C#

[DataMember]
public class MyClass : List<SomeOtherClass>
{
     [DataMember]
     public int SomeIntProp { get; set; }
}

// step 1 - win32 api functions used
[StructLayout(LayoutKind.Sequential)]
public struct COPYDATASTRUCT {
  public IntPtr dwData;
  public int cbData;
  [MarshalAs(UnmanagedType.LPStr)] public string lpData;
}
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, ref COPYDATASTRUCT lParam);

public const int WM_COPYDATA = 0x004A;

...

// step 2 - in your Main()
var running_already = Process.GetProcessesByName("Yourapp").ToDictionary(x => x.Id, x => x);
running_already.Remove(Process.GetCurrentProcess().Id);
if (running_already.Count > 0) {
    // there's another instance running
    if (args.Length == 0)
        // just let the other instance run
        return;
    // for the other instance - what to open
    string open = args[0];

    var cds = new win32.COPYDATASTRUCT {
        dwData = new IntPtr(0),
        cbData = open.Length * 2 + 1,
        lpData = open
    };
    var handle = running_already.First().Value.MainWindowHandle;
    win32.SendMessage(handle, win32.WM_COPYDATA, IntPtr.Zero, ref cds);
    return;
}

// step 3 - in your main window
protected override void WndProc(ref Message m) {
	if (m.Msg == win32.WM_COPYDATA) {
		var st = (win32.COPYDATASTRUCT) Marshal.PtrToStructure(m.LParam, typeof (win32.COPYDATASTRUCT));
		string open = st.lpData;
		util.postpone(() => on_file_drop(open), 100);
	}

	base.WndProc(ref m);
}