debugging - Attach debugger in C# to another process

Question

I'd like to be able to automatically attach a debugger, something like: System.Diagnostics.Debugger.Launch(), except rather than the current process to another named process. I've got a process name and PID to identify the other process.

Is this possible?

Answer 1

Edit: GSerjo offered the correct solution. I'd like to share a few thoughts on how to improve it (and an explanation). I hope my improved answer will be useful to to others who experience the same problem.

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Attaching the VS Debugger to a Process

Manually

1. Open the Windows Task Manager (Ctrl + Shift + Esc).
2. Go to the Tab Processes.
3. Right click the process.
4. Select Debug.

Or, within Visual Studio, select Debug > Attach to Process....

Results will vary depending on whether you have access to the source code.

Automatically with C#

A note of caution: The following code is brittle in the sense that certain values, such as the Visual Studio Version number, are hard-coded. Keep this in mind going forward if you are planning to distribute your program.

First of all, add a reference to EnvDTE to your project (right click on the references folder in the solution explorer, add reference). In the following code, I'll only show the unusual using directives; the normal ones such as using System are omitted.

Because you are interacting with COM you need to make sure to decorate your Main method (the entry point of your application) with the STAThreadAttribute.

Then, you need to define the IOleMessageFilter Interface that will allow you to interact with the defined COM methods (note the ComImportAttribute). We need to access the message filter so we can retry if the Visual Studio COM component blocks one of our calls.

using System.Runtime.InteropServices;

[ComImport, Guid("00000016-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IOleMessageFilter
{
    [PreserveSig]
    int HandleInComingCall(int dwCallType, IntPtr hTaskCaller, int dwTickCount, IntPtr lpInterfaceInfo);

    [PreserveSig]
    int RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType);

    [PreserveSig]
    int MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType);
}

Now, we need to implement this interface in order to handle incoming messages:

public class MessageFilter : IOleMessageFilter
{
    private const int Handled = 0, RetryAllowed = 2, Retry = 99, Cancel = -1, WaitAndDispatch = 2;

    int IOleMessageFilter.HandleInComingCall(int dwCallType, IntPtr hTaskCaller, int dwTickCount, IntPtr lpInterfaceInfo)
    {
        return Handled;
    }

    int IOleMessageFilter.RetryRejectedCall(IntPtr hTaskCallee, int dwTickCount, int dwRejectType)
    {
        return dwRejectType == RetryAllowed ? Retry : Cancel;
    }

    int IOleMessageFilter.MessagePending(IntPtr hTaskCallee, int dwTickCount, int dwPendingType)
    {
        return WaitAndDispatch;
    }

    public static void Register()
    {
        CoRegisterMessageFilter(new MessageFilter());
    }

    public static void Revoke()
    {
        CoRegisterMessageFilter(null);
    }

    private static void CoRegisterMessageFilter(IOleMessageFilter newFilter)
    {
        IOleMessageFilter oldFilter;
        CoRegisterMessageFilter(newFilter, out oldFilter);
    }

    [DllImport("Ole32.dll")]
    private static extern int CoRegisterMessageFilter(IOleMessageFilter newFilter, out IOleMessageFilter oldFilter);
}

I defined the return values as constants for better readability and refactored the whole thing a bit to get rid of some of the duplication from the MSDN example, so I hope you'll find it self-explanatory. extern int CoRegisterMessageFilter is our connection to the unmanaged message filter code - you can read up on the extern keyword at MSDN.

Now all that's left is some code illustrating the usage:

using System.Runtime.InteropServices;
using EnvDTE;

[STAThread]
public static void Main()
{
    MessageFilter.Register();
    var process = GetProcess(7532);
    if (process != null)
    {
        process.Attach();
        Console.WriteLine("Attached to {0}", process.Name);
    }
    MessageFilter.Revoke();
    Console.ReadLine();
}

private static Process GetProcess(int processID)
{
    var dte = (DTE)Marshal.GetActiveObject("VisualStudio.DTE.10.0");
    var processes = dte.Debugger.LocalProcesses.OfType<Process>();
    return processes.SingleOrDefault(x => x.ProcessID == processID);
}