Implementing DLL plugins

Question

I'm planning to extend my application. At the moment, I have Data.dll that uses Data Access that uses SQL Server with Stored Procedure. What I would to do is to make it flexible where changing the DLL will change the Data Access to any provider like MySQL or Oracle. The goal is to avoid making changes on the Main Application. I'm just not sure what is the best way to implement this in C#. At the moment, I'm using win32 but wanted to use the same concept on my other ASP.NET Core API. I'm planning to use App.config for Win32 or Web.config for the WebApi to make changes on the configuration.

-----UPDATE to MAKE IT MORE CLEARER ---------

I've just created 3 Projects Project# 1 - A DLL that will Perform CRUD on SQL Server and produce a file called DLLClassLibrary.dll

public class Data
{
    public string GetName()
    {
        return "NAME DLL SQL SERVER";
    }
}

Project# 2 - A DLL that will Perform CRUD on MySQL and produce a file called DLLClassLibrary.dll

public class Data
{
    public string GetName()
    {
        return "NAME DLL MYSQL";
    }
}

Project# 3 - The MAIN app where I will consume the DLL. On the program, I've created a reference to a file called DLLClassLibrary.dll.

public partial class frmMain : Form
{
    DLLClassLibrary.Data data = new DLLClassLibrary.Data();
    public frmMain()
    {
        InitializeComponent();
    }

    private void frmMain_Load(object sender, EventArgs e)
    {
        txtOut.Text = data.GetName();
    }
}

Test Done using Project # 1 generated DLL

Test Done using Project # 2 generated DLL

As you can see by placing the same dll file the behaviour changes. The question is what is the best way to implement these 2 DLLs without removing one another and without Hard Coding it on my Main. Or is that the only option?

Answer 1

I read this question as meaning you'd like to be able to change the DLL used for DB access by providing the DLL and changing your *.config file without recompiling.

To do that you would have to put the name of the DLL or the path to it in your config file and load the DLL dynamically from your application.

I'm not sure if you're changing your database often enough to make it worth it, but let's assume it is.

The way to do this is different between the old .NET framework for Windows and .NET Core.

In the classic .NET framework, you would use AppDomains to dynamically load DLLs at runtime, and then reflection or dynamics can be used to access the types and methods in the DLL.

Loading DLLs at runtime in C#

    var DLL = Assembly.LoadFile(@"C:\visual studio 2012\Projects\ConsoleApplication1\ConsoleApplication1\DLL.dll");

    var class1Type = DLL.GetType("DLL.Class1");

    //Now you can use reflection or dynamic to call the method. I will show you the dynamic way

    dynamic c = Activator.CreateInstance(class1Type);
    c.Output(@"Hello");

In .NET Core, AppDomains aren't a thing, and you'd use an AssemblyLoadContext (only in .NET Core 3.0 and above).

There's a nice tutorial on Microsoft Docs on how to do that to realize plugins for your application:

https://learn.microsoft.com/dotnet/core/tutorials/creating-app-with-plugin-support