Simple Generic misunderstanding

Question

Could you help me to understand the error in those case ?

public interface IGeneralInterface
{
}


public class A : IGeneralInterface
{
}

public class B : IGeneralInterface
{
}

public class SomeClass<TGenericType> where TGenericType : IGeneralInterface
{
    private TGenericType internalValue;

    public SomeClass(TGenericType InitValue)
    {
        internalValue = InitValue;
    }

    public TGenericType CreateAnother()
    {
       TGenericType val1 = new B();   //Error here: class B() could not be converted to TGenericType
       return val1;
    }
}

Even if I build the SomeClass<T> as

SomeClass<IGeneralInterface> someClass = new SomeClass<IGeneralInterface>();

I explicity pass base interface to include all (?) cases and it still throw an error

Answer 1

Change

 TGenericType val1 = new B();   //Error here: class B() could not be converted to TGenericType

To

  IGeneralInterface val1 = new B();   

You are trying to TypeCast IGeneralInterface to TGenericType which is the cause of error.

TGenericType could have other constraints, like it inherits from ISpecificInterface from which B donot inherit. In this case the assignment becomes invalid.

Example:

public class SomeClass< TGenericType> where TGenericType : IGeneralInterface, ISpecificInterface
TGenericType val1 = new B(); // TGenericType should be ISpecificInterface also, B is not.

For above to run. IGenericInterface should always be more specific than TGenericType.

 public class SomeClass <IGenericInterface> 

Alternatively you could use is keyword to find out whether the object is assignable to TGenericType and then use the casting.

TGenericType val1 = default(TGenericType);
var val = new B();
if ( val is TGenericType)
{
  val1 = (TGenericType)val;
}

EDIT For the below comment

how it could at runtime have additional requirements ? Everything I put in compiler listed here

CreateAnother() creates instance of Type B which is not generic. Take below example

SomeClass<C> c = new SomeClass<C();
C another = c.CreateAnother(); // C is not assignable from B. (C is below). But It would be valid, if compiler did not flag the error

public class C : IGeneralInterface, IDisposable
{
}