAbstractBase.AnotherMethod()
        SomeType b = new SomeType();
    }
}

Summary:

By the time you account for all the restrictions in base types, you're so constrained that deriving from a generic parameter is pointless.