Suspend Process in C#

Here's my suggestion:

 [Flags]
    public enum ThreadAccess : int
    {
      TERMINATE = (0x0001),
      SUSPEND_RESUME = (0x0002),
      GET_CONTEXT = (0x0008),
      SET_CONTEXT = (0x0010),
      SET_INFORMATION = (0x0020),
      QUERY_INFORMATION = (0x0040),
      SET_THREAD_TOKEN = (0x0080),
      IMPERSONATE = (0x0100),
      DIRECT_IMPERSONATION = (0x0200)
    }

    [DllImport("kernel32.dll")]
    static extern IntPtr OpenThread(ThreadAccess dwDesiredAccess, bool bInheritHandle, uint dwThreadId);
    [DllImport("kernel32.dll")]
    static extern uint SuspendThread(IntPtr hThread);
    [DllImport("kernel32.dll")]
    static extern int ResumeThread(IntPtr hThread);
    [DllImport("kernel32", CharSet = CharSet.Auto,SetLastError = true)]
    static extern bool CloseHandle(IntPtr handle);


private static void SuspendProcess(int pid)
{
  var process = Process.GetProcessById(pid); // throws exception if process does not exist

  foreach (ProcessThread pT in process.Threads)
  {
    IntPtr pOpenThread = OpenThread(ThreadAccess.SUSPEND_RESUME, false, (uint)pT.Id);

    if (pOpenThread == IntPtr.Zero)
    {
      continue;
    }

    SuspendThread(pOpenThread);

    CloseHandle(pOpenThread);
  }
}

public static void ResumeProcess(int pid)
{
  var process = Process.GetProcessById(pid);

  if (process.ProcessName == string.Empty)
    return;

  foreach (ProcessThread pT in process.Threads)
  {
    IntPtr pOpenThread = OpenThread(ThreadAccess.SUSPEND_RESUME, false, (uint)pT.Id);

    if (pOpenThread == IntPtr.Zero)
    {
      continue;
    }

    var suspendCount = 0;
    do
    {
      suspendCount = ResumeThread(pOpenThread);
    } while (suspendCount > 0);

    CloseHandle(pOpenThread);
  }
}

Thanks to Magnus

After including the Flags, I modified the code a bit to be an extension method in my project. I could now use

var process = Process.GetProcessById(param.PId);
process.Suspend();

Here is the code for those who might be interested.

public static class ProcessExtension
{
    [DllImport("kernel32.dll")]
    static extern IntPtr OpenThread(ThreadAccess dwDesiredAccess, bool bInheritHandle, uint dwThreadId);
    [DllImport("kernel32.dll")]
    static extern uint SuspendThread(IntPtr hThread);
    [DllImport("kernel32.dll")]
    static extern int ResumeThread(IntPtr hThread);

    public static void Suspend(this Process process)
    {
        foreach (ProcessThread thread in process.Threads)
        {
            var pOpenThread = OpenThread(ThreadAccess.SUSPEND_RESUME, false, (uint)thread.Id);
            if (pOpenThread == IntPtr.Zero)
            {
                break;
            }
            SuspendThread(pOpenThread);
        }
    }
    public static void Resume(this Process process)
    {
        foreach (ProcessThread thread in process.Threads)
        {
            var pOpenThread = OpenThread(ThreadAccess.SUSPEND_RESUME, false, (uint)thread.Id);
            if (pOpenThread == IntPtr.Zero)
            {
                break;
            }
            ResumeThread(pOpenThread);
        }
    }
}

I have a utility done which I use to generally suspend/kill/list a process. Full source is on Git

So really, what the other answer's are showing is suspending thread's in the process, there is no way to really suspend the process (i.e. in one call)....

A bit of a different solution would be to actually debug the target process which you are starting, see Mike Stall's blog for some advice how to implement this from a managed context.

If you implement a debugger, you will be able to scan memory or what other snap-shotting you would like.

However, I would like to point out, that technically, there is now way to really do this. Even if you do debugbreak a target debuggee process, another process on your system may inject a thread and will be given some ability to execute code regardless of the state of the target process (even let's say if it's hit a breakpoint due to an access violation), if you have all thread's suspended up to a super high suspend count, are currently at a break point in the main process thread and any other such presumed-frozen status, it is still possible for the system to inject another thread into that process and execute some instructions. You could also go through the trouble of modifying or replacing all of the entry point's the kernel usually calls and so on, but you've now entered the viscous arm's race of MALWARE ;)...

In any case, using the managed interfaces for debugging seems' a fair amount easier than p/invoke'ng a lot of native API call's which will do a poor job of emulating what you probably really want to be doing... using debug api's ;)