IF条件下如何消除对空值数据表的检查

Question

In my Page I am fetching a value from the Database & filling the values in the DataTable. I am then comparing that values with the mac String in the IF. Based upon the condition in the Query there will be no records fetched, it is stuck in the IF condition and throws the No row at Position 0 Exception rather than going into the Else part.

My code is:

  string mac = GetMac();
        string Qry = "Select VUserid,Password from passtable where VUserid='" + UserName.Text + "' and Flag='A'";
        string qry = "Select VUserid,Password from passtable where Flag='A'";
        string strq = "Select Mac_id from Sysinfo Where Appflag='A'";
        using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["EvalCon"].ConnectionString))
        {
            try
            {

                SqlCommand cmd = new SqlCommand(Qry, conn);
                SqlCommand cmd1 = new SqlCommand(qry, conn);
                SqlCommand cmd2 = new SqlCommand(strq, conn);
                conn.Open();
                SqlDataAdapter da = new SqlDataAdapter(cmd);
                SqlDataAdapter daa = new SqlDataAdapter(cmd1);
                SqlDataAdapter dap = new SqlDataAdapter(cmd2);
                DataTable dt = new DataTable();
                DataTable dtt = new DataTable();
                DataTable tab = new DataTable();
                da.Fill(dt);
                daa.Fill(dtt);
                dap.Fill(tab);
                for (int i = 0; i < tab.Rows.Count; i++)
                {
                    for (int x = 0; x <= dtt.Rows.Count - 1; x++)
                    {
                        if (mac == tab.Rows[i]["Mac_id"].ToString() || tab.Rows.Count != 0)
                        {
                            if (UserName.Text == dtt.Rows[x]["VUserid"].ToString() && Password.Text == dtt.Rows[x]["Password"].ToString())
                            {
                                Response.Redirect("~/Changepass.aspx");
                                break;
                            }
                            else
                            {
                                lblMessage.Visible = true;
                                lblMessage.ForeColor = System.Drawing.Color.Red;
                                lblMessage.Text = "Invalid Username or Password !!!";

                            }
                        }
                        else
                        {
                            lblMessage.Visible = true;
                            lblMessage.ForeColor = System.Drawing.Color.Red;
                            lblMessage.Text = "Invalid Access Point for Evaluation !!!";
                        }
                    }
                }

            }
            finally
            {
                conn.Close();
                conn.Dispose();
            }
        }

Answer 1

First of all, you may want to give some more meaningful names to your variables.

On a side note, you may want to change your for loops into a foreach loop:

foreach (DataRow tabRow in tab.Rows.Count)
{
    foreach (DataRow dttRow in dtt.Rows.Count)
    {
        // logic here
        // tab.Rows[i]["Mac_id"] becomes tabRow["Mac_id"]
        // and
        // dtt.Rows[x]["VUserid"] becomes dttRow["VUserid"]
        // and so on...
    }
}

This way if there are no records fetched it won't go in.

After that, you may want to check the conditions on RowCount > 0 for the datatables before going inside the loops and act outside the loop if the RowCount is 0.

Answer 2

Just swap your OR condition in If statement:

if (tab.Rows.Count != 0 || mac == tab.Rows[i]["Mac_id"].ToString())
{
    ...
    ...
}

Answer 3

If you need to check against a null result I'd wrap my if statement in another if statement that does a simple check for a null result before doing anything else. Something like this will check if it's not null:

if(tab.rows[i]["Mac_id"] != null)
{
//logic here
}

You could add this into your current if statement check so:

if(mac == tab.Rows[i]["Mac_id"].ToString() || tab.Rows.Count != 0)

becomes:

if(mac == tab.Rows[i]["Mac_id"].ToString() || tab.Rows.Count != 0 && tab.rows[i]["Mac_id"] != null)

Though as Tallmaris says it might be better to restructure it using a foreach loop instead.