写入：

/// <summary>
/// 向txt文件中写入字符串
/// </summary>
/// <param name="value">内容</param>
/// <param name="isClearOldText">是否清除旧的文本</param>
private void Wriete(string value, bool isClearOldText = true)
{
    string path = "txt文件的路径";
    //是否清空旧的文本
    if (isClearOldText)
    {
        //清空txt文件
        using (FileStream stream = File.Open(path, FileMode.OpenOrCreate, FileAccess.Write))
        {
            stream.Seek(0, SeekOrigin.Begin);
            stream.SetLength(0);
        }
    }
    //写入内容
    using (StreamWriter writer = new StreamWriter(path, true))
    {
        writer.WriteLine(value);
    }
}

读取：

/// <summary>
/// 读取txt文件，并返回文件中的内容
/// </summary>
/// <returns>txt文件内容</returns>
private string ReadTxTContent()
{
    try
    {
        string s_con = string.Empty;
        // 创建一个 StreamReader 的实例来读取文件 
        // using 语句也能关闭 StreamReader
        using (StreamReader sr = new StreamReader("txt文件的路径"))
        {
            string line;
            // 从文件读取并显示行，直到文件的末尾 
            while ((line = sr.ReadLine()) != null)
            {
                s_con += line;
            }
        }
        return s_con;
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
        return null;
    }
}