How to match two comboboxes

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C#

WinForms

I’ve two comboboxes which should contain two different informations.
 
1.cb1: select table_name from information_schema.tables (this display multiple tables)
2.cb2: should populate it with a column name from the selectedvalue at cb1

This code below is working and populating my cb1 (combobox).

string C = ConfigurationManager.ConnectionStrings[""].ConnectionString;
        SqlConnection con = new SqlConnection(C);
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = ("SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES ORDER BY TABLE_NAME ASC");
        try
        {
            // Open connection, Save the results in the DT and execute the spProc & fill it in the DT
            con.Open();
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            dt = new DataTable();
            adapter.Fill(dt);
            cbTbl.DisplayMember = "TABLE_NAME";
            cbTbl.ValueMember = "TABLE_NAME";
            //Fill combobox with data in DT
            cbTbl.DataSource = dt;
            // Empty bzw. clear the combobox
            cbTbl.SelectedIndex = -1;

The next code is almost correct and also populating cb2 BUT with System.Data.DataView.
What's wrong?
I want to get in cb2 a specific column of the selected table in cb1

private void comboBox1_SelectedIndexValue(object sender, EventArgs e)
        {
if (cbTbl.SelectedValue != null) 
{ 
string C = ConfigurationManager.ConnectionStrings[""].ConnectionString; SqlConnection con = new SqlConnection(C); 
SqlCommand sqlCmd = new SqlCommand(); 
sqlCmd.Connection = con; 
sqlCmd.CommandText = (" SELECT [Projektdefinition] FROM " + cbTbl.SelectedValue.ToString() + ""); 
SqlDataAdapter sqlDataAdap = new SqlDataAdapter(sqlCmd);
 try 
{ 
con.Open(); 
dt = new DataTable(); // If i debug there are no records in there
sqlDataAdap.Fill(dt); 
comboBox1.DataSource = dt; 
} 
catch (Exception ex) 
{
 MessageBox.Show(ex.Message); 
} 
finally 
{ 
con.Close(); 
}
 
  }

Thnx