How can I find the state of NumLock, CapsLock and ScrollLock in .NET?

With Framework 2.0 and above, you can use a framework function:

Control.IsKeyLocked(Keys) Method

public static bool NumlockActive()
{
    return Control.IsKeyLocked(Keys.NumLock);
}

Import the WinAPI function GetKeyState:

[DllImport("user32.dll", CharSet = CharSet.Auto, ExactSpelling = true, CallingConvention = CallingConvention.Winapi)]
public static extern short GetKeyState(int keyCode);

And then you can use it like this:

bool CapsLock = (((ushort)GetKeyState(0x14)) & 0xffff) != 0;
bool NumLock = (((ushort)GetKeyState(0x90)) & 0xffff) != 0;
bool ScrollLock = (((ushort)GetKeyState(0x91)) & 0xffff) != 0;

It is for framework 1.1. For framework 2.0 (and later) you can use:

Control.IsKeyLocked

Check State

To check state of CapsLock, NumLock and ScrollLock keys you can use Control.IsKeyLocked method:

var capsLockIsOn = Control.IsKeyLocked(Keys.CapsLock);

Actively Show The State in UI in status bar

Since the lock keys can be turn on or turn off when your application doesn't have focus handling keyboard events of the form is not enough to detect changes on the key lock state and you should also put your logic in some other places like activation event of your form or you need to register a global keyboard hook.

As a simple and reliable solution you can check their status in Application.Idle event. You must detach your idle event handler when your form closed.

public Form1()
{
    InitializeComponent();
    Application.Idle += Application_Idle;
}

void Application_Idle(object sender, EventArgs e)
{
    if (Control.IsKeyLocked(Keys.CapsLock))
        toolStripStatusLabel1.Text = "CapsLock is On";
    else
        toolStripStatusLabel1.Text = "";
}

protected override void OnFormClosed(FormClosedEventArgs e)
{
    Application.Idle -= Application_Idle;
    base.OnFormClosed(e);
}

If anyone comes across this thread while developing in WPF, you can use the Keyboard.IsToggled method that was introduced in .NET 3.0:

var isNumLockToggled = Keyboard.IsKeyToggled(Key.NumLock);
var isCapsLockToggled = Keyboard.IsKeyToggled(Key.CapsLock);
var isScrollLockToggled = Keyboard.IsKeyToggled(Key.Scroll);

You'll have to add the following using directive to the top of your class, if it's not already there:

using System.Windows.Input;

Internally, the IsToggled() method checks to see whether or not the KeyStates.Toggled flag is set for the specified key.

[Flags]
public enum KeyStates : byte
{
    None = (byte) 0,
    Down = (byte) 1,
    Toggled = (byte) 2,
}