I have Winforms application written in C# which connects to a SQL Server Database. When I connect to SQL Server I build up a connection string which can contain either SQL Server login details or use Windows Authentication, in which case I omit the user name and password and use the
Connection strings have nothing to do with this.
a user requested that they have the option to connect to MS SQL Server using an Active Directory Service Account as opposed to a Network User account
What that means is that the user has requested your application to run as a service account, not as the currently logged in user. One easy way to do it is to simply start the application under
runas /netonly /user:domain\serviceaccount MyWinFormsApp.exe
This way your application runs as a domain service account on the network and it will connect to the SQL Server using the
domain\serviceaccount credentials. This would satisfy your client's requirement, at least on a shallow surface cursory look.
If the solution of using runas is not satisfactory (client may legitimatly complain that it requires users that start the applciation to know the domain\serviceaccount password) then things get a bit more complicated. The correct way of doing it is o split your application in two, an UI presentation layer .exe application that runs under the logged in user credentials and a business logic layer component that runs, as a service, under the domain\serviceaccount credentials. The two components communicate using your IPC of choice (usually WCF). As you probably realize, this require s major rewrite of your application.
Some may suggest to use have your application impersonate the domain\serviceaccount before opening the connections to the database. I would strongly discourage that because of the serviceaccount password storage/retrieveal mess. Since the app will be required to know the serviceaccount password in order to impersonate it, the user logged on running the application will either know that password or easily be able to find it (there is no way to prevent him form find it if the application can find it). Since the domainservice password is accessible to the logged in user anyway, he may just use the
runas /netonly solution. And that finally explain why the
runas solution is just a shallow smoke and mirrors solution: the only reason your client may had requested what he requested is that he wants to separate the privileges of the users logged in from the privileges of the application (ie. don't give SQL Servera ccess to every employee). Since the
runas solution (as well as impersonating in the app) require the logged in user to know the service account password, the separation of privileges does not really occur since hte logged in user can use any time it wishes the service account password and elevate his privileges to access the SQL Server database at will. Therefore the only solution worth talking about is the separation of the application in two parts.