Develop Reference

What's the point of OpenClipboard API?

After reading quite a bit on the clipboard being blocked when trying to use it, I tried using OpenClipboard() directly, to capture the clipboard and be able to use it from my window.
public partial class Form1 : Form {
public Form1() {
InitializeComponent();
}
[DllImport("user32.dll", SetLastError=true)]
private static extern bool OpenClipboard(IntPtr hWndNewOwner);
[DllImport("user32.dll", SetLastError=true)]
private static extern bool CloseClipboard();
private int idx = 0;
private void refresh_Tick(object sender, EventArgs e) {
switch (idx++) {
case 0:
OpenClipboard(Handle);
break;
default:
Clipboard.SetText(" ");
break;
}
}
}
When using SetText, I will get the infamous error:
A first chance exception of type 'System.Runtime.InteropServices.ExternalException' occurred in System.Windows.Forms.dll
Additional information: Requested Clipboard operation did not succeed.
So, the questions:
Does OpenClipboard() actually work?
What is the argument for OpenClipboard() for? The rest of the API does not need a handle to any window, so why does OpenClipboard()? In other words, the Clipboard is supposed to be shared between PROCESSES, not WINDOWS - but I don't see a way to lock it for my current process.
I can call OpenClipboard(IntPtr.Zero), which MSDN says:
If this parameter is NULL, the open clipboard is associated with the current task.
What is 'task' supposed to mean?

Does OpenClipboard() actually work?
OpenClipboard() works, but definitely not in the way you showed. It is a raw Win32 API. Here is an example of using it to copy string to clipboard (C/C++).
What is the argument for OpenClipboard() for?
To be honest, I'm not sure about this. Usually we just pass NULL aka IntPtr.Zero and let it associate with the current task. I guess it retrieves the thread that created the specified window, and then associates it with the clipboard.
What is task? Well here is my understanding. Back in Win16 days, instead of 'Thread' or 'Process', they used the terminology 'Task'. There was a function to GetCurrentTask().
Then the function is replaced by GetCurrentProcess() and GetCurrentThread().
Since it makes more sense that only one thread can access a shared resource at one time, I would say that 'Task' actually means 'Thread'.
The rest of the API does not need a handle to any window, so why does OpenClipboard()?
If my assumption above is correct, that is, only one thread can access Clipboard at a time, then chances are, Windows will use the thread/window you specified with OpenClipboard() previously, until you call CloseClipboard() or the thread/window becomes invalid. That's why you don't need to specify the HWND all the time.
In other words, the Clipboard is supposed to be shared between PROCESSES, not WINDOWS - but I don't see a way to lock it for my current process.
It locks, though I'm not sure about C# P/Invoke. If you have access to Win32/C development toolkit, compile and run the following code:
#include <conio.h>
#include <Windows.h>
int main()
{
OpenClipboard(GetForegroundWindow());
_getch();
return 0;
}
And see what happen if you use Clipboard in other program. :) Close the console window to restore.

WinApi - GetLastError vs. Marshal.GetLastWin32Error

I tested a lot. But I found no disadvantages of those 2!
But see the accepted answer.
I read here that calling GetLastError in managed code is unsafe because the Framework might internally "overwrite" the last error. I have never had any noticeable problems with GetLastError and it seems for me that the .NET Framework is smart enough not to overwrite it. Therefore I have a few questions on that topic:
in [DllImport("kernel32.dll", SetLastError = true)] does the SetLastError attribute make the Framework store the error code for the use of Marshal.GetLastWin32Error() ?
is there an example where plain GetLastError fails to give the correct result ?
do I really HAVE to use Marshal.GetLastWin32Error() ?
is this "problem" Framework version related ?
public class ForceFailure
{
[DllImport("kernel32.dll")]
static extern uint GetLastError();
[DllImport("kernel32.dll", SetLastError = true)]
static extern bool SetVolumeLabel(string lpRootPathName, string lpVolumeName);
public static void Main()
{
if (SetVolumeLabel("XYZ:\\", "My Imaginary Drive "))
System.Console.WriteLine("It worked???");
else
{
// the first last error check is fine here:
System.Console.WriteLine(GetLastError());
System.Console.WriteLine(Marshal.GetLastWin32Error());
}
}
}
Producing errors:
if (SetVolumeLabel("XYZ:\\", "My Imaginary Drive "))
Console.WriteLine("It worked???");
else
{
// bad programming but ok GetlLastError is overwritten:
Console.WriteLine(Marshal.GetLastWin32Error());
try
{
using (new FileStream("sdsdafsdfsdfs sdsd ", FileMode.Open)) { }
}
catch { }
Console.WriteLine(GetLastError());
}
if (SetVolumeLabel("XYZ:\\", "My Imaginary Drive "))
Console.WriteLine("It worked???");
else
{
// bad programming and Marshal.GetLastWin32Error() is overwritten as well:
Console.WriteLine(GetLastError());
try
{
using (new FileStream("sdsdafsdfsdfs sdsd ", FileMode.Open)) { }
}
catch { }
Console.WriteLine(Marshal.GetLastWin32Error());
}
// turn off concurrent GC
GC.Collect(); // doesn't effect any of the candidates
Console.WriteLine(" -> " + GetLastError());
Console.WriteLine(" -> " + GetLastError());
Console.WriteLine(Marshal.GetLastWin32Error());
Console.WriteLine(Marshal.GetLastWin32Error());
// when you exchange them -> same behaviour just turned around
I don't see any difference! Both behave the same except Marshal.GetLastWin32Error stores results from App->CLR->WinApi calls as well and GetLastError stores only results from App->WinApi calls.
Garbage Collection seems not to call any WinApi functions overwriting the last error code
GetLastError is thread-safe. SetLastError stores an error code for each thread calling it.
since when would GC run in my threads ?

You must always use the Marshal.GetLastWin32Error. The main problem is the garbage collector. If it runs between the call of SetVolumeLabel and the call of GetLastError then you will receive the wrong value, because the GC has surely overwritten the last result.
Therefore you always need to specify the SetLastError=true in the DllImport-Attribute:
[DllImport("kernel32.dll", SetLastError=true)]
static extern bool SetVolumeLabel(string lpRootPathName, string lpVolumeName);
This ensures that the marhsallling stub calls immediately after the native function the "GetLastError" and stores it in the local thread.
And if you have specified this attribute then the call to Marshal.GetLastWin32Error will always have the correct value.
For more info see also "GetLastError and managed code" by Adam Nathan.
Also other function from .NET can change the windows "GetLastError". Here is an example which produces different results:
using System.IO;
using System.Runtime.InteropServices;
public class ForceFailure
{
[DllImport("kernel32.dll")]
public static extern uint GetLastError();
[DllImport("kernel32.dll", SetLastError = true)]
private static extern bool SetVolumeLabel(string lpRootPathName, string lpVolumeName);
public static void Main()
{
if (SetVolumeLabel("XYZ:\\", "My Imaginary Drive "))
System.Console.WriteLine("It worked???");
else
{
System.Console.WriteLine(Marshal.GetLastWin32Error());
try
{
using (new FileStream("sdsdafsdfsdfs sdsd ", FileMode.Open)) {}
}
catch
{
}
System.Console.WriteLine(GetLastError());
}
}
}
Also it seems that this is depended on the CLR which you are using! If you compile this with .NET2, it will produce "2 / 0"; if you switch to .NET 4, it will output "2 / 2"...
So it is depended on the CLR version, but you should not trust the native GetLastError function; always use the Marshal.GetLastWin32Error.

TL;DR
Do use [DllImport(SetLastError = true)] and Marshal.GetLastWin32Error()
perform the Marshal.GetLastWin32Error() immediately after a failing Win32 call and on the same thread.
Argumentation
As i read it, the official explanation why you need Marshal.GetLastWin32Error can be found here:
The common language runtime can make internal calls to APIs that overwrite the GetLastError maintained by the operating system.
To say it in other words:
Between your Win32 call which sets the error, the CLR may "insert" other Win32 calls which could overwrite the error.
Specifying [DllImport(SetLastError = true)] makes sure that the CLR retrieves the error code before the CLR executes any unexpected Win32 calls.
To access that variable we need to use Marshal.GetLastWin32Error.
Now what #Bitterblue found is that these "inserted calls" don't happen often - he couldn't find any. But that's not really surpising. Why? Because it's extremely difficult to "black box test" whether GetLastError works reliably:
you can detect unreliability only if a CLR-inserted Win32 call actually fails in the meantime.
failure of these calls may be dependent on internal/external factors. Such as time/timing, memory pressure, devices, state of computer, windows version...
insertion of Win32 calls by CLR may be dependent on external factors. So under some circumstances the CLR inserts a Win32 call, under others it doesn't.
behavior can change with different CLR versions as well
There's is one specific component - the Garbage collector (GC) - which is known to interrupt a .net thread if there's memory pressure and do some processing on that thread (see What happens during a garbage collection). Now if the GC were to execute a failing Win32 call, this would break your call to GetLastError.
To sum it up, you have a plethora of unknown factors which can influence the reliability of GetLastError. You'll most likely not find an unreliability problem when developing/testing, but it might blow up in production at any time. So do use [DllImport(SetLastError = true)] and Marshal.GetLastWin32Error() and improve your sleep quality ;-)

in [DllImport("kernel32.dll", SetLastError = true)] does the SetLastError attribute make the Framework store the error code for the use of Marshal.GetLastWin32Error() ?
Yes, as is documented in DllImportAttribute.SetLastError Field
is there an example where plain GetLastError fails to give the correct result ?
As documented in Marshal.GetLastWin32Error Method, if the framework itself (e.g. the garbage collector) calls any native method that sets an error value between your calls to the native method and GetLastError you would get the error value of the framework's call instead of your call.
do I really HAVE to use Marshal.GetLastWin32Error() ?
Since you can't ensure that the framework will never call a native method between your call and the call to GetLastError, yes. Also, why not?
is this "problem" Framework version related ?
It could definitely be (e.g. changes in the garbage collector), but it doesn't have to.

How to implement Global Hotkeys in C#?

I need to write an application which globally intercepts Alt+Shift+S.
What I did is I created a DLL which sets global hooks:
namespace Hotkeydll
{
public class MyHotKey
{
public static void setHooks()
{
KeyboardHookProcedure = new HookProc(KeyboardHookProc);
hKeyboardHook = SetWindowsHookEx(WH_KEYBOARD_LL, KeyboardHookProcedure, Marshal.GetHINSTANCE(Assembly.GetExecutingAssembly().GetModules()[0]), 0);
}
private int KeyboardHookProc(int nCode, Int32 wParam, IntPtr lParam)
{
//write something into file
}
}
}
Then I created a program which loads this DLL and set the hook:
using Hotkeydll;
namespace IWFHotkeyStarter
{
class Program
{
static void Main(string[] args)
{
MyHotKey.setHooks();
}
}
}
Now the problem is that the hotkey doesn't work.
It looks like the DLL is not loaded permanently into memory. I see that I can delete the dll file from file system.
So please advise what I am doing wrong?
Should I use a different approach?
Thank you.

Your Main() method sets the hooks, then immediately exits and terminates the program. Furthermore, you need a message loop to make the hook callback work. That requires a Windows Forms or WPF app. Using a real hot key instead of a hook now also becomes an option. Check this thread for an example, C# is further down the page.

Keyboard hooks are usually not the right way to get global hotkeys.
Use RegisterHotkey whenever possible.

How to use Control.FromHandle?

I saw a method called Control.FromHandle which (should) give you the access to it.
Now, I wanted to try it using this code
[DllImport("user32.dll", SetLastError = true)]
static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
// Find window by Caption only. Note you must pass IntPtr.Zero as the first parameter.
[DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
static extern IntPtr FindWindowByCaption(IntPtr ZeroOnly, string lpWindowName);
[DllImport("user32.dll")]
private static extern IntPtr GetDC(IntPtr hwnd);
[DllImport("user32.dll")]
private static extern bool ReleaseDC(IntPtr hwnd, IntPtr hdc);
public Form1()
{
InitializeComponent();
}
private void Form1_Load(object sender, EventArgs e)
{
IntPtr ptr = FindWindowByCaption(IntPtr.Zero, "Download");
Control f = Control.FromHandle(ptr);
f.Text = "Something";
}
but it won't, obviously, work.
I checked personally that the handle is correct... but the method returns a null control.
Any explaining?

This method only works if the handle you pass in actually is a Control in your application.

In your particular case, since you just want to set the text, call SetWindowText from user32.dll

For anyone else googling that found this answer and wondered why this behavior is the case this post http://www.codeguru.com/forum/showthread.php?t=443191 is particularly enlightening, specifically the last post from 'MadHatter':
well just from looking over what goes on in Control.FromHandle in reflector, it looks like as windows are added to the .net world, it stores a table of what handles it has loaded, the problem comes when you pass in a handle that is not listed in its tables. There may be some hack that would allow you to register the window through all the subsystems that are used when creating windows from within the .net app, but it would probably be better / more consistent to wrap whatever functionality you need directly through the windows api then try to hack in Control.FromHandle to allow you to access / manipulate the window of some other process.
Reading more into your question it seems like you are trying to do some automation or at very least manipulate the window in some way. Might I recommend looking at the Managed Windows API project on SourceForge. It is pretty well written and we've used it for the purposes you are describing.

How do I get a System.Windows.Form instance from its Win32 handle?

The following code implements a simple singleton that ensures only 1 instance of my application can be run. However, if another instance is started, I need to be able to grab that instance's command-line arguments, pass them to the initial instance, then terminate the second instance.
The issue comes in when I'm attempting to get hold of the first instance of the application. Once I've found the handle of that instance's main form, I pass it to the Control.FromHandle() method, expecting to get back a MainForm. Instead, the return value is always null. (Control.FromChildHandle() gives the same result.)
Therefore, my question is simply: what am I doing wrong? And is this even possible in .NET?
public class MainForm : Form
{
[DllImport("user32")]
extern static int ShowWindowAsync(IntPtr hWnd, int nCmdShow);
[DllImport("user32")]
extern static bool SetForegroundWindow(IntPtr hWnd);
private Mutex singletonMutex;
private void MainForm_Load(object sender, EventArgs e)
{
bool wasCreated;
singletonMutex = new Mutex(false, Application.ProductName + "Mutex", out wasCreated);
// returns false for every instance except the first
if (!wasCreated)
{
Process thisProcess = Process.GetCurrentProcess();
Process[] peerProcesses = Process.GetProcessesByName(thisProcess.ProcessName.Replace(".vshost", string.Empty));
foreach (Process currentProcess in peerProcesses)
{
if (currentProcess.Handle != thisProcess.Handle)
{
ShowWindowAsync(currentProcess.MainWindowHandle, 1); // SW_NORMAL
SetForegroundWindow(currentProcess.MainWindowHandle);
// always returns null !!!
MainForm runningForm = (MainForm) Control.FromHandle(currentProcess.MainWindowHandle);
if (runningForm != null)
{
runningForm.Arguments = this.Arguments;
runningForm.ProcessArguments();
}
break;
}
}
Application.Exit();
return;
}
}

Single-instance apps are well supported by the .NET framework. Check this thread for an example that does exactly what you need.

Control.FromHandle isn't going to work because the control you're looking for is in another process (and therefore in another appdomain).
You already have the WindowHandle but it's use is limited to the Win32 API. Nothing from WinForms is going to work.
You can send (WM_) messages but it's hard to get data across.
Options
use something low-level with a
temp-file.
use remoting (WCF)

Try the following
var form = (Form)(Control.FromHandle(myHandle));
EDIT
Re-read your question and realized you are looking at a handle in another process. There is no way to convert a handle in another process to a Form instance in the current process. My solution will only work for handles in the same process.
The only way to get ahold of the Form instance is to use Remoting. But that will require cooperation on the part of both processes which does not appear to be what you are looking for.

You are really trying to implement a singleton application. There are a few examples in the Internet (sorry, haven't really tried myself), e.g.
http://www.codeproject.com/KB/cs/SingletonApplication.aspx
http://www.nathanm.com/csharp-wpf-singleton-application/

You can't call code in another process directly, you need to use some form of inter-process communication
If you are communication only between processes started by the same user on the same computer you can use window messages (using WinAPI PostMessage and overriding WndProc), otherwise I think remoting is the easiest to use in .net

I use the Microsoft.VisualBasic.dll library described in the thread that nobugz pointed to. Yes, you can use it in C#. You just override the OnStartupNextInstance and pass the command line into your program in whatever way works best for you.
This is a whole lot easier than messing around with the threads manually.