Form loading take time

I think it depends upon your Query Execution time, that depends on both Visual Studio and Sql Server. And Also Sometimes it may depend on your processor speed, RAM etc.

But i Suggest you to use DataReader instead of DataTable. It will be Speed enough.

SqlConnection con = SqlConnection("Your Connection String");
con.Open();
SqlCommand com = new SqlCommand("select invoiceno from Only_invoices order by invoiceno asc");
Com.Connection = con;
SqlDataReader reader = cmd.ExecuteReader();

While( reader.Read())
{
  this.cmbinvoice.Items.Add(reader[0].ToString());
}

Thank You

@ Sam Hobbs...

sir there is only following code on page load..
adpt = new SqlDataAdapter("select invoiceno from Only_invoices order by invoiceno asc", conn);
dt = new DataTable();
adpt.Fill(dt);
if (dt.Rows.Count > 0)
{
 cmbinvoice.BeginUpdate();
  foreach (DataRow row in dt.Rows)
  {
  this.cmbinvoice.Items.Add(row["invoiceno"].ToString());
  }
  cmbinvoice.EndUpdate();
}

You need to use the BeginUpdate and EndUpdate methods. That will greatly improve performance and speed.

http://msdn.microsoft.com/en-us/library/system.windows.forms.combobox.beginupdate.aspx

@Mamta M...

yes there are thousands invoice to display in combobox..
..

Try to use background worker to load the data from the invoices from database.

1)Are there many invoices that you are retrieving? How many?
2) Find out ways to speed up your data retrieval operation.
3) Find out if you can place the code to get the data in any other method, instead of Load.