C# Signature:

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

User-Defined Types:

VK

Notes:

Here are some sample values for nVirtKey:

const int VK_SHIFT = 0x10;
const int VK_CONTROL = 0x11;
const int VK_MENU = 0x12;

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

Alternative Managed API:

Do you know one? Please contribute it!