Try this.
void List(){
SQLConnection.MsSql con = new SQLConnection.MsSql();
System.Data.SqlClient.SqlConnection objConnd = new System.Data.SqlClient.SqlConnection();
System.Data.SqlClient.SqlDataAdapter dtAdapterd = new System.Data.SqlClient.SqlDataAdapter();
DataTable dtd = new DataTable();
String strConnString = con.conn;
objConnd = new System.Data.SqlClient.SqlConnection(strConnString);
objConnd.Open();
String strSQL;
strSQL = "SELECT Name FROM User";
dtAdapterd = new System.Data.SqlClient.SqlDataAdapter(strSQL, objConnd);
dtAdapterd.Fill(dtd);
dtAdapterd = null;
objConnd.Close();
objConnd = null;
this.comboboxFilter.DataSource = dtd;
this.comboboxFilter.ValueMember = "Name";
this.comboboxFilter.DisplayMember = "Name";
this.comboboxFilter.SelectedIndex = -1;
}