Question

I have an application that loads plugins. I have a plugin that has complete access to a form instance. If I have a function in a form that needs to be overridden, but is not a virtual function, is there another way to override it?

Here is a very generic example:

//Form I am modifying
public partial class MyForm : Form
{
    public int myVariable1;
    public int myVariable2;

    //Constructor and other methods here

    private void setVar(int replacementValue)
    {
        myVariable1 = replacementValue;
    }
}

…then in a separate dll…

//My plugin
public class MyPlugin : IMyPluginBase
{
    MyForm theForm; //Reference to the form in the main application

    //Constructor and other methods here

    private void setVar(int replacementValue)
    {
        theForm.myVariable2 = replacementValue;
    }
}

In this example the function in the form sets ‘myVariable1’, but the ‘setVar’ function in the plugin sets ‘myVariable2’.

So, the question is, in the case of this example, can I replace/override the form’s ‘setVar’ function with the one in the plugin? Maybe with messages or reflection?

Answer 1

No. You cannot “replace” or overide private non-virtual methods in C#.

The C# language (and .NET runtime) don’t support dynamic replacement of methods in the manner you describe. Very few languages support this capability, to my knowledge (I believe that SmallTalk and Objective-C both do).

If this is the only place in your application where you need this kind of extensibility, you can achieve it through an interface, delegate, or inhertance+virtual methods. Any of these approaches could work … which one you choose depends on what kind of extensibility you desire.

If you expect to have many such extensibility points in your app, then you should probably take a look at the Managed Extensibility Framework (MEF). It provides a Microsoft-supported model for creating plug-in architectures using patterns and technique that work well in .NET.