String和StringBuilder
语法：
//声明一个空的StringBuilder对象
StingBuilder对象名称 = new StringBuilder();
//声明一个StringBuilder对象，值为"字符串初始值"
StringBuilder 对象名称 = new StringBuilder("字符串的初始值");
StringBuilder类： 用来定义可变字符串
StringBuilder sb = new StringBuilder("");
//追加字符串
sb.Append("World");
sb.Append("!");
//W2orld
sb.Insert(2, "2");
//原字符串：Wo2rld! 截取之后：W2rld!
sb.Remove(1, 2);
//ToString()
Console.WriteLine(sb.ToString());
查询学生记录数 //打开数据库连接
con.Open();
//使用StringBuilder追加SQL语句
StringBuilder sb = new StringBuilder();
sb.Append("select ");
sb.Append(" Count(*) ");
sb.Append(" from ");
sb.Append("[Student]");
Console.WriteLine(sb.ToString());
//创建一个SqlCommand对象
SqlCommand com = new SqlCommand(sb.ToString(),con);
Console.WriteLine((int)com.ExecuteScalar());
DataReader：从数据源中检索只读、只进的数据流，每次读取一行数据
StringBuilder sb = new StringBuilder();
sb.AppendLine("select");
sb.AppendLine("[StudentNo]");
sb.AppendLine(",[StudentName]");
sb.AppendLine("from");
sb.AppendLine("[Student]");
SqlCommand com = new SqlCommand(sb.ToString(), con);
//从数据源中检索只读、只进的数据流
return com.ExecuteReader();
SqlDataReader reader=GetStudentInfo();
while (reader.Read())
{
Console.WriteLine("{0}\t{1}",reader["StudentNo"],reader["StudentName"]);
}
reader.Close();
ExecuteNonQuery():
StringBuilder sb = new StringBuilder();
sb.AppendLine("Insert into");
sb.AppendLine("[Grade]([GradeName])");
sb.AppendLine("Values('" + gradeName + "')");
//3.创建一个SqlCommand
SqlCommand com = new SqlCommand(sb.ToString(),con);
//4.返回执行结果
return com.ExecuteNonQuery();
Student stu = new Student();
Console.WriteLine("请输入年级名称：");
string gradename = Console.ReadLine();
int count = stu.AddGrade(gradename);
if (count > 0)
{
Console.WriteLine("success！");
} else
{
Console.WriteLine("success mother!");
}