vs2010 Mixed mode assemblies in windows installer custom actions - visual-studio-2010

Question

I have a Windows installer project which includes a custom action. This custom action uses SMO to configure a databse. The installer project is .Net 4. When executing the custom action, I get 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.
I could run the database update code in a separate executable or rewrite the custom action so SMO is not used but I'd rather keep the code as it is if possible.
I know how to fix this in console and Winforms apps by adding the following to the app.config file.
<?xml version="1.0"?>
<configuration>
<startup useLegacyV2RuntimeActivationPolicy="true">
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>
</configuration>
The question is how on earth do I do this or something similar with an installer project? I still get this issue when all the referenced assemblies in the custom action are frame work 2 so it must be caused by the MSI itself which is .net4 according to the Fusion Log Viewer.

Answer 1

In the end I decided to launch a concole app from the installer's custom action to do the necessary updates to the databse. It works well
the code is to call the console app from the installer is :
public enum ReturnCode
{
Error = -1,
Updated = 0,
UpdateNotRequired = 1,
ServerNotAvailable = 2,
DatabaseNotAvailable = 3
}
private int UpdateSchema(string installationPath)
{
const string exeName = #"SchemaUpdater";
string executablePath = Path.Combine(installationPath, exeName);
LogModule.Log_NewLogEntry("Starting Schema Updater");
var myProcessStartInfo = new ProcessStartInfo(executablePath);
myProcessStartInfo.UseShellExecute = false;
myProcessStartInfo.CreateNoWindow = true;
Process process = Process.Start(myProcessStartInfo);
process.WaitForExit();
int exitCode = process.ExitCode;
LogModule.Log_NewLogEntry(string.Format("SchemaUpdate returned {0}", exitCode));
return exitCode;
}
public override void Install(IDictionary stateSaver)
{
string installationPath = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
int dbUpdaterReturnVal = UpdateSchema(installationPath);
if (dbUpdaterReturnVal < 0)
{
throw new Exception("Schema Update returned with an error");
}
if (dbUpdaterReturnVal == (int)ReturnCode.ServerNotAvailable)
{
throw new Exception("SqlServer Not available. Aborting Install");
}
base.Install(stateSaver);
}