Resolving Compatibility: Referencing a .NET 2.0 Mixed Mode Assembly in a .NET 4.0 Project

Understanding the Challenge

When referencing a .NET 2.0 mixed mode assembly (containing both managed and unmanaged code) in a .NET 4.0 project, you'll encounter a compatibility hurdle. .NET 4.0 uses a different Common Language Runtime (CLR) version than .NET 2.0. By default, the CLR won't load the assembly due to this version mismatch.

The Solution: App.config Configuration

To bridge this gap, you need to add a specific configuration setting to the .config file (typically named App.config) of your .NET 4.0 project. This configuration instructs the CLR on how to handle the .NET 2.0 mixed mode assembly.

Here's the key configuration element you'll need:

<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" />
  </startup>
</configuration>

Explanation of the Configuration:

• <startup> element: This section groups settings related to application startup.
• useLegacyV2RuntimeActivationPolicy="true" attribute: This is the critical part. It tells the CLR to use its legacy behavior (from the .NET 2.0 era) when activating the .NET 2.0 mixed mode assembly. This essentially forces the CLR to use the available version (which might be a lower version) to load the assembly.
• <supportedRuntime version="v4.0" /> element: While seemingly contradictory, this element specifies that your application's primary runtime version is 4.0. It coexists with the useLegacyV2RuntimeActivationPolicy setting.

Important Considerations

• Compatibility Risks: Using useLegacyV2RuntimeActivationPolicy can introduce potential compatibility issues with other parts of your .NET 4.0 application that rely on newer runtime features. It's generally recommended to refactor your code or find alternative libraries that are compatible with .NET 4.0 if possible.
• Limited Scope: This configuration only affects the specific .NET 2.0 mixed mode assembly being referenced. Other assemblies in your project will continue to use the .NET 4.0 runtime by default.

3. Add the configuration element: If the .config file doesn't already have a <startup> section, create one. Then, paste the following configuration element within the <startup> section:

   <useLegacyV2RuntimeActivationPolicy="true">
       <supportedRuntime version="v4.0" />
   </useLegacyV2RuntimeActivationPolicy>
   

1. Upgrade the Mixed Mode Assembly:

• The ideal solution is to upgrade the .NET 2.0 mixed mode assembly to a version that targets the .NET 4.0 runtime. This ensures compatibility and access to newer features. Check with the library or component's maintainer for an updated version.

1. Find an Alternative Library:

• Explore if there are alternative libraries or components that provide similar functionality and are compatible with .NET 4.0. This approach avoids the need for compatibility workarounds.

1. Use Interop (if applicable):

• If the .NET 2.0 assembly exposes functionality through COM interfaces, you might be able to use Platform Invoke (P/Invoke) or COM Interop to interact with it from your .NET 4.0 project. This approach requires more development effort but can be an option if upgrading or finding alternatives isn't feasible.

1. Create a Wrapper Assembly (Advanced):

• As an advanced approach, you could create a new .NET 4.0 class library project that acts as a wrapper around the .NET 2.0 mixed mode assembly. This wrapper would handle the compatibility concerns and provide a clean interface for your .NET 4.0 project to consume. Be aware that this method requires significant development effort.

Choosing the Right Method:

The best method depends on several factors, including:

• Availability of an upgraded version of the .NET 2.0 assembly
• Ease of finding an alternative library
• The complexity of the .NET 2.0 assembly and its interaction with your code
• Your development resources and expertise