a.SelectCommand.Parameters["@username"].Value = username;处提示未将对象引用设置到对象的实例。。。。。求

[dylanliu9301]

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Web.Configuration;

namespace SkySharp
{
public partial class WebLogonForm : System.Web.UI.Page
{
string constr = "Data Source=admin-pc;Initial Catalog=skyShark;Integrated Security=true";

protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
using (SqlConnection con = new SqlConnection(constr))
{
DataSet dataSet1 = new DataSet();
string username, password;
int datarows = 0;
try
{
SqlCommand cmd = new SqlCommand("select * from dtUsers" + "where Username=@username and Password=@password", con);
username = txtUserName.Text.Trim();
cmd.Parameters.Add(new SqlParameter("@username", SqlDbType.NVarChar, 15));
username = txtUserName.Text.Trim();
cmd.Parameters.Add("@password", SqlDbType.NVarChar, 15);
password =txtPassword.Text.Trim();
con.Open();
SqlDataAdapter a = new SqlDataAdapter();
a.SelectCommand.Parameters["@username"].Value = username;
a.SelectCommand = cmd;

datarows = a.Fill(dataSet1, "dtUsers");
if (datarows==0)
lblMessage.Text = "Incorrect user name";
else
{
if (dataSet1.Tables["dtUsers"].Rows[0][1].ToString().Trim() == password)
{
lblMessage.Text = "good";
}
else
lblMessage.Text = "Incorrect password";
}
}
catch (SqlException) { }
finally
{
con.Close();
}
}

}
}
}

[dylanliu9301]

嗯，谢谢。倒过来OK了，现在输入数据库中不存在用户名却没有出现预期的incorre user name，请问哪里出错了？

[蝶恋花雨]

判断if (datarows==0)

改成下面的

if(dataSet1.tables.rows.count==0)
lblMessage.Text = "Incorrect user name";

[蝶恋花雨]

dataSet1.Tables[0].Rows.Count 刚才手误。

[licai1210]

[Quote=引用 3 楼 的回复:]
嗯，谢谢。倒过来OK了，现在输入数据库中不存在用户名却没有出现预期的incorre user name，请问哪里出错了？
[/Quote]

            datarows = a.Fill(dataSet1, "dtUsers");
            datarows = dataSet1.Tables["dtUsers"].Rows.Count;

多看看基础

[蝶恋花雨]

也改成下面
SqlDataAdapter a = new SqlDataAdapter();
a.SelectCommand = cmd;
a.SelectCommand.Parameters["@username"].Value = username;
a.Fill(dataSet1, "dtUsers");
datarows = dataSet1.Tables[0].Rows.Count;
if (datarows==0)
lblMessage.Text = "Incorrect user name";

[dylanliu9301]

求解还是不行？？
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;
using System.Web.Configuration;

namespace SkySharp
{
public partial class WebLogonForm : System.Web.UI.Page
{
string constr = "Data Source=admin-pc;Initial Catalog=skyShark;Integrated Security=true";

protected void Page_Load(object sender, EventArgs e)
{
}
protected void Button1_Click(object sender, EventArgs e)
{
using (SqlConnection con = new SqlConnection(constr))
{
try
{

DataSet dataSet1 = new DataSet();
string username, password;
int datarows;
con.Open();
SqlCommand cmd = new SqlCommand("select * from dtUsers" + "where Username=@username and Password=@password", con);



cmd.Parameters.Add(new SqlParameter("@username", SqlDbType.NVarChar, 15));
username = txtUserName.Text.Trim();
cmd.Parameters.Add("@password", SqlDbType.NVarChar, 15);
password = txtPassword.Text.Trim();
SqlDataAdapter a = new SqlDataAdapter();
a.SelectCommand = cmd;
a.SelectCommand.Parameters["@username"].Value = username;
a.Fill(dataSet1, "dtUsers");
datarows = dataSet1.Tables["dtUsers"].Rows.Count;
if (datarows== 0)
lblMessage.Text = "Incorrect user name";
else
{
if (dataSet1.Tables["dtUsers"].Rows[0][1].ToString().Trim() == password)
{
lblMessage.Text = "good";
}
else
lblMessage.Text = "Incorrect password";
}
}
catch (SqlException) { }
finally
{
con.Close();
}
}

}
}
}

[dylanliu9301]

[Quote=引用 7 楼 的回复:]

也改成下面
SqlDataAdapter a = new SqlDataAdapter();
a.SelectCommand = cmd;
a.SelectCommand.Parameters["@username"].Value = username;
a.Fill(dataSet1, "dtUsers");
datarows = dataSet1.Tables[0].Row……
[/Quote]
嗯嗯，但还是没有incorrect user name

[licai1210]

    /// <summary>
    /// 检测用户是否存在
    /// </summary>
    /// <param name="userName">登录帐号</param>
    /// <param name="userPwd">登录密码</param>
    /// <returns>如果存在返回true,否则返回false</returns>
    public bool CheckLogin(string userName, string userPwd)
    {
        string constr = "Data Source=admin-pc;Initial Catalog=skyShark;Integrated Security=true";
        string sql = "select count(*) from dtUsers" + "where Username=@username and Password=@password";
        SqlConnection con = new SqlConnection(constr);
        SqlCommand cmd = new SqlCommand(sql, con);
        cmd.Parameters["@username"].Value = userName;
        cmd.Parameters["@password"].Value = userPwd;
        con.Open();
        int result = Convert.ToInt32(cmd.ExecuteScalar());
        con.Close();
        return (result > 0);
    }

//看看这个可以吗

[licai1210]

    /// <summary>
    /// 检测用户是否存在
    /// </summary>
    /// <param name="userName">登录帐号</param>
    /// <param name="userPwd">登录密码</param>
    /// <returns>如果存在返回true,否则返回false</returns>
    public bool CheckLogin(string userName, string userPwd)
    {
        string constr = "Data Source=admin-pc;Initial Catalog=skyShark;Integrated Security=true";
        string sql = "select count(*) from dtUsers where Username=@username and Password=@password";
        SqlConnection con = new SqlConnection(constr);
        SqlCommand cmd = new SqlCommand(sql, con);
        cmd.Parameters["@username"].Value = userName;
        cmd.Parameters["@password"].Value = userPwd;
        con.Open();
        int result = Convert.ToInt32(cmd.ExecuteScalar());
        con.Close();
        return (result > 0);
    }

[dylanliu9301]

不行啊，我直接把if (datarows== 0)
lblMessage.Text = "Incorrect user name";
改为
lblMessage.Text = "Incorrect user name"；
if (datarows== 0)
；单击button还是没有"Incorrect user name"
但是我
放到protected void Button1_Click(object sender, EventArgs e)
{lblMessage.Text = "Incorrect user name"；
单击button就OK！为什么？？？？？？？？？

[dylanliu9301]

[Quote=引用 11 楼 的回复:]

C# code
/// <summary>
/// 检测用户是否存在
/// </summary>
/// <param name="userName">登录帐号</param>
/// <param name="userPwd">登录密码</param>
/// <returns>如果存在返回true,否则返回false</returns>
……
[/Quote]尝试了，还是不行

[dylanliu9301]

问题查明了原来是try的原因

[licai1210]

a.SelectCommand.Parameters["@username"].Value = username;
a.SelectCommand = cmd;换成cmd.Parameters["@username"].Value = username;

[licai1210]

 protected void Button1_Click(object sender, EventArgs e)
  {
  using (SqlConnection con = new SqlConnection(constr))
  {
  DataSet dataSet1 = new DataSet();
  string username, password;
  int datarows = 0;
  try
  {
  SqlCommand cmd = new SqlCommand("select * from dtUsers" + "where Username=@username and Password=@password", con);
  username = txtUserName.Text.Trim();
  cmd.Parameters.Add(new SqlParameter("@username", SqlDbType.NVarChar, 15));
  username = txtUserName.Text.Trim();
  cmd.Parameters.Add("@password", SqlDbType.NVarChar, 15);
  password =txtPassword.Text.Trim();
  con.Open();
  SqlDataAdapter a = new SqlDataAdapter();
//次序换下就正确了
a.SelectCommand = cmd;

  a.SelectCommand.Parameters["@username"].Value = username;
  
  datarows = a.Fill(dataSet1, "dtUsers");
  if (datarows==0)
  lblMessage.Text = "Incorrect user name";
  else
  {
  if (dataSet1.Tables["dtUsers"].Rows[0][1].ToString().Trim() == password)
  {
  lblMessage.Text = "good";
  }
  else
  lblMessage.Text = "Incorrect password";
  }
  }
  catch (SqlException) { }
  finally
  {
  con.Close();
  }
  }

  }
  }
}