error in odbc data provider

Question

Hi..

I am working on desktop application with sql server 2000 database i am using ADODB for database connection.
but when i run project i am getting following error.
System.NullReferenceException was unhandled
Message: Object reference not set to an instance of an object.

code is bellow
        this.combo1.Items.Clear();
            this.combo1.Text = "Select";
            if (!this.optinvoice.Checked)
                return;
            this.gbdate.Visible = false;
            this.gbname.Visible = true;
            this.gbname.Text = "By Invoice#";
           
            this.adodc1.CommandType = CommandTypeEnum.adCmdText;
            this.adodc1.RecordSource = "select serial from purchase_invoice";
            this.adodc1.Refresh();
            if (this.adodc1.Recordset.RecordCount <= 0)
                return;
            this.adodc1.Recordset.MoveFirst();
            for (this.cnt = 1; this.cnt <= this.adodc1.Recordset.RecordCount; ++this.cnt)
            {
                this.combo1.Items.Add(this.adodc1.Recordset.get_Collect((object)0));
                this.adodc1.Recordset.MoveNext();
            }
while same code in framework 1.1 working nice..