How to return derived class with C# generics

Question

I am trying to generate some test data using NBuilder for automation for all the classes that extends a base class wherein each class will have different fields.

Here is my code:

public interface Interface
{
    T method<T>() where T : BaseClass;
}

public class DrivedClass: BaseClass,Interface
{
    public T method<T>() where T : BaseClass
    {
        var derviedObj = Builder<DrivedClass>.CreateNew().Build();

        return derviedObj;
    }
}

return derviedObj giving error cannot implicity convert derviedObj to T

Since the method uses an open type T, you have to return an instance of such type rather than instance of a concrete type DerivedClass. — Wiktor Zychla, May 31 '16 at 05:25

smoksnes · Answer 1 · 2016-05-31T05:55:23.560

I'm not really familiar with nbuilder, but it looks like you could try something like this:

public class DrivedClass: BaseClass,Interface
{
    public T method<T>() where T : BaseClass
    {
        var derviedObj = Builder<T>.CreateNew().Build();

        return derviedObj;
    }
}

Or this, if that's what you're really after:

public class DrivedClass: BaseClass,Interface
{
    public T method<T>() where T : BaseClass
    {
        var derviedObj = Builder<DrivedClass>.CreateNew().Build();

        return derviedObj as T;
    }
}

Then you will get T, but there's really no chance of knowing that it will succeed. What happens when you add another implementation that doesn't inherit DrivedClass?

public class MoreDrivedClass : BaseClass, FooInterface
{

}

Then this cast will fail:

var drivedClass = new DrivedClass();
var foo = drivedClass.method<MoreDrivedClass>();

Then you will not be able to cast DrivedClass to MoreDrivedClass. Foo will be null.

score 1 · Answer 2 · answered May 31 '16 at 05:28

The method can't return DerivedClass because there is no way to know if T is that class or some other one:

SomeOtherDerived v = (new DrivedClass()).method<SomeOtherDerived>();

It is not clear what you trying to achieve, possibly something like following sample:

public interface Interface<T> where T : BaseClass
{
    T method();
}

public class DrivedClass: BaseClass,Interface<DerivedClass>
{
  public DerivedClass method()
  {
      DerivedClass derviedObj = Builder<DrivedClass>.CreateNew().Build();
      return derviedObj ;
  }
}

Muhammad Ashikuzzaman · Answer 3 · 2016-05-31T06:02:23.307

0

Try with Activator.CreateInstance.

public class DrivedClass: BaseClass,Interface
{
  public T method<T>() where T : BaseClass
    {
      var derviedObj = Activator.CreateInstance(typeof(T));
      return (T)derviedObj ; 

    }
}


 DrivedClass drOb = new DrivedClass ();
 var v = drOb.method<desiredType_T>();

edited May 31 '16 at 06:02

answered May 31 '16 at 05:53

Muhammad Ashikuzzaman

3,075
6
29
53

How to return derived class with C# generics

3 Answers3