C# Signature:

[DllImport("user32.dll")]
static extern short GetKeyState(VirtualKeyStates nVirtKey);

User-Defined Types:

VK

Notes:

Here are some sample values for nVirtKey:

private enum VirtualKeyStates : int
{
    VK_LBUTTON       = 0x01,
    VK_RBUTTON       = 0x02,
    VK_CANCEL    = 0x03,
    VK_MBUTTON       = 0x04,
    //
    VK_XBUTTON1      = 0x05,
    VK_XBUTTON2      = 0x06,
    //
    VK_BACK      = 0x08,
    VK_TAB       = 0x09,
    //
    VK_CLEAR     = 0x0C,
    VK_RETURN    = 0x0D,
    //
    VK_SHIFT     = 0x10,
    VK_CONTROL       = 0x11,
    VK_MENU      = 0x12,
    VK_PAUSE     = 0x13,
    VK_CAPITAL       = 0x14,
    //
    VK_KANA      = 0x15,
    VK_HANGEUL       = 0x15,  /* old name - should be here for compatibility */
    VK_HANGUL    = 0x15,
    VK_JUNJA     = 0x17,
    VK_FINAL     = 0x18,
    VK_HANJA     = 0x19,
    VK_KANJI     = 0x19,
    //
    VK_ESCAPE    = 0x1B,
    //
    VK_CONVERT       = 0x1C,
    VK_NONCONVERT    = 0x1D,
    VK_ACCEPT    = 0x1E,
    VK_MODECHANGE    = 0x1F,
    //
    VK_SPACE     = 0x20,
    VK_PRIOR     = 0x21,
    VK_NEXT      = 0x22,
    VK_END       = 0x23,
    VK_HOME      = 0x24,
    VK_LEFT      = 0x25,
    VK_UP        = 0x26,
    VK_RIGHT     = 0x27,
    VK_DOWN      = 0x28,
    VK_SELECT    = 0x29,
    VK_PRINT     = 0x2A,
    VK_EXECUTE       = 0x2B,
    VK_SNAPSHOT      = 0x2C,
    VK_INSERT    = 0x2D,
    VK_DELETE    = 0x2E,
    VK_HELP      = 0x2F,
    //
    VK_LWIN      = 0x5B,
    VK_RWIN      = 0x5C,
    VK_APPS      = 0x5D,
    //
    VK_SLEEP     = 0x5F,
    //
    VK_NUMPAD0       = 0x60,
    VK_NUMPAD1       = 0x61,
    VK_NUMPAD2       = 0x62,
    VK_NUMPAD3       = 0x63,
    VK_NUMPAD4       = 0x64,
    VK_NUMPAD5       = 0x65,
    VK_NUMPAD6       = 0x66,
    VK_NUMPAD7       = 0x67,
    VK_NUMPAD8       = 0x68,
    VK_NUMPAD9       = 0x69,
    VK_MULTIPLY      = 0x6A,
    VK_ADD       = 0x6B,
    VK_SEPARATOR     = 0x6C,
    VK_SUBTRACT      = 0x6D,
    VK_DECIMAL       = 0x6E,
    VK_DIVIDE    = 0x6F,
    VK_F1        = 0x70,
    VK_F2        = 0x71,
    VK_F3        = 0x72,
    VK_F4        = 0x73,
    VK_F5        = 0x74,
    VK_F6        = 0x75,
    VK_F7        = 0x76,
    VK_F8        = 0x77,
    VK_F9        = 0x78,
    VK_F10       = 0x79,
    VK_F11       = 0x7A,
    VK_F12       = 0x7B,
    VK_F13       = 0x7C,
    VK_F14       = 0x7D,
    VK_F15       = 0x7E,
    VK_F16       = 0x7F,
    VK_F17       = 0x80,
    VK_F18       = 0x81,
    VK_F19       = 0x82,
    VK_F20       = 0x83,
    VK_F21       = 0x84,
    VK_F22       = 0x85,
    VK_F23       = 0x86,
    VK_F24       = 0x87,
    //
    VK_NUMLOCK       = 0x90,
    VK_SCROLL    = 0x91,
    //
    VK_OEM_NEC_EQUAL = 0x92,   // '=' key on numpad
    //
    VK_OEM_FJ_JISHO  = 0x92,   // 'Dictionary' key
    VK_OEM_FJ_MASSHOU= 0x93,   // 'Unregister word' key
    VK_OEM_FJ_TOUROKU= 0x94,   // 'Register word' key
    VK_OEM_FJ_LOYA   = 0x95,   // 'Left OYAYUBI' key
    VK_OEM_FJ_ROYA   = 0x96,   // 'Right OYAYUBI' key
    //
    VK_LSHIFT    = 0xA0,
    VK_RSHIFT    = 0xA1,
    VK_LCONTROL      = 0xA2,
    VK_RCONTROL      = 0xA3,
    VK_LMENU     = 0xA4,
    VK_RMENU     = 0xA5,
    //
    VK_BROWSER_BACK       = 0xA6,
    VK_BROWSER_FORWARD    = 0xA7,
    VK_BROWSER_REFRESH    = 0xA8,
    VK_BROWSER_STOP       = 0xA9,
    VK_BROWSER_SEARCH     = 0xAA,
    VK_BROWSER_FAVORITES  = 0xAB,
    VK_BROWSER_HOME       = 0xAC,
    //
    VK_VOLUME_MUTE    = 0xAD,
    VK_VOLUME_DOWN    = 0xAE,
    VK_VOLUME_UP      = 0xAF,
    VK_MEDIA_NEXT_TRACK   = 0xB0,
    VK_MEDIA_PREV_TRACK   = 0xB1,
    VK_MEDIA_STOP     = 0xB2,
    VK_MEDIA_PLAY_PAUSE   = 0xB3,
    VK_LAUNCH_MAIL    = 0xB4,
    VK_LAUNCH_MEDIA_SELECT= 0xB5,
    VK_LAUNCH_APP1    = 0xB6,
    VK_LAUNCH_APP2    = 0xB7,
    //
    VK_OEM_1     = 0xBA,   // ';:' for US
    VK_OEM_PLUS      = 0xBB,   // '+' any country
    VK_OEM_COMMA     = 0xBC,   // ',' any country
    VK_OEM_MINUS     = 0xBD,   // '-' any country
    VK_OEM_PERIOD    = 0xBE,   // '.' any country
    VK_OEM_2     = 0xBF,   // '/?' for US
    VK_OEM_3     = 0xC0,   // '`~' for US
    //
    VK_OEM_4     = 0xDB,  //  '[{' for US
    VK_OEM_5     = 0xDC,  //  '\|' for US
    VK_OEM_6     = 0xDD,  //  ']}' for US
    VK_OEM_7     = 0xDE,  //  ''"' for US
    VK_OEM_8     = 0xDF,
    //
    VK_OEM_AX    = 0xE1,  //  'AX' key on Japanese AX kbd
    VK_OEM_102       = 0xE2,  //  "<>" or "\|" on RT 102-key kbd.
    VK_ICO_HELP      = 0xE3,  //  Help key on ICO
    VK_ICO_00    = 0xE4,  //  00 key on ICO
    //
    VK_PROCESSKEY    = 0xE5,
    //
    VK_ICO_CLEAR     = 0xE6,
    //
    VK_PACKET    = 0xE7,
    //
    VK_OEM_RESET     = 0xE9,
    VK_OEM_JUMP      = 0xEA,
    VK_OEM_PA1       = 0xEB,
    VK_OEM_PA2       = 0xEC,
    VK_OEM_PA3       = 0xED,
    VK_OEM_WSCTRL    = 0xEE,
    VK_OEM_CUSEL     = 0xEF,
    VK_OEM_ATTN      = 0xF0,
    VK_OEM_FINISH    = 0xF1,
    VK_OEM_COPY      = 0xF2,
    VK_OEM_AUTO      = 0xF3,
    VK_OEM_ENLW      = 0xF4,
    VK_OEM_BACKTAB   = 0xF5,
    //
    VK_ATTN      = 0xF6,
    VK_CRSEL     = 0xF7,
    VK_EXSEL     = 0xF8,
    VK_EREOF     = 0xF9,
    VK_PLAY      = 0xFA,
    VK_ZOOM      = 0xFB,
    VK_NONAME    = 0xFC,
    VK_PA1       = 0xFD,
    VK_OEM_CLEAR     = 0xFE,
}

Tips & Tricks:

The Enum Keys in System.Windows.Forms has the key code used for this function, so you don't need to define const

Sample Code:

This Sample use this API to create a new textbox to support overwrite mode, testing the state of insert key.

Imports System.Windows.Forms

Public Class MinhaNovaTextbox

  Inherits System.Windows.Forms.TextBox

  Dim bInserting As Boolean = True

  Private Declare Function GetKeyState _
      Lib "user32" (ByVal nVirtKey As Long) As Integer

  Public Sub New()
    MyBase.New()
    bInserting = GetKeyState(Keys.Insert)
  End Sub

  Protected Overrides Sub OnKeyPress(ByVal e As System.Windows.Forms.KeyPressEventArgs)
    bInserting = GetKeyState(Keys.Insert)
    If Not bInserting Then
      Me.SelectionLength = 1
    End If
    MyBase.OnKeyPress(e)
  End Sub

End Class

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