Why Does My Web Application Throw a 'System.MissingMethodException' Even Though the Method Exists?

System.MissingMethodException: Troubleshooting Missing Methods in Web Applications

Web applications occasionally encounter the frustrating "System.MissingMethodException: Method not found" error, even if the method seems to exist in the code. This article explores a common cause and solution for this perplexing problem.

Consider this example from a generic handler:

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

    public void DoThis(){ ... }
}</code>

The runtime fails to recognize DoThis(), despite its clear definition. The reason? Often, outdated assemblies are to blame.

The Root Cause: Outdated Assemblies

This error frequently occurs when an older version of a DLL persists in the application's environment. As your application evolves, changes to classes, methods, and assemblies accumulate. If an older assembly remains, the runtime might load it instead of the updated version, resulting in the "method not found" error.

The Solution: A Fresh Build and Deployment

To rectify this, follow these steps:

1. Clean Build Output: Remove all files generated by the build process. This includes DLLs, EXEs, and temporary files. A thorough cleanup is crucial.
2. Rebuild and Redeploy: A complete rebuild and redeployment guarantees that only the latest assemblies are deployed, eliminating any residual outdated files.

After completing these steps, the "System.MissingMethodException" should be resolved. This comprehensive approach ensures the runtime consistently accesses the most current application assemblies, preventing method not found errors from hindering your application's performance.

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