如何用TextBox.GetPositionFromCharIndex按需显示垂直滚动条及方法异常排查

Let's break down why your second method fails with just "\r\n" and how to fix it, plus handle scrollbar visibility correctly.

Why NumberOfPhysicalLinesInTextBox_2 Fails for "\r\n" #

When you have only "\r\n" in the TextBox:

• The '\r' (index 0) and '\n' (index 1) are treated as a single newline by the WinForms TextBox. Their drawn positions share the same Y-coordinate because they don't occupy separate vertical space on their own—they act as a single marker to move to the next line.
• This means lastCharY - firstCharY equals 0, so your calculation gives 1 + 0 = 1 instead of the correct 2 lines.

Corrected Method for Physical Line Count #

Here's an adjusted version that handles edge cases like pure newlines, empty text, and standard content:

internal static int NumberOfPhysicalLinesInTextBox_Fixed(TextBox tb)
{
    // Handle empty text case first
    if (string.IsNullOrEmpty(tb.Text))
        return 0;

    int lines = 1;
    int firstCharY = tb.GetPositionFromCharIndex(0).Y;
    int lastCharIndex = tb.Text.Length - 1;
    int lastCharY = tb.GetPositionFromCharIndex(lastCharIndex).Y;

    // Calculate base line difference
    int lineHeight = tb.Font.Height;
    lines += (lastCharY - firstCharY) / lineHeight;

    // Special case: check if the last character is a newline (or part of one)
    // and if the cursor is on a new line after it
    char lastChar = tb.Text[lastCharIndex];
    if ((lastChar == '\n' || lastChar == '\r') && tb.GetFirstCharIndexFromLine(lines) != -1)
    {
        lines++;
    }

    // Fallback to the first method's logic for edge cases to ensure accuracy
    int fallbackLineCount = 0;
    while (tb.GetFirstCharIndexFromLine(fallbackLineCount) != -1)
    {
        fallbackLineCount++;
    }
    return Math.Max(lines, fallbackLineCount);
}

Key Fixes: #

• Empty Text Handling: Returns 0 immediately instead of incorrect values.
• Newline Edge Case: Checks if the last character is a newline and verifies if there's an extra line after it using GetFirstCharIndexFromLine.
• Fallback Logic: Leverages your original reliable method as a safety net to cover edge scenarios where position-based calculation might fall short.

Handling Vertical Scrollbar Visibility #

To show the vertical scrollbar only when needed:

1. Calculate the total physical lines using the fixed method above.
2. Get the number of visible lines in the TextBox (based on control height and font size):

   internal static int GetVisibleLineCount(TextBox tb)
   {
       // Calculate usable client height by subtracting padding
       int clientHeight = tb.ClientSize.Height - tb.Padding.Top - tb.Padding.Bottom;
       return clientHeight / tb.Font.Height;
   }
   

3. Toggle the scrollbar based on line count comparison:

   internal void UpdateVerticalScrollbar(TextBox tb)
   {
       int totalLines = NumberOfPhysicalLinesInTextBox_Fixed(tb);
       int visibleLines = GetVisibleLineCount(tb);
   
       // Show scrollbar if total lines exceed visible lines
       tb.ScrollBars = totalLines > visibleLines ? ScrollBars.Vertical : ScrollBars.None;
   }
   

Notes: #

• Call UpdateVerticalScrollbar whenever the TextBox content changes (e.g., in the TextChanged event) or when the control is resized.
• Ensure the TextBox's Multiline property is set to true for scrollbars to function properly.

内容的提问来源于stack exchange，提问作者silviubogan