How Can I Resolve the 'Mixed Mode Assembly' Error When Referencing a .NET 2.0 Assembly in a .NET 4.0 Project?

Troubleshooting .NET 2.0 Assemblies in .NET 4.0 Projects

Integrating older .NET libraries into newer projects can sometimes lead to compatibility problems. This guide addresses a common issue encountered when using a .NET 2.0 System.Data.SQLite framework within a .NET 4.0 application.

The Problem:

Attempting to utilize a mixed-mode assembly (typically C /CLI) built for the .NET 2.0 runtime (.NET Framework, Version v2.0.50727) within a .NET 4.0 project may result in the following error:

"Mixed mode assembly is built against version 'v2.0.50727' of the runtime and cannot be loaded in the 4.0 runtime without additional configuration information."

The Solution:

This error is resolved by adding a specific configuration section to your application's App.Config file. This configuration instructs the Common Language Runtime (CLR) on how to handle the older assembly. Add the following XML snippet within the <configuration> element of your App.Config:

<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime sku=".NETFramework,Version=v4.0" version="v4.0"/>
  </startup>
</configuration>

The key here is useLegacyV2RuntimeActivationPolicy="true". This setting enables the CLR to load the mixed-mode assembly using the .NET 4.0 runtime, overcoming the version mismatch.

Important Consideration:

This configuration is only required for mixed-mode assemblies. Purely managed (.NET) assemblies compiled for .NET 2.0 generally load without needing this App.Config modification.

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