Control.KeyDown Event (System.Windows.Forms)

KeyDown Event

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Microsoft Visual Studio 2010/.NET Framework 4

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.NET Framework Class Library

Control..::.KeyDown Event

Occurs when a key is pressed while the control has focus.

Namespace: System.Windows.Forms
Assembly: System.Windows.Forms (in System.Windows.Forms.dll)

Syntax

Visual Basic

Public Event KeyDown As KeyEventHandler

public event KeyEventHandler KeyDown

Visual C++

public:
 event KeyEventHandler^ KeyDown {
    void add (KeyEventHandler^ value);
    void remove (KeyEventHandler^ value);
}

member KeyDown : IEvent<KeyEventHandler,
    KeyEventArgs>

Remarks

Key events occur in the following order:

KeyDown
KeyPress
KeyUp

To handle keyboard events only at the form level and not enable other controls to receive keyboard events, set the KeyPressEventArgs..::.Handled property in your form's KeyPress event-handling method to true. Certain keys, such as the TAB, RETURN, ESC, and arrow keys are handled by controls automatically. To have these keys raise the KeyDown event, you must override the IsInputKey method in each control on your form. The code for the override of the IsInputKey would need to determine if one of the special keys is pressed and return a value of true. Instead of overriding the IsInputKey method, you can handle the PreviewKeyDown event and set the IsInputKey property to true. For a code example, see the PreviewKeyDown event.

For more information about handling events, see Consuming Events.

Examples

The following code example uses the KeyDown event to determine the type of character entered into the control.

Visual Basic

' Boolean flag used to determine when a character other than a number is entered.
Private nonNumberEntered As Boolean = False

' Handle the KeyDown event to determine the type of character entered into the control.
Private Sub textBox1_KeyDown(sender As Object, e As System.Windows.Forms.KeyEventArgs) _
Handles textBox1.KeyDown
' Initialize the flag to false.
nonNumberEntered = False

' Determine whether the keystroke is a number from the top of the keyboard.
If e.KeyCode < Keys.D0 OrElse e.KeyCode > Keys.D9 Then
' Determine whether the keystroke is a number from the keypad.
If e.KeyCode < Keys.NumPad0 OrElse e.KeyCode > Keys.NumPad9 Then
' Determine whether the keystroke is a backspace.
If e.KeyCode <> Keys.Back Then
' A non-numerical keystroke was pressed.
' Set the flag to true and evaluate in KeyPress event.
nonNumberEntered = True
End If
End If
End If
'If shift key was pressed, it's not a number.
If Control.ModifierKeys = Keys.Shift Then
nonNumberEntered = true
End If
End Sub 'textBox1_KeyDown

' This event occurs after the KeyDown event and can be used
' to prevent characters from entering the control.
Private Sub textBox1_KeyPress(sender As Object, e As System.Windows.Forms.KeyPressEventArgs) _
Handles textBox1.KeyPress
' Check for the flag being set in the KeyDown event.
If nonNumberEntered = True Then
' Stop the character from being entered into the control since it is non-numerical.
e.Handled = True
End If
End Sub 'textBox1_KeyPress

        // Boolean flag used to determine when a character other than a number is entered.
        private bool nonNumberEntered = false;

        // Handle the KeyDown event to determine the type of character entered into the control.
        private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
        {
            // Initialize the flag to false.
            nonNumberEntered = false;

            // Determine whether the keystroke is a number from the top of the keyboard.
            if (e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9)
            {
                // Determine whether the keystroke is a number from the keypad.
                if (e.KeyCode < Keys.NumPad0 || e.KeyCode > Keys.NumPad9)
                {
                    // Determine whether the keystroke is a backspace.
                    if(e.KeyCode != Keys.Back)
                    {
                        // A non-numerical keystroke was pressed.
                        // Set the flag to true and evaluate in KeyPress event.
                        nonNumberEntered = true;
                    }
                }
            }
            //If shift key was pressed, it's not a number.
            if (Control.ModifierKeys == Keys.Shift) {
                nonNumberEntered = true;
            }
        }

        // This event occurs after the KeyDown event and can be used to prevent
        // characters from entering the control.
        private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            // Check for the flag being set in the KeyDown event.
            if (nonNumberEntered == true)
            {
                // Stop the character from being entered into the control since it is non-numerical.
                e.Handled = true;
            }
        }

Visual C++

   // Boolean flag used to determine when a character other than a number is entered.
private:
   bool nonNumberEntered;

   // Handle the KeyDown event to determine the type of character entered into the control.
   void textBox1_KeyDown( Object^ /*sender*/, System::Windows::Forms::KeyEventArgs^ e )
   {
      // Initialize the flag to false.
      nonNumberEntered = false;

      // Determine whether the keystroke is a number from the top of the keyboard.
      if ( e->KeyCode < Keys::D0 || e->KeyCode > Keys::D9 )
      {
         // Determine whether the keystroke is a number from the keypad.
         if ( e->KeyCode < Keys::NumPad0 || e->KeyCode > Keys::NumPad9 )
         {
            // Determine whether the keystroke is a backspace.
            if ( e->KeyCode != Keys::Back )
            {
               // A non-numerical keystroke was pressed.
               // Set the flag to true and evaluate in KeyPress event.
               nonNumberEntered = true;
            }
         }
      }
      //If shift key was pressed, it's not a number.
      if (Control::ModifierKeys == Keys::Shift) {
         nonNumberEntered = true;
      }
   }

   // This event occurs after the KeyDown event and can be used to prevent
   // characters from entering the control.
   void textBox1_KeyPress( Object^ /*sender*/, System::Windows::Forms::KeyPressEventArgs^ e )
   {
      // Check for the flag being set in the KeyDown event.
      if ( nonNumberEntered == true )
      {         // Stop the character from being entered into the control since it is non-numerical.
         e->Handled = true;
      }
   }

Version Information

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference

Control Class

System.Windows.Forms Namespace

Other Resources

How Keyboard Input Works

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Annotations

Application wide key replacement

Nico Boey | Edit | Show History

If you need an application wide key replacement, e.g. replacing the numpad decimal point with the current culture's decimal separator (yes, there are countries where the decimal separator is a comma), you need a IMessageFilter; look here for an example: http://code.msdn.microsoft.com/Replace-numpad-decimalpoint-c1e0e6cd

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Redundant Code in the Example

Talha Munir | Edit | Show History

Here is a simpler solution. Just Control the input in keydown event of textbox as follows
private void txtBox1_KeyDown(object sender, KeyEventArgs e)
{
           if (Control.ModifierKeys == Keys.Shift)
            {
                e.SuppressKeyPress = true;
                return;
            }

            if (((e.KeyCode >= Keys.D0 &;;&;; e.KeyCode <= Keys.D9) ||
                 (e.KeyCode >= Keys.NumPad0 &;;&;; e.KeyCode <= Keys.NumPad9) ||
                  e.KeyCode == Keys.Decimal || e.KeyCode == Keys.OemPeriod ||
                  e.KeyCode == Keys.Back || e.KeyCode == Keys.Tab ||
                  e.KeyCode == Keys.Left || e.KeyCode == Keys.Right ||
                  e.KeyCode == Keys.End || e.KeyCode == Keys.Home ||
                  e.KeyCode == Keys.ShiftKey || e.KeyCode == Keys.Delete))
            {

            }
            else if (e.KeyCode >= Keys.A || e.KeyCode <= Keys.Z)
                e.SuppressKeyPress = true;
}

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Bad Example by MSDN and My Code

Chakravarthy | Edit | Show History

The given example is inappropriate for eliminating the key input. The given example doesn’t work for you, if your interest is to prevent the user to input anything other than the numbers. I’ve tried and came up with a simple workout for preventing the user to input other than numbers. Here is my code, please comment and revise if it can be more simplified.

Step 1: We need a small reusable function as mentioned below.
bool IsDigitInput(char keyValue)
{
return ((keyValue >= '0') &;;&;; (keyValue <= '9') || keyValue.Equals('\b'));
}

Step 2: Consume the above method at the KeyPress event of the text box as mentioned below
if (!IsDigitInput(e.KeyChar))
{
e.KeyChar = new char();
}

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