Execute package conflict between runtimes

I was using SQL Server 2008 R2 and had written a C# program using Microsoft C# Visual Studio 2008 that opened an SSIS package and, using files in a directory, modified the source and destination detail (SQL and table name) before executing the package. This it did for every file in the directory. Now I have upgraded to 2012 and Microsoft C# Visual Studio 2010. I have made the changes to the C# program's references so they all come from 110 rather than 100. But now the execution fails as per the message below. I can save the package and open it SQL Server Data Tools and it runs okay. I have tried modifying the app.config  in the same directory as the .cs to so it now reads:

<?xml version="1.0"?>
<configuration>
    <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
    </startup>
    <runtime>
        <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
            <dependentAssembly>
                <assemblyIdentity name="Microsoft.SqlServer.DTSRuntimeWrap" publicKeyToken="89845DCD8080CC91" culture="neutral"/>
                <bindingRedirect oldVersion="0.0.0.0-11.0.0.0" newVersion="11.0.0.0"/>
            </dependentAssembly>
        </assemblyBinding>
    </runtime>
</configuration>

I would appreciate some help.

Here's the error message:

The Execute method on the task returned error code 0x80131621 (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 Execute method must succeed, and indicate the result using an "out" Parameter.