TextBoxBase.Clear 方法

定義

命名空間:

System.Windows.Forms

組件:

System.Windows.Forms.dll

將所有文字從文字方塊控制項清除。

public:
 void Clear();

public void Clear ();

member this.Clear : unit -> unit

Public Sub Clear ()

範例

下列程式碼範例會使用 TextBox 衍生類別來建立事件的事件處理常式 TextChanged 。 事件處理常式內的程式碼會將資料限制為數字。 在控制項中輸入文字之後，程式碼會判斷輸入的文字是否為數字。 如果文字不是數位，程式碼會清除控制項中的文字，並 MessageBox 顯示 ，以通知使用者只接受數位。 此範例會要求 Boolean 名為 flag 的變數和 TextBox 呼叫 textBox1 的控制項定義在這個方法之外。 此範例示範如何使用旗標變數來避免事件中的 TextChanged 串聯事件。

private:
   bool flag;

private:
   void MyTextChangedHandler( System::Object^ sender, System::EventArgs^ e )
   {
      Int64 val;
      // Check the flag to prevent code re-entry. 
      if ( flag == false )
      {
         // Set the flag to True to prevent re-entry of the code below.
         flag = true;
         // Determine if the text of the control is a number.
         try
         {
            // Attempt to convert to long
            val = System::Convert::ToInt64( textBox1->Text );
         }
         catch ( Exception^ ) 
         {
            // Display a message box and clear the contents if not a number.
            MessageBox::Show( "The text is not a valid number. Please re-enter" );
            // Clear the contents of the text box to allow re-entry.
            textBox1->Clear();
         }
         // Reset the flag so other TextChanged events are processed correctly.
         flag = false;
      }
   }

private bool flag;

private void MyTextChangedHandler(System.Object sender, System.EventArgs e)
{
    long val;    
    // Check the flag to prevent code re-entry. 
    if(flag == false)
    {
       // Set the flag to True to prevent re-entry of the code below.
       flag = true;
       // Determine if the text of the control is a number.
       try {
          // Attempt to convert to long
          val = System.Convert.ToInt64(textBox1.Text);
       }
       catch {
          // Display a message box and clear the contents if not a number.
          MessageBox.Show("The text is not a valid number. Please re-enter");
          // Clear the contents of the text box to allow re-entry.
          textBox1.Clear();
       }
       // Reset the flag so other TextChanged events are processed correctly.
       flag = false;
    }        
 }

Private flag As Boolean    

Private Sub MyTextChangedHandler(sender As System.Object, e As System.EventArgs)
    ' Check the flag to prevent code re-entry. 
    If flag = False Then
        ' Set the flag to True to prevent re-entry of the code below.
        flag = True
        ' Determine if the text of the control is a number.
        If IsNumeric(textBox1.Text) = False Then
            ' Display a message box and clear the contents if not a number.
            MessageBox.Show("The text is not a valid number. Please re-enter")
            ' Clear the contents of the text box to allow re-entry.
            textBox1.Clear()
        End If
        ' Reset the flag so other TextChanged events are processed correctly.
        flag = False
    End If
End Sub

備註

您可以使用這個方法來清除 控制項的內容，而不是將屬性指派 Text 為空字串。

另請參閱

• Cut()
• Copy()
• Paste()
• CanUndo
• ClearUndo()