Maintaining backward compatibility is crucial when updates or changes are made to libraries or assemblies. Redirecting assembly versions allows developers to seamlessly transition between different versions of a .NET Framework assembly without breaking existing applications. In this blog post, we’ll explore the concept of assembly redirection and provide you with various examples to help you master this essential technique in C#.

Understanding Assembly Redirection

Assembly redirection is the process of mapping references to one version of an assembly to another version during runtime. It ensures that when an application requests a specific version of an assembly, it receives the appropriate version, even if a different version is installed on the system.

Redirecting Assembly Versions

One common way to redirect assembly versions is by using configuration files, namely App.config for Windows applications and Web.config for web applications. Below is an example of how to redirect an assembly version using the assemblyBinding element in an App.config file:

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="YourAssemblyName"
                          publicKeyToken="32ab4ba45e0a69a1"
                          culture="neutral" />
        <bindingRedirect oldVersion="1.0.0.0"
                         newVersion="2.0.0.0" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration>

In the example above, any reference to the assembly with the name “YourAssemblyName” and version “1.0.0.0” will be redirected to version “2.0.0.0” at runtime. This allows applications to utilize the updated version without modifying the source code.

Redirecting Programmatically

It is possible to programmatically perform a redirect using the following event: AppDomain.AssemblyResolve Event.

Common Errors that May Require an Assembly Redirect

The following errors can sometimes be confusing and not clear about what can be done to fix the issue. Often times, an assembly redirect is needed to resolve the problem.

  1. FileNotFoundException: This error occurs when the runtime cannot find the specified assembly version referenced by your application. It typically happens when the expected assembly version is not available on the system.

  2. VersionMismatchException: This error occurs when a referenced assembly is loaded with a different version than the one expected by the application. It can happen when multiple versions of an assembly are present in the application’s dependencies.

  3. AmbiguousMatchException: This error occurs when there are multiple versions of an assembly available, and the runtime cannot determine which version to load. It often happens when different components or libraries in the application have dependencies on different versions of the same assembly.

  4. TypeLoadException: This error occurs when a referenced type or member within an assembly cannot be loaded due to a version mismatch. It often happens when the type or member exists in different versions of the assembly.

Conclusion

Redirecting assembly versions in C# .NET Framework is a powerful technique that allows developers to manage versioning conflicts and ensure backward compatibility. Understanding assembly redirection is essential for maintaining the stability and functionality of your applications.

To read the full Microsoft guide to assembly redirects, follow this link: Redirecting assembly versions