Question

public abstract class BaseDocumentLayer : IDocumentLayer
{                                                       
   public IDocumentLayer NextLayer { get;set;}

   public virtual Document<T, U> ProcessDocument<T, U>(Document<T, U> document) {
      if(NextLayer != null)
         document = NextLayer.ProcessDocument(document);
      return document;          
   }
}

public override Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)
{       
    return base.ProcessDocument<GenericDatabaseDTO, 
}

public static class GenericDatabaseConstants 
{    
    public enum ActionType 
    {                
       CreateGeneric,
       ReadGeneric,
       UpdateGeneric,
       DeleteGeneric,               
    }
}

Why do I get the error: Type paramter declaration must be an identifier not a type?

Error1:

public override Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)
{   

    return base.ProcessDocument(document);
}

Error 1 ‘GenericDocumentLayer2.Document.GenericDatabase.ProcessLayer.ProcessDocument(GenericDocumentLayer2.TypeDefinition.Document)’: no suitable method found to override

I have not changed my origin BaseLayer.ProcessDocument method

UPDATE

public void Add(GenericDatabaseField field) {
            var dto = new GenericDatabaseDTO();
            dto.Field = field;
            dto.Id = "test";

            var document = new Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(dto, GenericDatabaseConstants.ActionType.CreateGeneric, null);
            IDocumentLayer layer = DocumentFactory.CreateDocumentLayers();
                                          // The generic ProcessDocument is never called just the base version
        document = layer.ProcessDocument<GenericDatabaseDTO, GenericDatabaseConstants.ActionType>(document);                
    }

Answer 1

You can’t override a generic method for a specific type. This is possible with C++’s templates but not with C#’s generics.

To make your code compile, change it to this:

public override Document<TDTO, TActionType> ProcessDocument<TDTO, TActionType>(Document<TDTO, TActionType> document) 

This will compile, but it will override the base type method for all generic arguments.

If you want to provide a special implementation just for one type, you can add a non-generic overload and internally call the generic version after the special handling:

public Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> ProcessDocument(Document<GenericDatabaseDTO, GenericDatabaseConstants.ActionType> document)  
{
    // some special handling for this special case here...

    return base.ProcessDocument(document);
}

However, please note that this will break in polymorphic scenarios like this one:

Assume the methods above are implemented in Derived. Derived is derived from Base

GenericDatabaseDTO dto = /*...*/;
GenericDatabaseConstants.ActionType actionType = /*...*/;

Derived d = new Derived();
Base b = d;

// calls the non-generic version with special handling from Derived:
d.ProcessDocument(dto, actionType);
// calls the generic version from Base:
b.ProcessDocument(dto, actionType);