In SqlDbType enum there are constants for all data types, supported by SQL Server (if you use OleDb namespace instead of SqlClient, use OleDbType enum).

I use this for ALL fields - there are situations, in which is impossible to do this in other way. Example: you construct SQL SELECT and add to WHERE clause condition for VARCHAR field:

"... WHERE SomeField = '" & NewString & "'"

Will this work if NewString = "asdf'ghjk" ???