如果行以条件 C# 开头，则替换特定 position 中的字符

Question

I am trying to modify a txt file, I need to change the 45 character with a P if the line starts with 8

for (int i = 0; i < textBox.Lines.Length; i++)//Loops through each line of text in RichTextBox
           {

               string text = textBox.Lines[i];
               if ((text.Contains("8") == true)) //Checks if the line contains 8.
               {
                   char replace = 'P';
                   int startindex = textBox.GetFirstCharIndexFromLine(i);
                   int endindex = text.Length;
                   textBox.Select(startindex, endindex);//Selects the text.
                   richTextBox1.Text = textBox.Text.Substring(0, textBox.SelectionStart) + (0, textBox.Lines) + replace + textBox.Text.Substring(textBox.SelectionStart + 45);
     }}             

Answer 1

To accomplish your goal the code could be changed in this way

//Loops through each line of text in RichTextBox
for (int i = 0; i < textBox.Lines.Length; i++)
{
    string text = textBox.Lines[i];
    //Checks if the line starts with "8".
    if (text.StartsWith("8")) 
    {
        // Find the 45th position from the start of the line
        int startindex = textBox.GetFirstCharIndexFromLine(i) + 45;
        // Starting from the found index select 1 char
        textBox.Select(startindex, 1);
        // Replace the selected char with the "P"
        textBox.SelectedText = "P";
    }
}

The key points changed are the way to select into a textbox. The Select method requires a starting index and the number of character to select , finally, once you have a SelectedText, (a read/write property) you can simply replace the current SelectedText with your own text. Lot easier than your current (and wrong) calculation.