First of all, do not concatenate data from controls to the SQL query. This leaves you open to
SQL injection - Wikipedia[
^] . The proper way is to use
SqlParameter Class (System.Data.SqlClient)[
^]
What comes to your query, I don't quite understand why you want to fetch the data you already have in the query below
SqlDataAdapter da = new SqlDataAdapter("select album_name from tb_Gallery where album_name like '" + DropDownList1.Text + "'", con);
But if the idea is to use LIKE comparison, you probably want to have a wildcard somewhere. With parameters, something like
SqlDataAdapter da = new SqlDataAdapter("select album_name from tb_Gallery where album_name like @albumname + '%'", con);
For more information, see
LIKE (Transact-SQL) | Microsoft Docs[
^]
ADDITION
--------
To use a parameter you need to define it and provide a value for it. For example
...
SqlDataAdapter da = new SqlDataAdapter("select album_name from tb_Gallery where album_name like @albumname + '%'", con);
da.SelectCommand.Parameters.Add(
"@albumname ", SqlDbType.VarChar, 100).Value = DropDownList1.Text;
....