Is writing "System.AppContext" in code behind not supported in .net 4.5.1?

I need long path support for my .net 4.5.1 WPF application.

This setting in App.config works (long paths are supported):

<runtime>
    <AppContextSwitchOverrides value="Switch.System.IO.UseLegacyPathHandling=false;Switch.System.IO.BlockLongPaths=false" />
</runtime>

So I tried to set in in code, but it does not change the behaviour:

public App()
{
    // returns correct type:
    Type type = Type.GetType("System.AppContext"); 
    if (type != null)
    {
        // returns correct switch:
        MethodInfo setSwitch = type.GetMethod("SetSwitch", BindingFlags.Public | BindingFlags.Static);
        setSwitch.Invoke(null, new object[] { "Switch.System.IO.UseLegacyPathHandling", false });
        setSwitch.Invoke(null, new object[] { "Switch.System.IO.BlockLongPaths", false });
    }
}

Is that "code behind setting" not supported in .net 4.5.1?