The project I have been working on this week to test computer environments needs to do various actions as a user other than the one running the application. For instance, it looks up an installed Windows Service, finds out who the startup user is, and tries to connect to a database as that Windows user. Later on, it will need to access a file in the context of the currently logged-in user. With ASP .NET, this is super-easy: just go into Web.Config and set up the "identity impersonate" node, which can either impersonate a named user or the one who had logged into the website if authentication was enabled. With Windows applications, this is not so straightforward. There may be something I am overlooking, but the limitation seems to be that you can only change the security context on the current thread: any threads spawned by the impersonated thread also inherit the impersonated credentials. Impersonation is easy enough to do, once you figure out how. Here is my code for impersonating a user on the current thread:

1.         using System;
2.         using System.ComponentModel;
3.         using System.Runtime.InteropServices;
4.         using System.Security.Principal;
5.         public class ImpersonateUser
6.         {
7.                 IntPtr userHandle;
8.   [DllImport("advapi32.dll", SetLastError = true)]
9.                 static extern bool LogonUser(
10.                         string lpszUsername,
11.                         string lpszDomain,
12.                         string lpszPassword,
13.                         LogonType dwLogonType,
14.                         LogonProvider dwLogonProvider,
15.                         out IntPtr phToken
16.                         );
17.  
18.  
19.                 [DllImport("kernel32.dll", SetLastError = true)]
20.                 static extern bool CloseHandle(IntPtr hHandle);
21.  
22.  
23.                 enum LogonType : int
24.                 {
25.                         Interactive = 2,
26.                         Network = 3,
27.                         Batch = 4,
28.                         Service = 5,
29.                         NetworkCleartext = 8,
30.                         NewCredentials = 9,
31.                 }
32.  
33.  
34.                 enum LogonProvider : int
35.                 {
36.                         Default = 0,
37.                 }
38.                 public static WindowsImpersonationContext Impersonate(string user, string domain, string password)
39.                 {
40.   IntPtr userHandle = IntPtr.Zero;
41.                         bool loggedOn = LogonUser(
42.                                 user,
43.                                 domain,
44.                                 password,
45.                                 LogonType.Interactive,
46.                                 LogonProvider.Default,
47.                                 out userHandle);
48.  
49.  
50.  
51.                         if (!loggedOn)
52.                         throw new Win32Exception(Marshal.GetLastWin32Error());
53.  
54.                         WindowsIdentity identity = new WindowsIdentity(userHandle);
55.                         WindowsPrincipal principal = new WindowsPrincipal(identity);
56.                         System.Threading.Thread.CurrentPrincipal = principal;
57.                         return identity.Impersonate();
58.   }
59.         }
60.  
61. /* Call impersonation */
62. ImpersonateUser.Impersonate("UserName","DomainName","Password");
63. /* When you want to go back to the original user */
64. WindowsIdentity.Impersonate(IntPtr.Zero);

When you want to stop impersonating, you can call Impersonate() again with a null pointer. This will allow you to simulate a variety of different Windows users from the same applicaiton.