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Capture Keypress in WPF C# Application when not in focus

I want to capture the PrintScreen button even if the application is minimised or not on focus at all. I've seen some snippets but none of these worked in WPF, only in Winforms. Anyone can give some code to start?

Please refer to the following blog post for an example of how you could implement global hot keys in a WPF application.
Implementing global hot keys in WPF: https://blog.magnusmontin.net/2015/03/31/implementing-global-hot-keys-in-wpf/.
A global, or system-wide, hot key is a key or a combination of keys that is associated with a particular window, which will receive messages whenever the user presses this key or key combination from anywhere in the system.
There are indeed no .NET API:s to register hot keys for your application but there is a native RegisterHotKey method that you can call from managed code using P/Invoke. Please refer to the link for more information.

What you want is called "Global Keyboard Hooks"
there is a pretty good solution here: http://www.codeproject.com/KB/cs/globalhook.aspx

Capture keyboard input after a hotkey is pressed, without app focus

I have a bar code scanner that presents itself as a USB-HID keyboard.
How do I capture scanned bar codes in my c# WPF application, if the app currently does not have focus?
When a barcode is scanned, the scanner sends one or more configurable prefix keypress events, then the barcode itself, then some postfix keypresses.
If the application has focus, I can use the prefix hotkeys to focus a specific textbox; then the barcode is automatically entered into this textbox.
If the application does not have focus, how can I receive the barcode?
I could register a global hotkey via pInvoking RegisterHotkey from User32.dll, as outlined e.g. here:
How can I register a global hot key to say CTRL+SHIFT+(LETTER) using WPF and .NET 3.5?
Global hotkeys in WPF working from every window
But how do I capture the following keyboard events containing the bar code?
I am a little dubious about this approach as well, is there a simpler way to capture a string prefixed by a specific hotkey, without having focus?

Control OS Mouse Event using Windows Form - C#

The program I am trying to do is to simulate the mouse event of a operating system using keyboard with Windows Form. Right now I am able to change the cursor and do different actions like mouse clicking inside the Form (when the Form is on the Top).
The problem is I would like to extend it to the whole Operating System, which means even if my Windows Form is not at the top, I am still able to control my cursor and do all sorts of mouse event on other applications while the Form is running. How should I do to implement this ?

You might want to look at this library Global System Hooks in .NET which uses global system hooks to detect all mouse and keyboard events include those outside of your application.

You can synthesize OS-wide keystrokes, mouse motions, and button clicks using the Win32 SendInput() API. You can call it from C# using P/Invoke. Sample code can be found here: SendInput on PInvoke.net.
I remember back in the day I used the SendInput (and a screenshot API) to create a Minesweeper bot in C# (2.0 I think). It could solve an Expert puzzle in about one second. I wish I still had the source code to sample here, but I don't.
EDIT: It appears someone has already created a nice .NET warpper for the SendInput(): Windows Input Simulator on CodePlex.

how to call onmousedown method on windows using c#

i need to perform the action done by using left mouse click when i press W for example:
i want to move the cruse to a folder and then press w so the folder open
i want to move the cruse to a file and press w and drag it and drop when i release w
i tried to use windowsFrom and performing the onKeyPress() and it calls onMouseDown() but of course it means that the onMouseDown() function will be called inside the form and what i need is a system behavior,,,, i am using windows 7 64 on VS2010 but i would like if it is some thing global for all windows
thanks a lot for helping,,,

The first step you'll need to capture Key Down events in an external application is define a global hotkey:
I recommend checking out some of the answers for this question for a global hot-key: Best way to tackle global hotkey processing in c#? or this CodeProject article: A Simple C# Global Low Level Keyboard Hook
Then, as M. Babcock said you'll need to send the mouse-down events. The link he provided in the comment is a good one. I'm repeating it here for completeness.
How to simulate Mouse Click in C#?
If you actually need to send keyboard inputs, you can use InputSimulator which is something I have advocated (and used in the past) and it provides a very flexible (and reliable) wrapper that is capable of simulating keyboard events.
It wraps SendInput under the hood but abstracts away all the PInvoke calls and other complexity.
InputSimulator.SimulateKeyDown(VirtualKeyCode.CTRL);
InputSimulator.SimulateKeyPress(VirtualKeyCode.KEYS_V);
InputSimulator.SimulateKeyUp(VirtualKeyCode.CTRL);
or
InputSimulator.SimulateModifiedKeyStroke(VirtualKeyCode.CONTROL, VirtualKeyCode.VK_C);

Use a boolean flag for the clicking, and set it on both by clicking or pressing your hotkey. For instance, this flag should be declared inside the form, and the events KeyDown and MouseDown set it true, while KeyUp and MouseUp set it false. For all your actions now you use this flag for verifying the click. (Not sure if it would works in Windows app, in XNA it works fine). For using outside the class, just set this flag as a public property.

Emulate keyboard

How can I emulate keyboard in application running in background (hidden in tray).
I want to press, hold, and relase keys on keyboard programatically.
Also, I would like to move mouse around and be able to click.

You can use SendInput API function

You can also try the Windows.Forms.SendKeys API
Here is a How-To: How to: Simulate Mouse and Keyboard Events in Code