How to: Handle Keyboard Input at the Form Level
Windows Forms provides the ability to handle keyboard messages at the form level, before the messages reach a control. This topic shows how to accomplish this task.
To handle a keyboard message at the form level
Handle the KeyPress or KeyDown event of the startup form, and set the KeyPreview property of the form to true so that keyboard messages are received by the form before they reach any controls on the form. The following code example handles the KeyPress event by detecting all of the number keys and consuming '1', '4', and '7'.
' Detect all numeric characters at the form level and consume 1, ' 4, and 7. Note that Form.KeyPreview must be set to true for this ' event handler to be called. Sub Form1_KeyPress(ByVal sender As Object, _ ByVal e As KeyPressEventArgs) Handles Me.KeyPress If e.KeyChar >= ChrW(48) And e.KeyChar <= ChrW(57) Then MessageBox.Show(("Form.KeyPress: '" + _ e.KeyChar.ToString() + "' pressed.")) Select Case e.KeyChar Case ChrW(49), ChrW(52), ChrW(55) MessageBox.Show(("Form.KeyPress: '" + _ e.KeyChar.ToString() + "' consumed.")) e.Handled = True End Select End If End Sub// Detect all numeric characters at the form level and consume 1, // 4, and 7. Note that Form.KeyPreview must be set to true for this // event handler to be called. void Form1_KeyPress(object sender, KeyPressEventArgs e) { if (e.KeyChar >= 48 && e.KeyChar <= 57) { MessageBox.Show("Form.KeyPress: '" + e.KeyChar.ToString() + "' pressed."); switch (e.KeyChar) { case (char)49: case (char)52: case (char)55: MessageBox.Show("Form.KeyPress: '" + e.KeyChar.ToString() + "' consumed."); e.Handled = true; break; } } }// Detect all numeric characters at the form level and consume 1, // 4, and 7. Note that Form.KeyPreview must be set to true for this // event handler to be called. private: void Form1_KeyPress(Object^ sender, KeyPressEventArgs^ e) { if ((e->KeyChar >= '0') && (e->KeyChar <= '9')) { MessageBox::Show("Form.KeyPress: '" + e->KeyChar.ToString() + "' pressed."); switch (e->KeyChar) { case '1': case '4': case '7': MessageBox::Show("Form.KeyPress: '" + e->KeyChar.ToString() + "' consumed."); e->Handled = true; break; } } }
Example
The following code example is the entire application for the above example. The application includes a TextBox along with several other controls that allow you to move focus from the TextBox. The KeyPress event of the main Form consumes '1', '4', and '7', and the KeyPress event of the TextBox consumes '2', '5', and '8' while displaying the remaining keys. Compare the MessageBox output when you press a number key while the TextBox has focus with the MessageBox output when you press a number key while focus is on one of the other controls.
Imports System
Imports System.Drawing
Imports System.Windows.Forms
Namespace KeyboardInputForm
Class Form1
Inherits Form
Private WithEvents TextBox1 As New TextBox()
<STAThread()> _
Public Shared Sub Main()
Application.EnableVisualStyles()
Application.Run(New Form1())
End Sub
Public Sub New()
Me.AutoSize = True
Dim panel As New FlowLayoutPanel()
panel.AutoSize = True
panel.FlowDirection = FlowDirection.TopDown
panel.Controls.Add(TextBox1)
Me.Controls.Add(panel)
Me.KeyPreview = True
End Sub
' Detect all numeric characters at the form level and consume 1,
' 4, and 7. Note that Form.KeyPreview must be set to true for this
' event handler to be called.
Sub Form1_KeyPress(ByVal sender As Object, _
ByVal e As KeyPressEventArgs) Handles Me.KeyPress
If e.KeyChar >= ChrW(48) And e.KeyChar <= ChrW(57) Then
MessageBox.Show(("Form.KeyPress: '" + _
e.KeyChar.ToString() + "' pressed."))
Select Case e.KeyChar
Case ChrW(49), ChrW(52), ChrW(55)
MessageBox.Show(("Form.KeyPress: '" + _
e.KeyChar.ToString() + "' consumed."))
e.Handled = True
End Select
End If
End Sub
' Detect all numeric characters at the TextBox level and consume
' 2, 5, and 8.
Sub TextBox1_KeyPress(ByVal sender As Object, _
ByVal e As KeyPressEventArgs) Handles TextBox1.KeyPress
If e.KeyChar >= ChrW(48) And e.KeyChar <= ChrW(57) Then
MessageBox.Show(("Control.KeyPress: '" + _
e.KeyChar.ToString() + "' pressed."))
Select Case e.KeyChar
Case ChrW(50), ChrW(53), ChrW(56)
MessageBox.Show(("Control.KeyPress: '" + _
e.KeyChar.ToString() + "' consumed."))
e.Handled = True
End Select
End If
End Sub
End Class
End Namespace
using System;
using System.Drawing;
using System.Windows.Forms;
namespace KeyboardInputForm
{
class Form1 : Form
{
TextBox TextBox1 = new TextBox();
[STAThread]
public static void Main()
{
Application.EnableVisualStyles();
Application.Run(new Form1());
}
public Form1()
{
this.AutoSize = true;
FlowLayoutPanel panel = new FlowLayoutPanel();
panel.AutoSize = true;
panel.FlowDirection = FlowDirection.TopDown;
panel.Controls.Add(TextBox1);
this.Controls.Add(panel);
this.KeyPreview = true;
this.KeyPress +=
new KeyPressEventHandler(Form1_KeyPress);
TextBox1.KeyPress +=
new KeyPressEventHandler(TextBox1_KeyPress);
}
// Detect all numeric characters at the form level and consume 1,
// 4, and 7. Note that Form.KeyPreview must be set to true for this
// event handler to be called.
void Form1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar >= 48 && e.KeyChar <= 57)
{
MessageBox.Show("Form.KeyPress: '" +
e.KeyChar.ToString() + "' pressed.");
switch (e.KeyChar)
{
case (char)49:
case (char)52:
case (char)55:
MessageBox.Show("Form.KeyPress: '" +
e.KeyChar.ToString() + "' consumed.");
e.Handled = true;
break;
}
}
}
// Detect all numeric characters at the TextBox level and consume
// 2, 5, and 8.
void TextBox1_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar >= 48 && e.KeyChar <= 57)
{
MessageBox.Show("Control.KeyPress: '" +
e.KeyChar.ToString() + "' pressed.");
switch (e.KeyChar)
{
case (char)50:
case (char)53:
case (char)56:
MessageBox.Show("Control.KeyPress: '" +
e.KeyChar.ToString() + "' consumed.");
e.Handled = true;
break;
}
}
}
}
}
#using <System.Drawing.dll>
#using <System.Windows.Forms.dll>
#using <System.dll>
using namespace System;
using namespace System::Drawing;
using namespace System::Windows::Forms;
using namespace System::Security::Permissions;
namespace KeyboardInputForm
{
public ref class Form1 sealed: public Form, public IMessageFilter
{
// The following Windows message value is defined in Winuser.h.
private:
static const int WM_KEYDOWN = 0x100;
private:
TextBox^ inputTextBox;
public:
Form1()
{
inputTextBox = gcnew TextBox();
this->AutoSize = true;
Application::AddMessageFilter(this);
FlowLayoutPanel^ panel = gcnew FlowLayoutPanel();
panel->AutoSize = true;
panel->FlowDirection = FlowDirection::TopDown;
panel->Controls->Add(gcnew Button());
panel->Controls->Add(gcnew RadioButton());
panel->Controls->Add(inputTextBox);
this->Controls->Add(panel);
this->KeyPreview = true;
this->KeyPress +=
gcnew KeyPressEventHandler(this, &Form1::Form1_KeyPress);
inputTextBox->KeyPress +=
gcnew KeyPressEventHandler(this,
&Form1::inputTextBox_KeyPress);
}
// Detect all numeric characters at the
// application level and consume 0.
[SecurityPermission(SecurityAction::LinkDemand,
Flags=SecurityPermissionFlag::UnmanagedCode)]
virtual bool PreFilterMessage(Message% m)
{
// Detect key down messages.
if (m.Msg == WM_KEYDOWN)
{
Keys keyCode = (Keys)((int)m.WParam) & Keys::KeyCode;
// Determine whether the keystroke is a number from the top of
// the keyboard, or a number from the keypad.
if (((keyCode >= Keys::D0) && (keyCode <= Keys::D9))
||((keyCode >= Keys::NumPad0)
&& (keyCode <= Keys::NumPad9)))
{
MessageBox::Show(
"IMessageFilter.PreFilterMessage: '" +
keyCode.ToString() + "' pressed.");
if ((keyCode == Keys::D0) || (keyCode == Keys::NumPad0))
{
MessageBox::Show(
"IMessageFilter.PreFilterMessage: '" +
keyCode.ToString() + "' consumed.");
return true;
}
}
}
// Forward all other messages.
return false;
}
// Detect all numeric characters at the form level and consume 1,
// 4, and 7. Note that Form.KeyPreview must be set to true for this
// event handler to be called.
private:
void Form1_KeyPress(Object^ sender, KeyPressEventArgs^ e)
{
if ((e->KeyChar >= '0') && (e->KeyChar <= '9'))
{
MessageBox::Show("Form.KeyPress: '" +
e->KeyChar.ToString() + "' pressed.");
switch (e->KeyChar)
{
case '1':
case '4':
case '7':
MessageBox::Show("Form.KeyPress: '" +
e->KeyChar.ToString() + "' consumed.");
e->Handled = true;
break;
}
}
}
// Detect all numeric characters at the TextBox level and consume
// 2, 5, and 8.
private:
void inputTextBox_KeyPress(Object^ sender, KeyPressEventArgs^ e)
{
if ((e->KeyChar >= '0') && (e->KeyChar <= '9'))
{
MessageBox::Show("Control.KeyPress: '" +
e->KeyChar.ToString() + "' pressed.");
switch (e->KeyChar)
{
case '2':
case '5':
case '8':
MessageBox::Show("Control.KeyPress: '" +
e->KeyChar.ToString() + "' consumed.");
e->Handled = true;
break;
}
}
}
};
}
[STAThread]
int main()
{
Application::EnableVisualStyles();
Application::Run(gcnew KeyboardInputForm::Form1());
}
Compiling the Code
This example requires:
- References to the System, System.Drawing and System.Windows.Forms assemblies.
For information about building this example from the command line for Visual Basic or Visual C#, see Building from the Command Line (Visual Basic) or Command-line Building With csc.exe. You can also build this example in Visual Studio by pasting the code into a new project. How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio
How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio
How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio
How to: Compile and Run a Complete Windows Forms Code Example Using Visual Studio