Setting the Top Line of a RichTextBox in VB6
Setting the top-most visible line in a VB6 RichTextBox isn't a direct property setting like you might find in other richer text editors. The RichTextBox control lacks a property to explicitly control the top visible line. Instead, you need to manipulate the SelStart property in conjunction with the SelLength property to achieve the desired effect.
This involves a bit of calculation and understanding how the RichTextBox handles text selection and scrolling. Here's how you can accomplish this:
Understanding the Approach
The core idea is to simulate a selection that starts at the desired line and extends for zero length. This selection's position will then force the RichTextBox to scroll to display that line at the top. We'll need to determine the character index corresponding to the beginning of the target line.
Determining the Character Index of the Desired Line
There's no direct method to get the character index for a specific line number. We need to iterate through the lines of text within the RichTextBox to find the starting index of the target line.
This code snippet iterates through the lines, counting characters until it reaches the desired line:
Private Function GetLineStartIndex(rtb As RichTextBox, lineNumber As Long) As Long
Dim i As Long
Dim lineIndex As Long
Dim currentLine As String
If lineNumber < 1 Or lineNumber > rtb.Lines.Count Then
Err.Raise vbError, , "Invalid line number."
End If
For i = 0 To lineNumber - 1
currentLine = rtb.Lines(i)
lineIndex = lineIndex + Len(currentLine) + 1 'Add 1 for the newline character
Next i
GetLineStartIndex = lineIndex
End Function
Setting the Top Line
Now, let's combine this function with the SelStart and SelLength properties to set the top line. This function takes the RichTextBox and the desired line number as input:
Private Sub SetTopLine(rtb As RichTextBox, lineNumber As Long)
Dim startIndex As Long
startIndex = GetLineStartIndex(rtb, lineNumber)
With rtb
.SelStart = startIndex
.SelLength = 0 'Zero length selection
End With
End Sub
Example Usage
To use this, you would call the SetTopLine function, passing in your RichTextBox control and the line number you want to display at the top. For example:
Private Sub Command1_Click()
SetTopLine RichTextBox1, 5 'Sets line 5 to the top
End Sub
Remember to handle potential errors, such as an invalid line number, as shown in the GetLineStartIndex function. This provides a robust and reliable method for managing the top-most visible line in your VB6 RichTextBox. Always test thoroughly with various amounts of text and line numbers to ensure correct behavior.
Handling Empty RichTextBox
If your RichTextBox is empty, calling GetLineStartIndex with a non-zero lineNumber will result in an error. It's crucial to handle this edge case. Here's an improved version of SetTopLine:
Private Sub SetTopLine(rtb As RichTextBox, lineNumber As Long)
On Error GoTo ErrHandler
If rtb.Text = "" Then Exit Sub 'Handles empty RichTextBox
Dim startIndex As Long
startIndex = GetLineStartIndex(rtb, lineNumber)
With rtb
.SelStart = startIndex
.SelLength = 0
End With
Exit Sub
ErrHandler:
MsgBox "Error setting top line: " & Err.Description, vbCritical
End Sub
This enhanced version gracefully handles empty RichTextBox situations, preventing runtime errors. Always prioritize robust error handling in your VB6 code.
