Question

C#登陆失败，未处理FormatException，输入字符串的格式不正确。

错误代码： int state = Convert.ToInt32(cmd.ExecuteScalar());

登陆代码
string strConnection = "Provider=Microsoft.Jet.OleDb.4.0;";

strConnection += @"Data Source=C:\Users\thtf\Desktop\新建文件夹\管理系统\管理系统\Database\xiangmu.mdb"; // 定义连接

OleDbConnection objConnection = new OleDbConnection(strConnection);

objConnection.Open();
string sql = "select * from login_user where username='" + this.txtusername.Text + "' and password='" + this.txtpassword.Text + "'";
OleDbCommand cmd = new OleDbCommand(sql, objConnection);
int state = Convert.ToInt32(cmd.ExecuteScalar());//执行sql语句，并返回获得值

if (state == 0)//如果数据中没有记录或有多条记录则抱错
{

MessageBox.Show("用户不存在，请检测用户名和密码是否正确！");

}
else
{
main2Form main2 = new main2Form();
main2.Show();
this.Hide();
}
objConnection.Close();

Answer 1

ExecuteScalar 方法 返回查询的值为第一行第一个单元格的值。。。
按照你的SQL来看，你是在查询是否存在录入账号密码对应的记录，所以应该用聚合函数COUNT来解决。
把SQL语句
select * from login_user where username='" + this.txtusername.Text + "' and password='" + this.txtpassword.Text + "'"
改成
select count（*） from login_user where username='" + this.txtusername.Text + "' and password='" + this.txtpassword.Text + "'"

Answer 2

把你的sql语句select * from login_user where username='" + this.txtusername.Text + "' and password='" + this.txtpassword.Text + "'"改成select count（*） from login_user where username='" + this.txtusername.Text + "' and password='" + this.txtpassword.Text + "'"

Answer 3

string sql =string.Format("select * from login_user where username='{0}' and password='{1}'", this.txtusername.Text , this.txtpassword.Text );

Answer 4

int state = Convert.ToInt32(cmd.ExecuteScalar().tostring());//执行sql语句，并返回获得值