MsiGetSummaryInformation always returns invalid parameter error

Answer 1

Get rid of the MsiInterop that you are using and use the interop found in WiX's DTF. The Microsoft.Deploymnet.WindowsInstaller namespace has a SummaryInformation Class that exposes a read/write string Template property. Way better object model and interop without worrying about all the P/Invoke details that your current interop makes you deal with.

I'm home now so here's a code snippet:

using Microsoft.Deployment.WindowsInstaller;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            using( var database = new Database(@"C:\orca.msi", DatabaseOpenMode.Direct  ))
            {
                database.SummaryInfo.Template = "Intel;666";
            }            
        }
    }
}

Notice the use of the using() clause. The Database class implements the IDisposable interface and automatically handles ( pun intended ) cleaning up all those pesky unmanaged handles for you.