当项目需要引用其他dll时

Question

I encountered strange behavior and don't know the reason. I have two projects A and B. A references System.Web assembly and B references A. I add using statement to projects B class using A . It works fine until I start using linq statements. I get compilation error :

The type 'System.Web.UI.Control' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Simplified example project. Two projects Parent and Child. Parent project references System.Web. Child project references Parent project. Parent project contains one class Parent:

using System.Web.UI;

namespace Parent
{
    public static class Parent
    {
        public static T DoStuff<T>(this object obj)
            where T : Control 
        {
            return null;
        }
    }
}

Child project contains one class Child:

using System.Collections.Generic;
using System.Linq;
using Parent;

namespace Child
{
    public class Child
    {
        void Test()
        {
            var strings = new List<string>();
            var string1 = strings.FirstOrDefault();
        }
    }
}

Exception is thrown on var string1 = strings.First(); . I would just add reference to System.Web in Child project and everything works fine. But I would like to understand why it works like this. If I change parent class to:

public static class Parent
{
    public static Control DoStuff(this object obj)
    {
        return null;
    }
}

Instead of using generics I use Control class directly. And it works fine. Any ideas?

edited: var string1 = strings.FirstOrDefault();

Answer 1

If you reference a project, it adds a reference to its compiled dll. However, only because you reference a dll doesn't makes your project to reference recursively all dll's that are referenced by the referenced dll.

Let's say you add a reference to project B to your project A. While project B has a reference to System.Web it won't automatically reference it in project A. If the system would recursively add references to a dll you would end up having a ton of references you don't need.

That is why you still have to reference all necessary assemblies manually in your project A in order to be able to use everything from Project B.