If you are looking for exact match then the code which you have written will work:
adp = new SqlDataAdapter("Select * from Prod_Tbl where ProductName IN (" + filter + ")", con);
But if the values which you are search are not exact values then you have to use the like operator.
something like
foreach (string name in year)
{
if (filter.Length > 0)
{
filter += "AND ";
}
filter += string.Format("ProductName LIKE '{0}'", name);}
Another option:
http://www.eggheadcafe.com/software/aspnet/29398160/combine-in--like.aspx[
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