Why Does My ASP.NET WebForms Application Throw a System.MissingMethodException, Even Though the Method Exists?

Solving the Mysterious System.MissingMethodException in ASP.NET WebForms

The dreaded System.MissingMethodException can be a frustrating roadblock in .NET development, especially when the missing method clearly exists within the same class. This often happens in ASP.NET WebForms applications, leaving developers scratching their heads.

Let's examine a scenario where the DoThis method, seemingly present in the MyHandler class, inexplicably triggers this exception:

public class MyHandler : IHttpHandler
{
    public void ProcessRequest(HttpContext context)
    {
      // Throws System.MissingMethodException: Method not found.
      this.DoThis(); 
    }

    public void DoThis() { ... }
}

The Culprit: Conflicting Assemblies

The root cause frequently lies in lingering, outdated assemblies. An older version of the DLL, lurking somewhere in your deployment path, can create a conflict, causing the runtime to load the incorrect version and thus fail to find the expected method.

The Solution: A Clean Rebuild and Redeployment

To resolve this issue, a thorough cleanup is necessary:

1. Completely remove all build artifacts: Delete the bin and obj folders within your project and solution directories. This ensures no remnants of previous builds interfere.

2. Rebuild the entire solution: A fresh build guarantees that the latest code is compiled and packaged correctly.

3. Redeploy the application: This step ensures that the updated assemblies completely replace the outdated ones on the server. Pay close attention to your deployment process to ensure a clean overwrite of the files.

By following these steps, you effectively eliminate the lingering effects of old assemblies, allowing your WebForms application to function correctly and avoid the System.MissingMethodException.

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