Impersonation.cs

// http://stackoverflow.com/q/31522642/1768303

using System;
using System.Diagnostics; 
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using System.Security;
using System.Security.Principal;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication
{
	class Program
	{
		static async Task TestAsync()
		{
			ShowIdentity();

			// substitute your actual test credentials
			using (ImpersonateIdentity(
				userName: "TestUser", domain: "sammy", password: "TestPassword"))
			{
				ShowIdentity();

				int threadId = -1;

				await Task.Run(() =>
				{
					ShowIdentity();
					Thread.Sleep(100);

					threadId = Environment.CurrentManagedThreadId;

					ImpersonateIdentity(userName: "TestUser1", domain: "sammy", password: "TestPassword1");

					ShowIdentity();
				}).ConfigureAwait(false);

				ShowIdentity();

				// switch to a non-pool thread
				await Task.Factory.StartNew(() => { }, TaskCreationOptions.LongRunning);
				ShowIdentity();

				GoThruThreads(threadId);
			}

			ShowIdentity();
		}

		static void GoThruThreads(int threadId)
		{
			var more = true;
			while (more)
			{
				var tcs = new TaskCompletionSource<object>();

				ThreadPool.UnsafeQueueUserWorkItem(_ => 
				{ 
					if (Environment.CurrentManagedThreadId == threadId)
					{
						more = false;
						ShowIdentity();
					}

					tcs.SetResult(Type.Missing);
				}, null);

				tcs.Task.Wait();
			}
		}

		static WindowsImpersonationContext ImpersonateIdentity(string userName, string domain, string password)
		{
			var userToken = IntPtr.Zero;
			
			var success = NativeMethods.LogonUser(
			  userName, 
			  domain, 
			  password,
			  (int)NativeMethods.LogonType.LOGON32_LOGON_INTERACTIVE,
			  (int)NativeMethods.LogonProvider.LOGON32_PROVIDER_DEFAULT,
			  out userToken);

			if (!success)
			{
				throw new SecurityException("Logon user failed");
			}
			try 
			{	        
				return WindowsIdentity.Impersonate(userToken);
			}
			finally
			{
				NativeMethods.CloseHandle(userToken);
			}
		}

		static void Main(string[] args)
		{
			TestAsync().Wait();
			Console.ReadLine();
		}

		static void ShowIdentity(
			[CallerMemberName] string callerName = "",
			[CallerLineNumber] int lineNumber = -1,
			[CallerFilePath] string filePath = "")
		{
			// format the output so I can double-click it in the Debuger output window
			Debug.WriteLine("{0}({1}): {2}", filePath, lineNumber,
				new { Environment.CurrentManagedThreadId, WindowsIdentity.GetCurrent().Name });
		}

		static class NativeMethods
		{
			public enum LogonType
			{
				LOGON32_LOGON_INTERACTIVE = 2,
				LOGON32_LOGON_NETWORK = 3,
				LOGON32_LOGON_BATCH = 4,
				LOGON32_LOGON_SERVICE = 5,
				LOGON32_LOGON_UNLOCK = 7,
				LOGON32_LOGON_NETWORK_CLEARTEXT = 8,
				LOGON32_LOGON_NEW_CREDENTIALS = 9
			};

			public enum LogonProvider
			{
				LOGON32_PROVIDER_DEFAULT = 0,
				LOGON32_PROVIDER_WINNT35 = 1,
				LOGON32_PROVIDER_WINNT40 = 2,
				LOGON32_PROVIDER_WINNT50 = 3
			};

			public enum ImpersonationLevel
			{
				SecurityAnonymous = 0,
				SecurityIdentification = 1,
				SecurityImpersonation = 2,
				SecurityDelegation = 3
			}

			[DllImport("advapi32.dll", SetLastError = true)]
			public static extern bool LogonUser(
					string lpszUsername,
					string lpszDomain,
					string lpszPassword,
					int dwLogonType,
					int dwLogonProvider,
					out IntPtr phToken);

			[DllImport("kernel32.dll", SetLastError=true)]
			public static extern bool CloseHandle(IntPtr hObject);
		}
	}
}