C#委托方法与用户控件问题：拖放UserControl至窗体时VS2017报错

Hey there! No worries about your English at all—let's work through this issue step by step.

From what you've described, your regular classes work perfectly with methods that get redefined at runtime, but turning Class1 into a UserControl and dragging it onto your main form throws a design-time error in Visual Studio 2017. Here are the most likely causes and fixes to try:

Common Causes & Solutions #

1. Runtime-Only Logic Running in Design Mode #

The Visual Studio designer creates an instance of your UserControl when you drag it onto a form. If your method redefinition code runs during this design-time instantiation (like in the control's constructor, Load event, or initialization logic), it will fail—since the design environment doesn't support runtime code modification (such as reflection or Emit operations).

Fix: Wrap your runtime-specific code with a check for the DesignMode property to ensure it only runs when the app is live:

public class Class1 : UserControl
{
    public Class1()
    {
        InitializeComponent();
        
        if (!DesignMode)
        {
            // Call your method redefinition logic here
            RedefineRuntimeMethod();
        }
    }

    private void RedefineRuntimeMethod()
    {
        // Your runtime method redefinition code goes here
    }
}

2. Missing Dependencies or Flawed Redefinition Logic #

If your method redefinition relies on assemblies, resources, or config settings that aren't available in the design-time context (or if the logic itself has bugs that only surface when instantiated by the designer), this will trigger errors.

Fix:

• Double-check that all dependencies for your method redefinition are accessible during design time (no missing DLLs, no runtime-only config values).
• Test your method redefinition logic in a standalone console app first to confirm it works outside of the UserControl context.

3. Incomplete Error Details #

The error snippet you shared is cut off—full details (like the exact exception message, stack trace, or affected line number) would make pinpointing the issue much easier. If you can grab the full error text, that will help narrow things down further.

内容的提问来源于stack exchange，提问作者Tara Rulez