How to Detect If a Debugger Is Attached to Another Process from C#

Is there a way to detect if a debugger is attached to another process from C#?

You will need to P/Invoke down to CheckRemoteDebuggerPresent. This requires a handle to the target process, which you can get from Process.Handle.

Attach debugger in C# to another process

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);
}

See if other assembly is being debugged

If you want to check if other applications have a debugger attached, use CheckRemoteDebuggerPresent

I've written an extension method for the processes:

public static class ProcessExtensions
{
    [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
    static extern bool CheckRemoteDebuggerPresent(IntPtr hProcess, ref bool isDebuggerPresent);

    public static bool IsDebuggerAttached(this Process process)
    {
        try
        {
            var isDebuggerAttached = false;
            CheckRemoteDebuggerPresent(process.Handle, ref isDebuggerAttached);

            return isDebuggerAttached;
        }
        catch (Exception)
        {
            return false;
        }
    }
}

How to detect if debugging

HttpContext is another HttpContext than you were used to since you are now using ASP.NET Core. That property can't be found in the ASP.NET Core variant. An explanation of the differences is given by poke.

I would use Debugger.IsAttached, which not only checks if debugging is enabled, but also actively being debugged.

How can I check if a process was started by Visual Studio 2015

The answer turned out to be in the comment provided by Hans Passant referencing his answer to another question: stackoverflow.com/a/2533287/17034

Using WMI as suggested by Hans, and checking for the new remote debugger MSVSMON rather than DEVENV as before, we get the answer we need:

using System.Management;

var myId = Process.GetCurrentProcess().Id;
var query = $"SELECT ParentProcessId FROM Win32_Process WHERE ProcessId = {myId}";
var search = new ManagementObjectSearcher("root\\CIMV2", query);
var results = search.Get().GetEnumerator();
results.MoveNext();
var queryObj = results.Current;
var parentId = (uint)queryObj["ParentProcessId"];
var parent = Process.GetProcessById((int)parentId);
bool isInVisualStudio = parent.ProcessName.ToLower() == "msvsmon";

This even allows us to dispense with call to Debugger.IsAttached, since if the parent process is the debugger than it must be attached.

I would have been more than happy to accept Hans' answer but he made it only as a comment. Therefore I am posting the answer in case someone else needs it but freely and very gratefully acknowledge that the answer came almost entirely from Hans Passant.

Is there a way to detect if the debugger has been used, from within the program being debugged?

The only thing I can think of is to use some sort of timing (which seems like a terrible idea). If the time it takes to complete a task is several magnitudes higher than usual, it could be a sign that something is being tampered with. This could result in lots of false positives if things like disk reading/writing are involved. Again, not a great idea, but just a thought

Detect if Debugger is Attached *and* stepping through

You can get the debugger state from the Debugger.CurrentMode property. But that's only going to tell you whether it is currently in the break state or if the program is running. There is no "step" mode, that's a fleeting moment which pretty much ensures that, if you could find about it, that acting on it would already be too late. Practically guaranteed to be a threading race.

Debugger.IsAttached only working when launched from visual studio

According to C# Detect if Debugger is Attached the property Debugger.IsAttached will only detect managed debuggers (which should include the visual studio debugger of course, thanks @René Vogt). You can use CheckRemoteDebuggerPresent instead, the documentation states:

The "remote" in CheckRemoteDebuggerPresent does not imply that the debugger necessarily resides on a different computer; instead, it indicates that the debugger resides in a separate and parallel process. Use the IsDebuggerPresent function to detect whether the calling process is running under the debugger.

[DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
static extern bool CheckRemoteDebuggerPresent(IntPtr hProcess, ref bool isDebuggerPresent);

In order to avoid any confusion about Debugger.IsAttached and IsDebuggerPresent - sorry I didn't mention this earlier in the tip:

IsDebuggerPresent = works for any running process and detects native debuggers too
Debugger.IsAttached = works only for the current process and detects only managed debuggers.

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