從RichTextBox獲取文本的一部分

Question

有人可以告訴我這有什麼問題。我試圖在脫字符和脫字符之前在幾個字符之間得到文本。「可比」不會超過RichTextBox中的實際文字。

這是我的代碼：

int coLen = comparable.Length; 
TextPointer caretBack = rtb.CaretPosition.GetPositionAtOffset(coLen, 
    LogicalDirection.Backward); 
TextRange rtbText = new TextRange(caretBack, rtb.CaretPosition); 
string text = rtbText.Text; 

這將返回text = ""

請幫幫忙！

Answer 1

可正常工作，我得到I a

一段代碼：

 RichTextBox rtb = new RichTextBox(); 
     rtb.AppendText("I am adding some texts to the richTextBox"); 
     rtb.CaretPosition = rtb.CaretPosition.DocumentEnd; 

     int coLen = 3; 
     TextPointer caretBack = rtb.CaretPosition.GetPositionAtOffset(-coLen); 
     TextRange rtbText = new TextRange(caretBack, rtb.CaretPosition); 
     string ttt = rtbText.Text; 

編輯

這裏是一個MSTest的方法來解釋符的行爲和閱讀：

[TestMethod] 
    public void TestRichtTextBox() 
    { 
     RichTextBox rtb = new RichTextBox(); 
     rtb.AppendText("I am adding some texts to the richTextBox"); 

     int offset = 3; 

     TextPointer beginningPointer = rtb.CaretPosition.GetPositionAtOffset(offset); 
     TextPointer endPointer = rtb.CaretPosition.DocumentEnd; 
     TextRange rtbText = new TextRange(beginningPointer, endPointer); 

     Assert.IsTrue(rtbText.Text == "m adding some texts to the richTextBox\r\n"); 

     // Now we if we keep the same beggining offset but we change the end Offset to go backwards. 

     beginningPointer = rtb.CaretPosition.GetPositionAtOffset(3); 
     endPointer = rtb.CaretPosition; // this one is the beginning of the text 
     rtbText = new TextRange(beginningPointer, endPointer); 
     Assert.IsTrue(rtbText.Text == "I a"); 

     // Nowe we want to read from the back three characters. 
     // so we set the end Point to DocumentEnd. 

     rtb.CaretPosition = rtb.CaretPosition.DocumentEnd; 
     beginningPointer = rtb.CaretPosition.GetPositionAtOffset(-offset); 
     endPointer = rtb.CaretPosition; // we already set this one to the end document 
     rtbText = new TextRange(beginningPointer, endPointer); 
     Assert.IsTrue(rtbText.Text == "Box"); 
    } 

加上這裏是從MSDN的負面指標註釋：

偏移類型：System.Int32的偏移，在符號，爲其 計算並傳回的位置。如果偏移量爲負數，則在邏輯方向上計算 位置，該邏輯方向與由LogicalDirection屬性指示的 相反。