C#

public class Units
        {
            public string Unit { get; set; }

        }


        private void column1DataGridViewTextBoxColumn_SelectedIndexChanged(object sender, DataGridViewCellEventArgs e)
        {

            string strPrimary = column1DataGridViewTextBoxColumn.ToString();
            Debug.WriteLine(strPrimary);
            if (strPrimary == "Test1")
            {

                //Build a list 
                var dataSource = new List<Units>();
                dataSource.Add(new Units() { Unit = "blah1" });
                dataSource.Add(new Units() { Unit = "blah2" });
                dataSource.Add(new Units() { Unit = "blah3" });

                //Setup data binding 
                this.column2DataGridViewTextBoxColumn.DataSource = dataSource;
                this.column2DataGridViewTextBoxColumn.DisplayMember = "Unit";
                
            }
        }