《编程的奥秘》读书笔记——读取按键

用户按下特定的按键，有三个事件可以读取按键，KeyPress、KeyDown、KeyUp。当用户按下并松开一个键时，三个事件的发生顺序：KeyDown、KeyPress、KeyUp。

e.Control = True And e.KeyCode = Keys.C ；用户同时按下了ctrl键和C键。

如果想在窗体级别处理键盘事件，比如textbox控件只接收数字，应在KeyPress事件中将KeyPressEventArgs.Handled 属性设置为 true。

   示例代码：   If Not Char.IsDigit(e.KeyChar) Then
                       e.Handled = True
                     End If

MSDN提供的方法：

' 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

首先在KeyDown事件中检测键值是否是数字和退格键，如果不是，将变量nonNumberEntered标记为True，其次在KeyPress事件中根据变量值决定是否处理键盘事件。