刚刚接触C#就遇到一系列的有关问题！求解释

刚刚接触C#就遇到一系列的问题！求解释
string conn = ConfigurationSettings.AppSettings.Get("connecionString");
SqlCommand cmd = new SqlCommand("select count(*) from tb_Admin where AdminUser='" + textBox1.Text + "' and Adminpassword='" + textBox2.Text + "'",conn);
int i = Convert.ToInt32(cmd.ExecuteScalar());
{
。。。
}
错误提示是：参数2无法转换成string类型
  最匹配的重载方法具有一些无效参数！
我不会改，求指教

[解决办法]
connecionString

没有拼写错误吗？
还是
connectionString

这个节点存在吗
配置文件里面appSettings节点里面有这个节点吗
[解决办法]

C# code

SqlConnection conn=new Sqlconnection();conn.ConnectionString = ConfigurationSettings.AppSettings.Get("connecionString");SqlCommand cmd......
[解决办法]
C# code
#region 连接字符串        const string CONNSTRING = "server=.;database=Library;Trusted_connection=true";        static SqlConnection conn = new SqlConnection(CONNSTRING);        public static SqlConnection Conn        {          get { return DBsql.conn; }        }        #endregion        #region 打开连接        public static void Open()        {            if (conn.State == ConnectionState.Closed)            {                conn.Open();            }            else if(conn.State== ConnectionState.Broken)            {                conn.Close();                conn.Open();            }        }        #endregion        #region 关闭连接        public static void Close()        {            if (conn.State == ConnectionState.Closed || conn.State == ConnectionState.Broken)            {                conn.Close();            }        }        #endregionpublic static void Ti(string sql,ref SqlDataReader reader)        {            SqlCommand comm = new SqlCommand(sql, conn);            Open();            reader = comm.ExecuteReader(CommandBehavior.CloseConnection);        }