I'm install an c# winforms application 3.5 to some client that references a mdf file which will in SQL server express.

I installed .internet 3.5 sp1 therefore the latest .internet data companies for SQL server ought to be there?? I get a mistake for system.data and also the above dll. Do you know the dependencies shall we be held missing?

You have to install SQL Server too :) The Us dot Internet Framework 3.5 doesn't include SQL Server.

And when its a customer used application, you ought to have used a nearby database, .sdf (SQL Server CE)

.mdf (server based databases) are utilized when lots of clients have to access just one shared database.

Use a .SDF but SQL Server must be installed and when the database live in the applying folder you'll need to alter the bond string too.