why does my C# client that uses Library A need to have a using statement for Library B (which A uses)

Question

Hi,

I have:

• Main Program Class - uses Library A
• Library A - has partial classes which mix in methods from Library B
• Library B - mix in methods & interfaces

So in Library B when I include a partial Node class which implements INode (defined in Library B) I suddenly get an error in my main class where it uses Node from Library A. The error tells me in the Main Class I have to have a using statement to Library B.

Any ideas?

EDIT - Except from code

    // *** PROGRAM ***
    class Program
    {
        static void Main(string[] args)
        {
            var context = new Model1Container();
            Node myNode;  // ** WITHOUT A using for Library B I have an error here ***
         }
     }


// ** LIBRARY A
namespace TopologyDAL
{
    public partial class Node
    {
        // Auto generated from EF
    }

    public partial class Node : INode<int>   // to add extension methods from Library B
    {
        public int Key
    }
}

// ** LIBRARY B
namespace ToplogyLibrary
{
    public static class NodeExtns
    {
        public static void FromNodeMixin<T>(this INode<T> node) {
           // XXXX
        }
    }
    public interface INode<T> 
    {
        // Properties
        T Key { get; }

        // Methods
    }

}

EDIT 2 - To clarify was it a reference or using error:

So the error that appears against the "Node myNode;" line is:

Error 1 The type 'Topology.INode`1' is defined in an assembly that is not referenced. You must add a reference to assembly 'Topology, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'. U:\My Dropbox\source\ToplogyLibrary\TopologyDAL_ConsoleTest\Program.cs 11 13 TopologyDAL_ConsoleTest

When I let VS fix it for me it adds the Library2 as a reference. That is there is no "using " either before or after in the client code. So the issue is a Reference not Using issue.

EDIT 3 - Not specifically about the question, however I now notice that in the program project, I can't see the mixin methods (from Library B) unless I have a using statement to Library B? I might create a separate question re this.

Answer 1

If LibraryA exposes properties, returns objects from methods, or take parameters of types that are defined in LibraryB, and you declare or use a variable in Main Program of one of those types, you'll need the using statement.

Answer 2

A partial class in c# is just a class that exists in multiple files. You cannot have 2 files that define a partial class in different namespaces.

Answer 3

Since Library A is using mix in method and interfaces from Library B, a using statement is required if Library A and Library B exist in different namespaces, this is just how C# works. The compiler needs to know where to find the Library B types that are used in Library A.

Answer 4

My understanding of what's happening is that you only reference Library A from Main program and compiler tells you to add reference to Library B because some types that Library A exposes are defined in Library B.

To fix this add a reference to Library B to Main program project.

Here is little diagram. If Library A exposes a type defined in Library B then Main must reference Library B as well. The below situation will not work:

_____________             _____________               _____________
| Main       |references  | Library A  |references    | Library B  |
|           -|------------|->         -|--------------|->          |
|            |            | public     |              | SomeType   |
|            |            |  SomeType  |              |            |
|            |            |            |              |            |
-------------             -------------               -------------

This is only an issue when a type defined in Library B is accessible through Library A. This will be in one of the following situations:

• EDITED Type defined in Library A (Node) derives from a type in Library B(INode<int>).
• Method defined in Library A uses a type from Library B as return type or as an argument.
• Type defined in Library A exposes a type from Library B as a property or a public field.

You will need to add a reference to Assembly3 from Assembly1 to make it compile.

_____________             _____________               _____________
| Main       |references  | Library A  |references    | Library B  |
|           -|------------|->         -|--------------|->          |
|            |            | public     |              | SomeType   |
|            |references  |  SomeType  |              |            |
|           -|------------|------------|--------------|->          |
|            |            |            |              |            |
-------------             -------------               -------------

Answer 5

All you're using is the Node class from Library A, that's true. However, part of the definition of the Node class is the fact that it implements the INode<int> interface from Library B.

The very fact of the existence of the Node class demands that you include a reference to Library B.