To have your singleton app process new command line arguments, just invoke it exactly the same way (e.g. on the command line or by using ProcessStart) without regard for whether the app is running or not. This way you "handle the second scenario" by capturing the "new" command line arguments before denying the new instance" and then send those new arguments to the running instance via a named pipe.
Here's what I mean: If no instance is running we take the command line args and display them in the title bar so we have some way of observing what's happening. This is the result of the if block having successfully executed in Main().
PS >> .\WinformsSingletonApp.exe orig cmd line args
private static Mutex? mutex;
private static MainForm? mainForm;
const string MUTEX = "{5F563B29-172F-4385-B813-21062155F22E}-MUTEX";
[STAThread]
static void Main()
{
bool createdNew;
using (mutex = new Mutex(true, MUTEX, out createdNew))
{
if (createdNew)
{
try
{
ListenForMessages(); // Running instance is now listening to pipe.
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
int N = 0;
var displayArgs = string.Join(
" ",
Environment.GetCommandLineArgs().Skip(1).Select(_ => $"[{N++}] {_}"));
mainForm = new MainForm
{
Text = $"Main Form {displayArgs}"
};
Application.Run(mainForm);
}
finally
{
mutex.ReleaseMutex();
}
}
else
{
var args = Environment.GetCommandLineArgs();
args[0] = Path.GetFileName(args[0]);
SendMessageToPipe(args);
}
}
}
private static async void ListenForMessages()
{
await Task.Run(() =>
{
while (true)
{
using (NamedPipeServerStream pipeServer = new NamedPipeServerStream(PIPE, PipeDirection.In))
{
pipeServer.WaitForConnection();
using (StreamReader reader = new StreamReader(pipeServer))
{
if(reader.ReadToEnd() is { } json)
{
mainForm?.OnPipeMessage(json);
}
}
}
}
});
}
Subsequent command line invocations
PS .\WinformsSingletonApp.exe new args orig instance
The app is already running, so this obviously doesn't actually start a new instance. However, the "new" command line arguments are transmitted to the running instance via a named pipe. This occurs in the else block in Main().
const string PIPE = "{5F563B29-172F-4385-B813-21062155F22E}-PIPE";
private static void SendMessageToPipe(Array args)
{
try
{
using (NamedPipeClientStream pipeClient = new NamedPipeClientStream(".", PIPE, PipeDirection.Out))
{
pipeClient.Connect(timeout: TimeSpan.FromSeconds(5));
using (StreamWriter writer = new StreamWriter(pipeClient) { AutoFlush = true })
{
writer.WriteLine(JsonConvert.SerializeObject(args, Formatting.Indented));
}
}
}
catch (Exception ex)
{
MessageBox.Show($"Failed to send arguments to the main instance: {ex.Message}", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
}
For demo purposes, when the singleton app receives a message, it pops up a message box showing the arguments.
namespace WinformsSingletonApp
{
public partial class MainForm : Form
{
public MainForm() => InitializeComponent();
public void OnPipeMessage(string json)
{
int N = 0;
if(JsonConvert.DeserializeObject<JArray>(json) is { } pm)
{
var displayArgs = string.Join(Environment.NewLine, pm.Select(_ => $"[{N++}] {_}"));
BeginInvoke(() => MessageBox.Show(this, $"Main Form {displayArgs}"));
// This helps pop it up when running from CLI
BeginInvoke(() => TopMost = true);
BeginInvoke(() => TopMost = false);
}
}
}
}
