C# Creating window - Defining Parent window

If I understood your question correctly you should be able to achieve what you want by using something like this:

class Win32Window : IWin32Window
{
    IntPtr handle;

    public Win32Window(IntPtr handle) { this.handle = handle; }

    public IntPtr Handle
    {
        get { return this.handle; }
    }
}

static void Main()
{
    IntPtr targetParent = // Get handle to the parent window

    new MainForm().ShowDialog(new Win32Window(targetParent));
}

This will turn MainForm a child window of the specified window making it always appear above it. I use ShowDialog in the example, but this should also work for Show. This is specific for Windows Forms.

In WPF you can try the following:

var helper = new WindowInteropHelper(/* your Window instance */);

helper.Owner = // Set with handle for the parent

I quickly tried this after showing the WPF window and it seemed to work as expected, but WPF knowledge is not that great.