how to sort data before dropdownlist.databind()

Change your code. Write:

for (int i = 0; i < ds1.Tables[0].Rows.Count; i++)
{
adds1.Add(ds1.Tables[0].Rows[i].ItemArray[0].ToString().ToLower());
name1 = ds1.Tables[0].Rows[i]["block_name"].ToString().ToLower();
}
adds1.Sort((x, y) => string.Compare(x, y));
DropDownList7.DataSource = adds1;
DropDownList7.DataBind();

ListItem li = new ListItem();
li.Text = "Select";
li.Value = "0";
DropDownList7.Items.Insert(0,li);
DropDownList7.SelectedIndex = 0;

This way you are adding all the sorted items to your dropdownlist and then inserting a new item "Select" at first position.

Try:

for (int i = 0; i < ds1.Tables[0].Rows.Count; i++)
{
    adds1.Add(ds1.Tables[0].Rows[i].ItemArray[0].ToString().ToLower());
    name1 = ds1.Tables[0].Rows[i]["block_name"].ToString().ToLower();
}
adds1.Sort((x, y) => string.Compare(x, y));
adds1.Insert(0, "SELECT");
DropDownList7.DataSource = adds1;
DropDownList7.DataBind();

Refer: MSDN: List<t>.Insert Method[^]

You need to add the select after you sort.Assuming adds1 is a List of some kind:

for (int i = 0; i < ds1.Tables[0].Rows.Count; i++)
{
    adds1.Add(ds1.Tables[0].Rows[i].ItemArray[0].ToString().ToLower());
    name1 = ds1.Tables[0].Rows[i]["block_name"].ToString().ToLower();
}
adds1.Sort((x, y) => string.Compare(x, y));
adds1.Insert(0, "SELECT");
DropDownList7.DataSource = adds1;
DropDownList7.DataBind();

Note that you need to use Insert rather than Add to specify where you want "Select" to go. If add1 is an array or collection that doesn't support Insert, the problem is very slightly harder. In this case you should copy the items from the dataset into a new variable (e.g. tempAdds), sort tempAdds
add "Select" to adds1, then copy the order values from tempAdds.