Control.PreviewKeyDown 事件

定义

命名空间:

System.Windows.Forms

程序集:

System.Windows.Forms.dll

在焦点位于此控件上的情况下，当有按键动作时发生（在 KeyDown 事件之前发生）。

public:
 event System::Windows::Forms::PreviewKeyDownEventHandler ^ PreviewKeyDown;

public event System.Windows.Forms.PreviewKeyDownEventHandler? PreviewKeyDown;

member this.PreviewKeyDown : System.Windows.Forms.PreviewKeyDownEventHandler 

Public Custom Event PreviewKeyDown As PreviewKeyDownEventHandler 

事件类型

PreviewKeyDownEventHandler

示例

下面的代码示例演示包含 Button 的 ContextMenuStrip。 Button当 具有焦点并且按向上键或向下键时，ContextMenuStrip将显示 。 事件处理程序 PreviewKeyDown 检测何时按下向上键或向下键，并将 属性设置为 IsInputKeytrue。 这会引发 KeyDown 事件，以便可以显示 ContextMenuStrip。 不应将任何逻辑放入事件处理程序中 PreviewKeyDown ，而不应设置 IsInputKey 属性。 相反，应将逻辑放入 KeyDown 事件处理程序中。

public Form1()
{
    InitializeComponent();

    // Form that has a button on it
    button1.PreviewKeyDown +=new PreviewKeyDownEventHandler(button1_PreviewKeyDown);
    button1.KeyDown += new KeyEventHandler(button1_KeyDown);
    button1.ContextMenuStrip = new ContextMenuStrip();
    // Add items to ContextMenuStrip
    button1.ContextMenuStrip.Items.Add("One");
    button1.ContextMenuStrip.Items.Add("Two");
    button1.ContextMenuStrip.Items.Add("Three");
}

// By default, KeyDown does not fire for the ARROW keys
void button1_KeyDown(object sender, KeyEventArgs e)
{
    switch (e.KeyCode)
    {
        case Keys.Down:
        case Keys.Up:
            if (button1.ContextMenuStrip != null)
            {
                button1.ContextMenuStrip.Show(button1,
                    new Point(0, button1.Height), ToolStripDropDownDirection.BelowRight);
            }
            break;
    }
}

// PreviewKeyDown is where you preview the key.
// Do not put any logic here, instead use the
// KeyDown event after setting IsInputKey to true.
private void button1_PreviewKeyDown(object sender, PreviewKeyDownEventArgs e)
{
    switch (e.KeyCode)
    {
        case Keys.Down:
        case Keys.Up:
            e.IsInputKey = true;
            break;
    }
}

Public Class Form1

    Public Sub New()
        InitializeComponent()
        ' Form that has a button on it
        AddHandler Button1.PreviewKeyDown, AddressOf Me.button1_PreviewKeyDown
        AddHandler Button1.KeyDown, AddressOf Me.button1_KeyDown
        Button1.ContextMenuStrip = New ContextMenuStrip
        ' Add items to ContextMenuStrip
        Button1.ContextMenuStrip.Items.Add("One")
        Button1.ContextMenuStrip.Items.Add("Two")
        Button1.ContextMenuStrip.Items.Add("Three")
    End Sub

    ' By default, KeyDown does not fire for the ARROW keys
    Private Sub button1_KeyDown(ByVal sender As Object, ByVal e As KeyEventArgs)
        Select Case (e.KeyCode)
            Case Keys.Down, Keys.Up
                If (Not (Button1.ContextMenuStrip) Is Nothing) Then
                    Button1.ContextMenuStrip.Show(Button1, _
                        New Point(0, Button1.Height), ToolStripDropDownDirection.BelowRight)
                End If
        End Select
    End Sub

    ' PreviewKeyDown is where you preview the key.
    ' Do not put any logic here, instead use the
    ' KeyDown event after setting IsInputKey to true.
    Private Sub button1_PreviewKeyDown(ByVal sender As Object, ByVal e As PreviewKeyDownEventArgs)
        Select Case (e.KeyCode)
            Case Keys.Down, Keys.Up
                e.IsInputKey = True
        End Select
    End Sub

End Class

注解

某些按键（如 TAB、RETURN、ESC 和箭头键）通常被某些控件忽略，因为它们不被视为输入按键。 例如，默认情况下， Button 控件会忽略箭头键。 按箭头键通常会导致焦点移动到上一个或下一个控件。 箭头键被视为导航键，按这些键通常不会引发 KeyDown 的事件 Button。 但是，按 的 Button 箭头键会引发 PreviewKeyDown 事件。 通过处理 PreviewKeyDown 的事件 Button 并将 属性设置为 IsInputKeytrue，可以在按下箭头键时引发 KeyDown 事件。 但是，如果处理箭头键，焦点将不再移动到上一个或下一个控件。

有关处理事件的详细信息，请参阅 处理和引发事件。