TXT文档的循环遍历之后，如何都是显示最后一行的呢？求解

TXT文档的循环遍历之后，怎么都是显示最后一行的呢？求解~
openFileDialog1.ShowDialog();
  openFileDialog1.Filter = "文本文件|*.txt|所有文件|*.*";
  string PathFile = openFileDialog1.FileName;
  string[] StrLine = File.ReadAllLines(PathFile);
  for (int i = 0; i < StrLine.Length; i++)
  {
  richTextBox1.Text = StrLine[i].ToString() + "\r\n";
   
  }

------解决方案--------------------
richTextBox1.Text += StrLine[i].ToString() + "\r\n";
------解决方案--------------------

C# code

openFileDialog1.ShowDialog();
openFileDialog1.Filter = "文本文件|*.txt|所有文件|*.*";
string PathFile = openFileDialog1.FileName;
string[] StrLine = File.ReadAllLines(PathFile);
StringBuilder sb = new StringBuilder();

foreach( string str in StrLine)
{
   sb.Append(str).Append("\r\n");
}

richTextBox1.Text = sb.ToString();