Hello
The SelectedValue property select the value of the CatID field.
Change your code for this:
Dropdownlist.SelectedValue = CatID;
If you want to use the CatName field to find your record in the control, use this code:
Dropdownlist.SelectedIndex = Dropdownlist.Items.IndexOf(Dropdownlist.Items.FindByText(CatName));
Now his final code would look like this:
Dropdownlist.DataSource = reader;
Dropdownlist.DataValueField = "CatID";
Dropdownlist.DataTextField = "CatName";
Dropdownlist.DataBind();
Dropdownlist.SelectedValue = CatID;
or
Dropdownlist.SelectedIndex = Dropdownlist.Items.IndexOf(Dropdownlist.Items.FindByText(CatName));
I hope I have helped you.