You can try something like:
SELECT * FROM YourTable WHERE voudate BETWEEN CONVERT(date, '1/1/2012',101) and CONVERT(date, '4/1/2012', 101)
In your program you would use SqlParameters, see
SqlParameter [
^] so your query in the C# program would look like:
SELECT * FROM YourTable WHERE voudate BETWEEN @startdate and @enddate
Notice that conversion isn't needed if parameter data types are defined correctly (date in this case)