Question

I’m trying to learn ADO.NET from “Programmers Heaven: C# School”

Thats the code I’ve created basing on that ebook, but running it causes:

System.NullReferenceException: Object reference not set to an instance of an object.

When trying to add Parameter (cmd.Parameters.Add(“@” + col, OleDbType.Char, 0, col);) to the Command.

Could someone please point me whats wrong?

EDIT:

added the problem description

using System;
using System.Collections.Generic;
using System.Drawing;
using System.Windows.Forms;
using System.Data;
using System.Data.OleDb;

namespace AdoNET
{
    public partial class MainForm : Form
    {
    private OleDbConnection conn;
    private OleDbDataAdapter dataAdapter;

    private DataTable dataTable;
    private DataSet ds;

    private int currRec = 0;
    private int totalRec = 0;
    private bool insertSelected;

    public MainForm()
    {
        InitializeComponent();
    }

    void BtnLoadTableClick(object sender, EventArgs e)
    {
        this.Cursor = Cursors.WaitCursor;

        //create connection string
        string connectionString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=db\\ProgrammersHeaven.mdb;";

        //create connection
        conn = new OleDbConnection(connectionString);

        //define command
        string commandString = "SELECT * FROM Article, Author WHERE Author.authorId = Article.authorId";

        //create Data Adapter for communication with DB
        dataAdapter = new OleDbDataAdapter(commandString, conn);

        //create Data Set to store data offline
        ds = new DataSet ();

        //fill dataset (table prog) with data from dataAdapter
        dataAdapter.Fill(ds, "prog");

        //create and fill table dataTable
        dataTable = ds.Tables["prog"];
        currRec = 0;
        totalRec = dataTable.Rows.Count;

        FillControls();
        InitializeCommands();
        ToggleControls(true);

        btnNext.Enabled = true;
        btnPrevious.Enabled = true;

        this.Cursor = Cursors.Default;
    }

    private void FillControls()
    {
        txtArticleID.Text = dataTable.Rows[currRec]["artId"].ToString();
        txtArticleTitle.Text = dataTable.Rows[currRec]["title"].ToString();
        txtArticleTopic.Text = dataTable.Rows[currRec]["topic"].ToString();
        txtAuthorId.Text = dataTable.Rows[currRec]["Author.authorId"].ToString();
        txtAuthorName.Text = dataTable.Rows[currRec]["name"].ToString();
        txtNumOfLines.Text = dataTable.Rows[currRec]["lines"].ToString();
        txtDateOfPublishing.Text = dataTable.Rows[currRec]["dateOfPublishing"].ToString();
    }

    void BtnNextClick(object sender, EventArgs e)
    {
        currRec++;
        if (currRec>=totalRec)
        {
            currRec=0;
        }
        FillControls();
    }

    void BtnPreviousClick(object sender, EventArgs e)
    {
        currRec--;
        if (currRec<0)
        {
            currRec=totalRec-1;
        }
        FillControls();
    }

    private void InitializeCommands()
    {
        //preparing INSERT command
        dataAdapter.InsertCommand = conn.CreateCommand();
        dataAdapter.InsertCommand.CommandText =
            "INSERT INTO article " +
            "(artId, title, topic, authorId, lines, dateOfPublishing) " +
            "VALUES (@artId, @title, @topic, @authorId, @lines, @dateOfPublishing)";
        AddParams(dataAdapter.InsertCommand, "artId", "title", "topic", "lines", "dateOfPublishing");

        //preparing UPDATE command
        dataAdapter.InsertCommand = conn.CreateCommand();
        dataAdapter.InsertCommand.CommandText =
            "UPDATE article SET" +
            "title = @title, topic = @topic, lines = @lines, dateOfPublishing = @dateOfPublishing" +
            "WHERE artId = @artId";
        AddParams(dataAdapter.UpdateCommand, "artId", "title", "topic", "authorId", "lines", "dateOfPublishing");

        //prepare DELETE command
        dataAdapter.InsertCommand = conn.CreateCommand();
        dataAdapter.InsertCommand.CommandText =
            "DELETE FROM article WHERE artId = @artId";
        AddParams(dataAdapter.DeleteCommand, "artId");
    }

    private void AddParams(OleDbCommand cmd, params string[] cols)
    {
        //adding hectic (?) parameters
        foreach (string col in cols)
        {
            cmd.Parameters.Add("@" + col, OleDbType.Char, 0, col);
        }
    }

    private void ToggleControls(bool val)
    {
        txtArticleTitle.ReadOnly = val;
        txtArticleTopic.ReadOnly = val;
        txtAuthorId.ReadOnly = val;
        txtNumOfLines.ReadOnly = val;
        txtDateOfPublishing.ReadOnly = val;

        btnLoadTable.Enabled = val;
        btnNext.Enabled = val;
        btnPrevious.Enabled = val;
        btnEditRecord.Enabled = val;
        btnInsertRecord.Enabled = val;
        btnDeleteRecord.Enabled = val;

        btnSave.Enabled = !val;
        btnCancel.Enabled = !val;
    }

    void BtnEditRecordClick(object sender, EventArgs e)
    {
        ToggleControls(false);
    }

    void BtnSaveClick(object sender, EventArgs e)
    {
        lblStatus.Text = "Saving Changes...";
        this.Cursor = Cursors.WaitCursor;
        DataRow row = dataTable.Rows[currRec];
        row.BeginEdit();
        row["title"] = txtArticleTitle.Text;
        row["topic"] = txtArticleTopic.Text;
        row["Article.authorId"] = txtAuthorId.Text;
        row["lines"] = txtNumOfLines.Text;
        row["dateOfPublishing"] = txtDateOfPublishing.Text;
        row.EndEdit();
        dataAdapter.Update(ds, "prog");
        ds.AcceptChanges();

        ToggleControls(true);
        insertSelected = false;
        this.Cursor = Cursors.Default;
        lblStatus.Text = "Changes Saved";
    }


    void BtnCancelClick(object sender, EventArgs e)
    {
        if (insertSelected)
        {
            BtnDeleteRecordClick(null,null);
            insertSelected=false;
        }

        FillControls();
        ToggleControls(true);
    }

    void BtnDeleteRecordClick(object sender, EventArgs e)
    {
        DialogResult res = MessageBox.Show("Are you sure you want to delete the current record?",
                                           "Confirm Record Deletion", MessageBoxButtons.YesNo);
        if (res == DialogResult.Yes)
        {
            DataRow row = dataTable.Rows[currRec];
            row.Delete();
            dataAdapter.Update(ds, "prog");
            ds.AcceptChanges();
            lblStatus.Text = "Record Deleted";
            totalRec--;
            currRec = totalRec - 1;
            FillControls();
        }
    }

    void BtnInsertRecordClick(object sender, EventArgs e)
    {
        insertSelected = true;
        //table must be loaded to create new row
        DataRow row = dataTable.NewRow();
        dataTable.Rows.Add(row);
        totalRec = dataTable.Rows.Count;
        currRec = totalRec-1;
        row["artId"] = totalRec;

        txtArticleID.Text = totalRec.ToString();
        txtArticleTitle.Text = "";
        txtArticleTopic.Text = "";
        txtAuthorId.Text = "";
        txtNumOfLines.Text = "";
        txtDateOfPublishing.Text = DateTime.Now.Date.ToString();

        ToggleControls(false);
    }
}

}

Answer 1

In the InitializeCommands method, without creating the Update and Delete command you’re sending into the AddParams method.

Change your dataAdapter.InsertCommand to dataAdapter.UpdateCommand and dataAdapter.DeleteCommand in the corresponding places.

Side Note: If you read the error message, it is evident that you are trying to use some reference which refers to no object in the heap. In such cases, run through your code to find the guy who has no definition.