How to select distinct year from a datetime column and add the result to a comboBox in C#?

You're not selecting tdate but you select Year(tdate)

I would modify the query to this:

string sql = "SELECT DISTINCT Year(tdate) AS tdate_year FROM saletransaction ORDER BY Year(tdate) DESC";

and access it with dr["tdate_year"]

You have missed to give the column name in sql query

try this

string sql = "SELECT DISTINCT Year(tdate) AS tdate FROM saletransaction ORDER BY Year(tdate) DESC";

Looks like you have not given an alias to your tdate query. Therefore when you try and reference tdate, the column does not exist and Visual Studio throws the error.

Change the query to:

 string sql = "SELECT DISTINCT Year(tdate) AS tdate FROM saletransaction ORDER BY Year(tdate) DESC";

Which will return all your results under the column name tdate.