insert item in combobox after binding it from a Dataset in c#

You have to Insert to the object you are databinding to rather than to the combobox. You can't insert directly into the combobox.

You can use this:

DataTable dt = new DataTable();

dt.Columns.Add("ID", typeof(int));
dt.Columns.Add("CategoryName");

DataRow dr = dt.NewRow();
dr["CategoryName"] = "Select";
dr["ID"] = 0;

dt.Rows.InsertAt(dr, 0);

cmbCategory.DisplayMember = "CategoryName";
cmbCategory.ValueMember = "ID";
cmbCategory.DataSource = dt;
cmbCategory.SelectedIndex = 0;

This is very straight forward example.

You cannot add items to a ComboBox after binding it to a data source. To add or remove items from a ComboBox with a bound data source, you have to do it through data source itself.

You can insert a DataRow into your table and it will be automatically added to your ComboBox. Try the following:

 DataRow dr = dsCat.Tables[0].NewRow();
 dr["CategoryName"] = "Select";
 dr["ID"] = 123;// Some ID
 dsCat.Tables[0].Rows.Add(dr);