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步骤 5：添加标签引用

程序需要跟踪玩家单击了哪些 Label 控件。在第一个标签被单击后，程序显示该标签的图标。在第二个标签被单击后，程序需要同时显示两个图标很短的时间，然后再次使这两个图标不可见。程序将使用引用变量跟踪第一次和第二次分别单击了哪个 Label 控件。

添加标签引用

  1. 通过使用下面的代码向窗体中添加标签引用。

    Public Class Form1

    ' firstClicked points to the first Label control 
    ' that the player clicks, but it will be Nothing 
    ' if the player hasn't clicked a label yet
    Dim firstClicked As Label = Nothing

    ' secondClicked points to the second Label control 
    ' that the player clicks
    Dim secondClicked As Label = Nothing

    public partial class Form1 : Form
{
    // firstClicked points to the first Label control 
    // that the player clicks, but it will be null 
    // if the player hasn't clicked a label yet
    Label firstClicked = null;

    // secondClicked points to the second Label control 
    // that the player clicks
    Label secondClicked = null;
    说明说明

    引用变量看上去类似于用来向窗体添加对象（如 Timer 对象、List 对象和 Random 对象）的语句。但是，这些语句不会导致窗体中显示两个额外的 Label 控件，因为这两个语句中没有任何一个语句具有 new。而没有 new 就不能创建对象。这就是将 firstClicked 和 secondClicked 称为引用变量的原因：它们只跟踪（或引用）Label 对象。
    说明说明

    当某变量不跟踪对象时，它设置为特殊的值：null（Visual C# 中）或 Nothing（Visual Basic 中）。因此，当程序启动时，firstClicked 和 secondClicked 都设置为 null 或 Nothing，这意味着该变量不会跟踪任何内容。

  2. 修改 Click 事件处理程序，以使用新的 firstClicked 引用变量。移除 label_Click() 事件处理程序方法中的最后一个语句 (clickedLabel.ForeColor = Color.Black;)，并将它替换为下面的 if 语句。（确保包含了注释和大括号之间的所有内容。）

    ''' <summary>
''' Every label's Click event is handled by this event handler
''' </summary>
''' <param name="sender">The label that was clicked</param>
''' <param name="e"></param>
''' <remarks></remarks>
Private Sub label_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label9.Click, Label8.Click, Label7.Click, Label6.Click, Label5.Click, Label4.Click, Label3.Click, Label2.Click, Label16.Click, Label15.Click, Label14.Click, Label13.Click, Label12.Click, Label11.Click, Label10.Click, Label1.Click

    Dim clickedLabel As Label = TryCast(sender, Label)

    If clickedLabel IsNot Nothing Then

        ' If the clicked label is black, the player clicked
        ' an icon that's already been revealed --
        ' ignore the click
        If (clickedLabel.ForeColor = Color.Black) Then
            Return
        End If

        ' If firstClicked is Nothing, this is the first icon 
        ' in the pair that the player clicked, 
        ' so set firstClicked to the label that the player
        ' clicked, change its color to black, and return
        If (firstClicked Is Nothing) Then
            firstClicked = clickedLabel
            firstClicked.ForeColor = Color.Black
            Return
        End If
    End If

End Sub

    /// <summary>
/// Every label's Click event is handled by this event handler
/// </summary>
/// <param name="sender">The label that was clicked</param>
/// <param name="e"></param>
private void label_Click(object sender, EventArgs e)
{
    Label clickedLabel = sender as Label;

    if (clickedLabel != null)
    {
        // If the clicked label is black, the player clicked
        // an icon that's already been revealed --
        // ignore the click
        if (clickedLabel.ForeColor == Color.Black)
            return;

        // If firstClicked is null, this is the first icon 
        // in the pair that the player clicked,
        // so set firstClicked to the label that the player 
        // clicked, change its color to black, and return
        if (firstClicked == null)
        {
            firstClicked = clickedLabel;
            firstClicked.ForeColor = Color.Black;

            return;
        }
    }
}

  3. 保存并运行程序。单击其中一个 Label 控件，它的图标将出现。

  4. 单击下一个 Label 控件，并注意是否没有任何反应。程序已跟踪玩家单击的第一个标签，因此 firstClicked 不等于 Visual C# 中的 null 或 Visual Basic 中的 Nothing。当 if 语句检查 firstClicked 以确定它是否等于 null 或 Nothing 时，它发现它不等于，因而不会在 if 语句中执行语句。因此，只有被单击的第一个图标变为黑色，其他图标是不可见的，如下图所示。

    显示一个图标的匹配游戏

    显示一个图标的匹配游戏

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