Question

I have what seems to be a simple question but its killing me trying to find out.

I have a form in which I have a ListView. In this ListView I would like to populate it with data from a SQL Server 2008 database table.

public void LoadList()
{
   DataTable dtable = budget_MainDataSetReceipt.Tables["Receipt"];
   listView1.Items.Clear();

   for (int i = 0; i < dtable.Rows.Count; i++)
   {
      DataRow drow = dtable.Rows[i];

      if (drow.RowState != DataRowState.Deleted)
      {
         ListViewItem lvi = new ListViewItem(drow["ReceiptID"].ToString());
         lvi.SubItems.Add(drow["DateCleared"].ToString());
         lvi.SubItems.Add(drow["CategoryID"].ToString());
         lvi.SubItems.Add(drow["Amount"].ToString());
         lvi.SubItems.Add(drow["Store"].ToString());
         lvi.SubItems.Add(drow["DateEntered"].ToString());

         listView1.Items.Add(lvi);
      }
   }
} 

I keep getting an

Object reference not set to an instance of an object

error, and I can’t figure out why. There are about 5 rows of data in my database, so in my mind, there should be 5 rows of data within the list view.

Can anyone tell me what I am missing? I can post more code if that would be helpful.

I have tried calling the LoadList() method in several ways:

• Before the method itself
• With the InitializeComponent() method

• I have tried the following syntax

  this.LoadList(); 
  this.Form1.LoadList();`
  

I have also tried to initialize the DataTables type with the following:

DataTables dt = new DataTables //did not work

Answer 1

My hunch would be: you’re assuming for all columns in your DataRow that they’re present and not null – that’s a bit of a dangerous assumption.

I would change your assignments to use a method that checks for DBNull before returning the string:

public string SafeGetString(DataRow row, string columnName)
{
    if(row[columnName] != null && row[columnName] != DBNull.Value)
    {
        return row[ColumName].ToString();
    }

    return string.Empty;
}

so your could would look like:

ListViewItem lvi = new ListViewItem(SafeGetString(drow, "ReceiptID"));
lvi.SubItems.Add(SafeGetString(drow, "DateCleared"));
// and so forth

This way, if any of the columns should contain a NULL, you would get back an empty string – instead of running into a NULL.ToString() that causes the error you’re seeing.