C#

dt.DefaultView.RowFilter = "area=" + areaname ;
DropDownList2.DataSource = dt.DefaultView ;

            System.Data.DataRow dr ;

// Define a State table
            System.Data.DataTable state = new System.Data.DataTable("State");
            state.Columns.Add(new System.Data.DataColumn("ID",typeof(int)));
            state.Columns.Add(new System.Data.DataColumn("Name",typeof(string)));

// Add some rows
            dr = state.NewRow() ;
            dr [ "ID" ] = 1 ;
            dr [ "Name" ] = "Arizona" ;
            state.Rows.Add ( dr ) ;

            dr = state.NewRow() ;
            dr [ "ID" ] = 2 ;
            dr [ "Name" ] = "California" ;
            state.Rows.Add ( dr ) ;

// Set the first ComboBox to display the table
            this.comboBox1.DataSource = state.DefaultView ;
            this.comboBox1.DisplayMember = "Name" ;

// Define a City table
            System.Data.DataTable city = new System.Data.DataTable("State");
            city.Columns.Add(new System.Data.DataColumn("ID",typeof(int)));
            city.Columns.Add(new System.Data.DataColumn("State",typeof(int)));
            city.Columns.Add(new System.Data.DataColumn("Name",typeof(string)));

// Add some rows
            dr = city.NewRow() ;
            dr [ "ID" ] = 1 ;
            dr [ "State" ] = 1 ;
            dr [ "Name" ] = "Phoenix" ;
            city.Rows.Add ( dr ) ;

            dr = city.NewRow() ;
            dr [ "ID" ] = 2 ;
            dr [ "State" ] = 1 ;
            dr [ "Name" ] = "Tucson" ;
            city.Rows.Add ( dr ) ;

            dr = city.NewRow() ;
            dr [ "ID" ] = 3 ;
            dr [ "State" ] = 2 ;
            dr [ "Name" ] = "San Diego" ;
            city.Rows.Add ( dr ) ;

            dr = city.NewRow() ;
            dr [ "ID" ] = 4 ;
            dr [ "State" ] = 2 ;
            dr [ "Name" ] = "Los Angeles" ;
            city.Rows.Add ( dr ) ;

// Set the second ComboBox to display the table
            this.comboBox2.DataSource = city.DefaultView ;
            this.comboBox2.DisplayMember = "Name" ;

// Add a SelectedIndexChanged handler
            this.comboBox1.SelectedIndexChanged += delegate ( object sender , System.EventArgs e )
            {
                System.Data.DataRowView item = this.comboBox1.SelectedItem as System.Data.DataRowView ;

                city.DefaultView.RowFilter = "State=" + item [ "ID" ] ;

                return ;
            } ;

// Select the first State
            this.comboBox1.SelectedIndex = 0 ;

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        DropDownList2.Items.Clear();
        string areaName = DropDownList1.Items[DropDownList1.SelectedIndex].ToString().Trim();
        DataRow[] city = dt.Select("area='" + areaName + "'");
        for (int i = 0; i < city.Count<DataRow>(); i++)
        {
            DropDownList2.Items.Add(city[i]["city_name"].ToString());
        }
    }