Display a message box when exe is not lauched from console

DepotDownloader/PlatformUtilities.cs

@@ -145,5 +145,35 @@ namespace DepotDownloader
                     : mode & ~ModeExecute)));
             }
         }
+
+        #region WIN32_CONSOLE_STUFF
+        [DllImport("kernel32.dll", SetLastError = true)]
+        static extern uint GetConsoleProcessList(uint[] processList, uint processCount);
+
+        [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
+        static extern int MessageBox(IntPtr hWnd, string text, string caption, uint type);
+
+        const uint MB_OK = 0x0;
+        const uint MB_ICONWARNING = 0x30;
+
+        public static void VerifyConsoleLaunch()
+        {
+            // Reference: https://devblogs.microsoft.com/oldnewthing/20160125-00/?p=92922
+            var processList = new uint[2];
+            var processCount = GetConsoleProcessList(processList, (uint)processList.Length);
+
+            if (processCount != 1)
+            {
+                return;
+            }
+
+            _ = MessageBox(
+                IntPtr.Zero,
+                "Depot Downloader is a console application; there is no GUI.\n\nIf you do not pass any command line parameters, it prints usage info and exits.\n\nYou must use this from a terminal/console.",
+                "DepotDownloader",
+                MB_OK | MB_ICONWARNING
+            );
+        }
+        #endregion
     }
 }

DepotDownloader/Program.cs

@@ -21,6 +21,12 @@ namespace DepotDownloader
             if (args.Length == 0)
             {
                 PrintUsage();
+
+                if (OperatingSystem.IsWindows())
+                {
+                    PlatformUtilities.VerifyConsoleLaunch();
+                }
+
                 return 1;
             }