How I do multiple selection in combo box?

CodeProject is your friend: "CheckBox ComboBox Extending the ComboBox Class and Its Items" [^], and read the comments here: [^]

SqlCommand cmd = new SqlCommand("Select * from Add_Stock", con);
con.Open();
SqlDataAdapter sda = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
sda.Fill(ds);

for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
{
comboBox1.Items.Add(ds.Tables[0].Rows[i]["pdetails"] + " " + ds.Tables[0].Rows[i]["batchno"] + " " + Convert.ToDateTime(ds.Tables[0].Rows[i]["expdate"]).ToShortDateString() + " " + ds.Tables[0].Rows[i]["sttokk"]);

}