pinvoke.net: Edit: user32.getasynckeystate

C# Signature:
[DllImport("user32.dll")]
static extern short GetAsyncKeyState(System.Windows.Forms.Keys vKey);

// FOR CONSOLE APPLICATIONS USE:
[DllImport("user32.dll")]
public static extern int GetAsyncKeyState(Keys vKeys);
VB.NET Signature:
    <DllImport("user32.dll")> _
    Shared Function GetAsyncKeyState(ByVal vKey As System.Windows.Forms.Keys) As Short
    End Function
VB Signature:
Declare Function GetAsyncKeyState Lib "user32" (ByVal vkey As Integer) As Short

User-Defined Constants:
VKs

http://msdn.microsoft.com/en-us/library/windows/desktop/dd375731(v=vs.85).aspx

User-Defined Types:
None.

Notes:
None.

Tips & Tricks:
This could be a good keylogger (= yeah !

Or for hotkeys and such. This works even if the window isn't focused.

This works not for keyboard only, for mouse also (VK_RBUTTON, VK_LBUTTON).

C# Sample Code:
    // this is import of libraries
    [DllImport("User32.dll")]
    private static extern short GetAsyncKeyState(System.Windows.Forms.Keys vKey); // Keys enumeration

[DllImport("User32.dll")]
    private static extern short GetAsyncKeyState(System.Int32 vKey);

string keyBuffer = string.Empty;

private void timer1_Tick(object sender, EventArgs e)
    {
        timer1.Interval = 3;
        foreach (System.Int32 i in Enum.GetValues(typeof(Keys)))
        {
            int x = GetAsyncKeyState(i);
            if ((x == 1) || (x == Int16.MinValue)) //Use constants (0x8000 and -32768 is Int16.MaxValue)
            {
                keyBuffer += Enum.GetName(typeof(Keys), i) + " ";//this is WinAPI listener of the keys
            }
        }
        if (keyBuffer != "")
        {
            // replacing of unreadable keys              
            keyBuffer = keyBuffer.Replace("Space", "_");
            keyBuffer = keyBuffer.Replace("Delete", "_Del_");
            keyBuffer = keyBuffer.Replace("LShiftKey", "_SHIFT_");
            keyBuffer = keyBuffer.Replace("ShiftKey", "");
            keyBuffer = keyBuffer.Replace("OemQuotes", "!");
            keyBuffer = keyBuffer.Replace("Oemcomma", "?");
            keyBuffer = keyBuffer.Replace("D8", "á");
            keyBuffer = keyBuffer.Replace("D2", "ě");
            keyBuffer = keyBuffer.Replace("D3", "š");
            keyBuffer = keyBuffer.Replace("D4", "č");
            keyBuffer = keyBuffer.Replace("D5", "ř");
            keyBuffer = keyBuffer.Replace("D6", "ž");
            keyBuffer = keyBuffer.Replace("D7", "ý");
            keyBuffer = keyBuffer.Replace("D9", "í");
            keyBuffer = keyBuffer.Replace("D0", "é");
            keyBuffer = keyBuffer.Replace("Back", "<==");
            keyBuffer = keyBuffer.Replace("LButton", "");
            keyBuffer = keyBuffer.Replace("RButton", "");
            keyBuffer = keyBuffer.Replace("NumPad", "");
            keyBuffer = keyBuffer.Replace("OemPeriod", ".");
            keyBuffer = keyBuffer.Replace("OemSemicolon", "ů");
            keyBuffer = keyBuffer.Replace("Oem4", "/");
            keyBuffer = keyBuffer.Replace("LControlKey", "");
            keyBuffer = keyBuffer.Replace("ControlKey", "_CTRL_");
            keyBuffer = keyBuffer.Replace("Enter", "");
            keyBuffer = keyBuffer.Replace("Shift", "____SHIFT___");
            keyBuffer = keyBuffer.ToLower();
            keyBuffer = keyBuffer.Replace(" ", "");
            textBox1.Text = keyBuffer;
        }
    }

C# Sample Code 2:
    [DllImport("user32.dll")]
    static extern ushort GetAsyncKeyState(int vKey);

public static bool IsKeyPushedDown(System.Windows.Forms.Keys vKey) {
        return 0 != (GetAsyncKeyState((int)vKey) & 0x8000);
    }

VB.NET Sample Code:
Imports System.Runtime.InteropServices

Public Class Form1
     <DllImport("user32.dll")> _
     Shared Function GetAsyncKeyState(ByVal vKey As System.Windows.Forms.Keys) As Short
     End Function

Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
     If GetAsyncKeyState(Keys.Return) <> 0 Then
         TextBox1.Text = "Enter pressed now"
     Else
         TextBox1.Text = "Enter not pressed now"
     End If
     End Sub
End Class

Alternative Managed API:
The ManagedWindowsApi project (http://mwinapi.sourceforge.net) provides a KeyboardKey class to detect keyboard state and synthesize keyboard events.

Windows Input Simulator
An open source managed .NET wrapper written in C# is available on Codeplex at http://inputsimulator.codeplex.com/. It has all of these definitions complete with documented code comments and can be used to determine key states as well. Thanks to all the contributors at pinvoke.