Invalid attempt to read when no data is present in dr

问题

I am trying to create a login form on my ASP.NET website. Currently, there's some problem. I am trying to incorporate the functionality such that the logged in user has the previlige to view only his profile. The code on the login page is this:

business.clsprofiles obj = new business.clsprofiles();
        Int32 a = obj.logincheck(TextBox3.Text, TextBox4.Text);
        if (a == -1)
        {
            Label1.Text = "Username/Password incorrect";
        }
        else
        {
            Session["cod"]= a;
            Response.Redirect("profile.aspx");
        }

After logging in, the user is moved to the page where the person can view his profile once logged in. Session is obtaining the value correctly of the logged in person from the login-page and successfully passing it on to the next page. But here on this profile page an error occurs and I think there is problem somewhere in the grid_bind() method below

public void grid_bind()
{
    business.clsprofiles obj = new business.clsprofiles();
    List<business.clsprofilesprp> objprp = new List<business.clsprofilesprp>();
    Int32 z = Convert.ToInt32(Session["cod"]);
    objprp = obj.fnd_profiles(z); //This line of code is passing an integer as required but does not get the desired result from the database
    GridView1.DataSource = objprp;
    GridView1.DataBind();
}

As the error in business logic says, " invalid attempt to read when no data is present in dr"

public List<clsprofilesprp> fnd_profiles(Int32 id)
        {
            if (con.State == ConnectionState.Closed)
            {
                con.Open();
            }
            SqlCommand cmd = new SqlCommand("fndpro", con);
            cmd.CommandType = CommandType.StoredProcedure;
            cmd.Parameters.Add("@id", SqlDbType.Int).Value = id;
            SqlDataReader dr = cmd.ExecuteReader();
            List<clsprofilesprp> obj = new List<clsprofilesprp>();
            while(dr.HasRows)
            {
                clsprofilesprp k = new clsprofilesprp();

                k.id = Convert.ToInt32(dr[0]);//Something wrong here?

                k.name = dr[1].ToString();
                k.password = dr[2].ToString();
                k.description = dr[3].ToString();
                k.created = Convert.ToDateTime(dr[4]);
                k.modified = Convert.ToDateTime(dr[5]);
                obj.Add(k);
            }
            dr.Close();
            cmd.Dispose();
            con.Close();
            return obj;
        }lesprp k = new clsprofilesprp();

        k.id = Convert.ToInt32(dr[0]);//Something wrong here?

                k.name = dr[1].ToString();
                k.password = dr[2].ToString();
                k.description = dr[3].ToString();
                k.created = Convert.ToDateTime(dr[4]);
                k.modified = Convert.ToDateTime(dr[5]);
                obj.Add(k);
            }
            dr.Close();
            cmd.Dispose();
            con.Close();
            return obj;

回答1:

You have to call DataReader.Read to fetch the result:

SqlDataReader dr = cmd.ExecuteReader();
dr.Read();
// ...

DataReader.Read returns a boolean, so if you have more than 1 result, you can do:

While (dr.Read())
{
  // read data for each record here
}

Moreover, you're trying to access the dr data when there's none in this part of the code:

k.id = Convert.ToInt32(dr[0]);//Something wrong here?
k.name = dr[1].ToString();
k.password = dr[2].ToString();
k.description = dr[3].ToString();
k.created = Convert.ToDateTime(dr[4]);
k.modified = Convert.ToDateTime(dr[5])

回答2:

You've got a problem with...

while (dr.HasRows)
{
   /* If this loop is entered, it will run 
    * indefinitely until the datareader miraculously 
    * loses all its rows in a hole somewhere */
}

This will either never enter, or will create an infinite loop... it's either got no rows or it has rows. What I think you meant was:

while (dr.Read())
{
   /* Do something with the current record */
}

dr.Read() loops to the next record and returns true or false depending on if there's a record to be read or not. When the data reader is initialized, the first record is not selected. It has to be selected by calling dr.Read() which will then return true if a first row is found, and indeed will return true until Read() is called when currently on the last row - i.e. there's no more rows left to read.

dr.HasRows is just a property that tells you if the datareader contains rows... which if it does will keep having rows from here until eternity. For instance, you would use this if you wanted to do something in the event it has rows and something else in the event it doesn't:

if (dr.HasRows)
{
    while (dr.Read())
    {
        /* Display data for current row */
    }
}
else
{
    Console.WriteLine("I didn't find any relevant data.");
}

回答3:

You are trying to access a row in the datareader though there are no rows. i.e if the dr doesn't enter the while loop then there are no rows in the datareader, however you are still accessing the fields where you have a comment "//Something wrong here? ".

来源：https://stackoverflow.com/questions/4628698/invalid-attempt-to-read-when-no-data-is-present-in-dr

标签

ASP.NET

datareader

no-data