Check if user exist

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Hi guys now am trying to check if the user exist in a Database and yes the code Behind works. The problems comes when Im using a Class, I get the following error: "Object reference not set to an instance of an object."
And it highlights this line in the code: "reader = dbCmd.ExecuteReader();"

Below is the code for my Class

C#

public class Class1
{
    OleDbConnection dbConn;
    DataSet ds;
    OleDbDataAdapter dbAdapter;
    OleDbCommand dbCmd;
    DataTable dataTable;
    OleDbDataReader reader;
    public void dbConnection()
    {
        string connString = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source= H:/SCHOOL OF ICT/ITP 2000/Semester 2/Database System/App_Data/dbICTWorxScheduling.mdb ";
        try
        {
            // set up and open connection
            dbConn = new OleDbConnection(connString);
            dbConn.Open();
        }
        catch (OleDbException exp)
        {
            exp.ToString();
        }
       
    }
	public Class1()
	{
		//
		// TODO: Add constructor logic here
		//
	}
    public DataTable CognosGrid()
    {
       
        dbConnection();
        string sql = "SELECT * FROM ICTWorxEducationMembers";
        dbAdapter = new OleDbDataAdapter();
        dbCmd = new OleDbCommand(sql, dbConn);
        dbAdapter.SelectCommand = dbCmd;
        reader = dbCmd.ExecuteReader();
        dataTable = new DataTable();
        reader.Close();
        dbConn.Close();
        dbAdapter.Fill(dataTable);
        
        return dataTable;
        
    }
}

This is the code behind the button

C#

protected void btnTest_Click(object sender, EventArgs e)
    {
       

        bool found = false;

        DataTable dataTable = myClass.CognosGrid();
         reader = dbCmd.ExecuteReader();

        while (reader.Read())
        {
   if (txtusername.Text == reader.GetString(0) &&     
        txtpassword.Text == reader.GetString(1))
            {
         MessageBox.Show("Welldone", " Found");

                Response.Redirect("Default.aspx");
                found = true;
            }

        }
        if (!found)
        {

            MessageBox.Show("The account does not exist. Please Register first", "Not Found");

        }

    }
}

Edit TR : Added <pre> block to clean up first code snippet.

Posted 21-Sep-10 0:19am

Anele Ngqandu

Updated 21-Sep-10 2:49am

Tony Richards

v2

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4 solutions

Solution 2

yes, the error line "Object reference not set to an instance of an object" suggests that objects are not instantiated, before they are being used. You can try two things here. Either make your method "CognosGrid()" static so that you can call it directly :

1. public static DataTable CognosGrid()
{
//your code
}

or make an object of the class in which the method resides and then use that object to call that particular method :

C#

protected void btnTest_Click(object sender, EventArgs e)
    {

        bool found = false;

          Class1 objClass= new Class1();                 //<---
        DataTable dataTable = objClass.CognosGrid();     //<---
         reader = dbCmd.ExecuteReader();
        while (reader.Read())
        {
   if (txtusername.Text == reader.GetString(0) &&
        txtpassword.Text == reader.GetString(1))
            {
         MessageBox.Show("Welldone", " Found");
                Response.Redirect("Default.aspx");
                found = true;
            }
        }
        if (!found)
        {
            MessageBox.Show("The account does not exist. Please Register first", "Not Found");
        }
    }

}

Hope, this would solve your problem. Please provide vote if this was useful.

Anurag
@cheers@

Posted 21-Sep-10 2:44am

@nuraGGupta@

Solution 4

Make sure that before clicking on your Test button your dbCmd object is created and it is not null , so write db connectivity code in !ISPOSTBACK

Posted 22-Sep-10 0:16am

unikapil

Solution 1

Many Mistakes in the Class.

C#

catch (OleDbException exp)
{
exp.ToString();
}

Use Alert or Lable Text to Display the Catched Message,

MIDL

reader = dbCmd.ExecuteReader();
dataTable = new DataTable();
reader.Close();
dbConn.Close();
dbAdapter.Fill(dataTable);

Close the connection after dbAdapter Fills Data to DataTable.

C#

reader = dbCmd.ExecuteReader();

No Need of this if you are Filling Data to Datatable using Adapter.

and Finally your Error Object reference not set to an instance of an object.

That is because Some of the Objects are not initialized and you are attempting to use that object.

Hope it Helps,

Mark as answer if Liked.

Posted 21-Sep-10 1:42am

Hiren solanki

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kaushal s · Accepted Answer · 2010-09-21T23:52:00

hey buddy , check this code . ur problem is solved. still if u find any prob let me know. and dont forget to vote if my answer is corret ;)

OleDbConnection dbConn;
       DataSet ds;
       OleDbDataAdapter dbAdapter;
       OleDbCommand dbCmd;
       DataTable dataTable;
       OleDbDataReader reader;
       public void dbConnection()
   {
       string connString = "Provider=Microsoft.Jet.OLEDB.4.0;" + "Data Source= H:/SCHOOL OF ICT/ITP 2000/Semester 2/Database System/App_Data/dbICTWorxScheduling.mdb ";
       try
       {
           // set up and open connection
           dbConn = new OleDbConnection(connString);
           dbConn.Open();
       }
       catch (OleDbException exp)
       {
           exp.ToString();
       }

   }

       public DataTable CognosGrid()
   {

       dbConnection();
       dataTable = new DataTable();

       string sql = "SELECT * FROM ICTWorxEducationMembers";
       dbAdapter = new OleDbDataAdapter();
       dbCmd = new OleDbCommand(sql, dbConn);
       dbAdapter.SelectCommand = dbCmd;

       dbAdapter.Fill(dataTable);

       dbConn.Close();
       return dataTable;

   }

protected void btnTest_Click(object sender, EventArgs e)
    {


        bool found = false;

        DataTable dataTable = myClass.CognosGrid();

        foreach (DataRow dr in dataTable.Rows)
        {
            if (txtusername.Text == dr["ColumnName"].ToString() && txtpassword.Text == dr["ColumnPassword"].ToString())
            {
                MessageBox.Show("Welldone", " Found");
                found = true;
                Response.Redirect("Default.aspx");
                
            }

        }

        if (!found)
            MessageBox.Show("The account does not exist. Please Register first", "Not Found");
        
    }