回答編集履歴

  古い行の削除にも需要はあると思いますので回答しておきます。

 

  [c# - Delete a specific line in a .NET RichTextBox - Stack Overflow](https://stackoverflow.com/questions/1329347/delete-a-specific-line-in-a-net-richtextbox/20983817#20983817)

  で、書式を維持したままの行削除を確認できました。

 

  using System;

  using System.Drawing;

  using System.Linq;

  using System.Windows.Forms;

 

  namespace Questions281426

  {

  public partial class Form1 : Form

  {

  private Timer timer1;

  private RichTextBox richTextBox1;

  private int c;

 

  public Form1()

  {

  InitializeComponent();

 

  timer1 = new Timer

  {

  Enabled = true,

  Interval = 1000,

  };

  timer1.Tick += Timer1_Tick;

 

  richTextBox1 = new RichTextBox

  {

  Dock = DockStyle.Fill,

  };

  Controls.Add(richTextBox1);

  }

 

  private void Timer1_Tick(object sender, EventArgs e)

  {

  // なんか追加

  richTextBox1.SelectionStart = richTextBox1.TextLength;

  richTextBox1.SelectionLength = 0;

  richTextBox1.SelectionColor = ++c % 2 == 0 ? Color.Red : Color.Blue;

  richTextBox1.AppendText($"あああああ{c}\n");

 

  // 10行以上あったら先頭から削除

  while(10 < richTextBox1.Lines.Count())

  {

  //[c# - Delete a specific line in a .NET RichTextBox - Stack Overflow](https://stackoverflow.com/questions/1329347/delete-a-specific-line-in-a-net-richtextbox/20983817#20983817)

  richTextBox1.SelectionStart = richTextBox1.GetFirstCharIndexFromLine(0);

  richTextBox1.SelectionLength = richTextBox1.Lines[0].Length + 1;

  richTextBox1.SelectedText = string.Empty;

  }

  }

  }

  }

  ```