It is simple way to bind the combobox to textbox try the code
this. combTest.DataSource = this.customerDS.Tables["CustomerDB"];
this. combTest.DisplayMember = "Cust_Code";
this. combTest.ValueMember = "Cust_Code";
this.txtBox.DataBindings.Add("Text", this. customerDS, "CustomerDB.Cust_Name");
In same way binding the other text box to other related fields.