c# Singleton Pattern with multiple DLLs

Question

My question is about a singleton pattern used in the following constellation of assemblies and libraries.

Imagine the following assemblies

• CommonLibrary.dll
• MainProcess.exe
• Plugin1.dll
• Plugin2.dll

MainProcess.exe, Plugin1.dll, and Plugin2.dll use the class Manager from CommonLibrary.dll. The CommonLibrary.dll is linked against the other assemblies at compile time. The Manager class uses the singleton pattern:

public class Manager
{
    private static Manager instance;

    public static Manager Instance
    {
        [MethodImpl(MethodImplOptions.Synchronized)]
        get { 
            if (instance == null)
            {
                instance = new Manager();
            }
            return instance;
        }
    }

    private Manager()
    {

    }

    // methods ...
}

The singleton pattern is used to create only one instance of the Manager class (obviously). This is important in the methods of Manager because access to resources needs to be coordinated with different locks. Eg by using lock:

// Method inside Manager
public void DoSomething()
{
    lock(someLockObject)
    {
         // do something
    }
}

MainProcess.exe, Plugin1.dll, and Plugin2.dll each have a reference to the Manager somewhere in their code by calling

Manager manager = Manager.Instance;
// do something with manager

Now imagine the following scenario: MainProcess.exe loads Plugin1.dll and Plugin2.dll using reflection during runtime using Assembly.LoadFrom();

Question: Do MainProcess.exe, Plugin1.dll, and Plugin2.dll share the same instance of Manager? This is important because if they do not share the same instance of Manager i need to do some inter process locking of resources.

Thanks.

Answer 1

Do MainProcess.exe, Plugin1.dll, and Plugin2.dll share the same instance of Manager?

Yes, they do. There will be only one instance of Manager in the application domain (and in the whole process, unless you explicitly create another AppDomain ).