C# :: Link Combobox With SQL Column

When I try to link combobox with sql column i get error !!

void comfunction() {
string constring = "Data Source=LC-VAIO\SQLEXPRESS;Initial Catalog=sample1;Integrated Security=True";
string query = "select * from tbltest";
SqlConnection cn = new SqlConnection(constring);
SqlCommand cmd = new SqlCommand(query,cn);
SqlDataReader dreader;

[Code] ....

I got error from "string sname = dreader.GetString("name"); "

And this how i try to link it...