combobox depend another combobox problem

Question

I wanna select from first combobox id of a train and in the second combobox give me the wagon that train has (number of a wagon ) I did a database: that first table stored the train id and the second table stored wagon id + id train foreign key i tried this code and its work but when i choose another id it doesn't change the wagon combobox result   What I have tried: adapter =  new  SqlDataAdapter( "select * from train" , cn);   SqlCommand cmd =  new  SqlCommand( "select * from train" , cn);   cmd.Parameters.Clear();   adapter.Fill(ds,  "train" );   comboBox1.DataSource = ds.Tables[ "train" ];   comboBox1.DisplayMember =  "id" ;   comboBox1.ValueMember =  "id" ;   adapter =  new  SqlDataAdapter( "select id_w from wagon where id='"  + comboBox1.SelectedValue +  "'" , cn);   adapter.Fill(ds,  "wagon" );   // adapter1.SelectCommand.Parameters.Clear();    comboBox2.DataSource = ds.Tables[ "wagon" ];   comboBox2.DisplayMember =  "id_w" ;   comboBox2.ValueMember =  "id_w" ;

Amit Gupta · Answer

Hey   You are just filling​ the combobox1 and not selected any value from it and on the next line you are binding it with combobox1.SelectedValue (which is null)   ....   comboBox1.DataSource = ds.Tables[ "train" ];   comboBox1.DisplayMember =  "id" ;   comboBox1.ValueMember =  "id" ;      if (combobox1.Items.Count>0)        combobox1.SelectedIndex =0;       adapter =  new  SqlDataAdapter( "select id_w from wagon where id='"  + comboBox1.SelectedValue +  "'" , cn);   ....