C# · December 5, 2011

Mixed Mode Assembly Error

Q: How do I fix the mixed mode assembly error?

Add an app.config file to the project containing a element with useLegacyV2RuntimeActivationPolicy="true" and a entry.

TL;DR

The mixed mode assembly error happens when a project references a library built for an older .NET runtime, such as an old MySql.Data build in a WPF 4.0 app. Adding an app.config with useLegacyV2RuntimeActivationPolicy="true" fixes it.

I first encountered this error when I was building a small media player using WPF 4.0 and the embeddable open source database MySql. I added a link to the MySql.Data.MySqlClient library which was built on a lower version of the .Net framework than the one which the current application is running on. The screen is as shown below

mixedmode

In order to correct the error, an app.config file must be added to the project and then the following code is copied into the configuration file.

mixedmodesolution

Frequently asked questions

What causes the "Mixed mode assembly is built against version v2.0.50727" error?

It occurs when an application references a library that was built on a lower version of the .NET framework than the one the application is running on. The author hit it in a WPF 4.0 app that referenced the MySql.Data.MySqlClient library.

How do I fix the mixed mode assembly error?

Add an app.config file to the project containing a <startup> element with useLegacyV2RuntimeActivationPolicy="true" and a <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/> entry.