Program.cs

using System;
using System.Diagnostics;
using System.Runtime.InteropServices;
using System.Windows.Forms;


namespace nwn2_Chatter
{
	/// <summary>
	/// Class with program entry point.
	/// </summary>
	sealed class Program
	{
		/// <summary>
		/// Program entry point.
		/// </summary>
		[STAThread]
		static void Main(string[] args)
		{
			logfile.CreateLog();

			Process proc = null;

			if (args.Length != 0) // else start a new instance of Yata
			{
				string label = Process.GetCurrentProcess().ProcessName;

				//int id = current.Id;
				//logfile.Log(". current process= " + id + " : " + label + " h= " + current.Handle);

				DateTime dt = DateTime.Now, dtTest;
				//logfile.Log(". dt= " + dt.Minute + ":" + dt.Second + ":" + dt.Millisecond);

				Process[] processes = Process.GetProcesses();
				//logfile.Log(". qty processes= " + processes.Length);

				foreach (var process in processes)
				{
					//logfile.Log(". . process= " + process.Id + "\t: " + process.ProcessName);

					if (process.MainWindowHandle != IntPtr.Zero // NOTE: 'current' won't have a MainWindow (ie. bypass current proc.)
						&& process.ProcessName == label)
					{
						//logfile.Log(". . . found other Yata process");
						//logfile.Log(". . . h= " + process.Handle + " MainWindowHandle= " + process.MainWindowHandle);

						// find longest-running instance -> use it
						dtTest = process.StartTime;
						//logfile.Log(". . . dtTest= " + dtTest.Minute + ":" + dtTest.Second + ":" + dtTest.Millisecond);
						if (dtTest < dt)
						{
							//logfile.Log(". . . . is less");
							dt = dtTest;
							proc = process;
						}
						//logfile.Log(". . . dt= " + dt);
						//logfile.Log(". . . proc h= " + proc.MainWindowHandle);
					}
				}
			}

			if (proc != null)
			{
				//logfile.Log(". FOUND INSTANCE proc= " + proc.Id + " : " + proc.ProcessName);

				// https://www.codeproject.com/Tips/1017834/How-to-Send-Data-from-One-Process-to-Another-in-Cs
				IntPtr ptrCopyData = IntPtr.Zero;
				try
				{
					string arg = args[0];

					// create the data structure and fill with data
					var copyData = new Crap.COPYDATASTRUCT();
					copyData.dwData = new IntPtr(Crap.CopyDataStructType);	// just a number to identify the data type
					copyData.cbData = arg.Length + 1;						// one extra byte for the \0 character
					copyData.lpData = Marshal.StringToHGlobalAnsi(arg);

					// allocate memory for the data and copy
					ptrCopyData = Marshal.AllocCoTaskMem(Marshal.SizeOf(copyData));
					Marshal.StructureToPtr(copyData, ptrCopyData, false);

					// send the message
					IntPtr ptrWnd = proc.MainWindowHandle;
					Crap.SendMessage(ptrWnd, Crap.WM_COPYDATA, IntPtr.Zero, ptrCopyData);
				}
				catch (Exception ex)
				{
					using (var ib = new Infobox(Infobox.Title_excep,
												"Failed to send file to Chatter.",
												ex.ToString(),
												InfoboxType.Error))
					{
						ib.ShowDialog(); // this better not throw ...
					}
				}
				finally // free the allocated memory after execution has returned
				{
					if (ptrCopyData != IntPtr.Zero)
						Marshal.FreeCoTaskMem(ptrCopyData);
				}
			}
			else
			{
				Application.EnableVisualStyles();
				Application.SetCompatibleTextRenderingDefault(false);	// ie, use GDI aka TextRenderer class. (perhaps, read:
																		// perhaps depends on the Control that's being drawn)
				Application.Run(new Chatter(args));
			}
		}
	}
}