Compiling with .NET 3.5 error

Hello, i have VS 2015 and i want compile app with .net 3.5 and when i run app, i get this error (after press button).

An unhandled exception of type 'System.IO.FileLoadException' occurred in System.Windows.Forms.dll

Additional information: 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.

i have app.config but still not working:

<?xml version="1.0"?>
<configuration>
<startup useLegacyV2RuntimeActivationPolicy="true">
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>
</configuration>

how can i fix this? Thanks :)
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