New Post: determine install dir from external UI

using External_UI/WinFormsSetup as the starting point, this is basically what I have done. Hopefully, it will save a new user some times implementing this solution.

In myproductSetup.cs, add this

public void StartInstallWithAgrument(string agrument)

    {
        //string agrument = string.Format("CUSTOM_UI=true INSTALLDIR=\"{0}\"", FullPath);
        StartInstall(argument);
    }

In MsiSetupForms.cs, modify installBtn_Click to call

void installBtn_Click(object sender, EventArgs e)
    {
        DisableButtons();
    string agrument = "CUSTOM_UI=true INSTALLDIR=\"c:\\whatever\"";
    session.StartInstallWithAgrument(agrument);
     }

The argument is hard coded for now, you can use FolderBrowserDialog to allow a user to select which folder to install.

Hope that helps