Recently I again stumbled into a situation where I just wanted to test, if a certain portion of my code would also work, if it ran under different Windows user accounts. But instead of switching users by hand, I wanted to impersonate them in my code. How to do this in Managed Code (e.g. C#), is described in quite a few online sources. Here are two examples:

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/frlrfSystemSecurityPrincipalWindowsIdentityClassImpersonateTopic.asp

http://www.codeproject.com/csharp/lsadotnet.asp

But what I find deplorable is, there are so few (or even no) copy&paste solutions for such small solutions. What I mean is: Authors mostly just explain the problem and a possible solution (with every little step) - but they don´t provide a ready to use component. Why don´t they put everything needed into just a class? Or if it becomes a little more complex, an assembly? It´s this extra level in abstraction or value, that I´m often missing from explanations found online or offline. For understanding I´m interested in every detail - but for my solution I just need a couple of lines explaining, how to use the solution.

(Before anyone get´s anry at me for my limited world view: Sure there are also many authors who not just think "explanation" but also "added value" and "component". But, alas, I can´t help thinking, it´s still too few.)

Never mind.

Since I badly needed a solution for the impersonation problem, I banged together a small "component" myself from what I found online. (Mainly from the Microsoft source.) So whoever is in the need for quick impersonation, here´s a simple VB.NET class to use. Just put it in your project and become a chameleon in just two lines of code:

Dim aa As New AliasAccount("mydomain\myusername", "mypassword")
aa.BeginImpersonation()
...
aa.EndImpersonation()

(See how short an explanation of how to do impersonation can be?)

It worked find for me within WebForms code behind and WinForms applications.

Enjoy!

PS: And here the length details. Read someplace else, what it all means. If you want to use it, just copy the class into your project and you´re set :-)

Public Class AliasAccount
    Private _username, _password, _domainname As String

Private _tokenHandle As New IntPtr(0)
    Private _dupeTokenHandle As New IntPtr(0)
    Private _impersonatedUser As System.Security.Principal.WindowsImpersonationContext

Public Sub New(ByVal username As String, ByVal password As String)
        Dim nameparts() As String = username.Split("\")
        If nameparts.Length > 1 Then
            _domainname = nameparts(0)
            _username = nameparts(1)
        Else
            _username = username
        End If
        _password = password
    End Sub

Public Sub New(ByVal username As String, ByVal password As String, ByVal domainname As String)
        _username = username
        _password = password
        _domainname = domainname
    End Sub

Public Sub BeginImpersonation()
        Const LOGON32_PROVIDER_DEFAULT As Integer = 0
        Const LOGON32_LOGON_INTERACTIVE As Integer = 2
        Const SecurityImpersonation As Integer = 2

Dim win32ErrorNumber As Integer

_tokenHandle = IntPtr.Zero
        _dupeTokenHandle = IntPtr.Zero

If Not LogonUser(_username, _domainname, _password, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, _tokenHandle) Then
            win32ErrorNumber = System.Runtime.InteropServices.Marshal.GetLastWin32Error()
            Throw New ImpersonationException(win32ErrorNumber, GetErrorMessage(win32ErrorNumber), _username, _domainname)
        End If

If Not DuplicateToken(_tokenHandle, SecurityImpersonation, _dupeTokenHandle) Then
            win32ErrorNumber = System.Runtime.InteropServices.Marshal.GetLastWin32Error()

CloseHandle(_tokenHandle)
            Throw New ImpersonationException(win32ErrorNumber, "Unable to duplicate token!", _username, _domainname)
        End If

Dim newId As New System.Security.Principal.WindowsIdentity(_dupeTokenHandle)
        _impersonatedUser = newId.Impersonate()
    End Sub

Public Sub EndImpersonation()
        If Not _impersonatedUser Is Nothing Then
            _impersonatedUser.Undo()
            _impersonatedUser = Nothing

If Not System.IntPtr.op_Equality(_tokenHandle, IntPtr.Zero) Then
                CloseHandle(_tokenHandle)
            End If
            If Not System.IntPtr.op_Equality(_dupeTokenHandle, IntPtr.Zero) Then
                CloseHandle(_dupeTokenHandle)
            End If
        End If
    End Sub

Public ReadOnly Property username() As String
        Get
            Return _username
        End Get
    End Property

Public ReadOnly Property domainname() As String
        Get
            Return _domainname
        End Get
    End Property

Public ReadOnly Property currentWindowsUsername() As String
        Get
            Return System.Security.Principal.WindowsIdentity.GetCurrent().Name
        End Get
    End Property

Region "Exception Class"

Public Class ImpersonationException
        Inherits System.Exception

Public ReadOnly win32ErrorNumber As Integer

Public Sub New(ByVal win32ErrorNumber As Integer, ByVal msg As String, ByVal username As String, ByVal domainname As String)
            MyBase.New(String.Format("Impersonation of {1}{0} failed! [{2}] {3}", username, domainname, win32ErrorNumber, msg))
            Me.win32ErrorNumber = win32ErrorNumber
        End Sub
    End Class

End Region

Region "External Declarations and Helpers"

Private Declare Auto Function LogonUser Lib "advapi32.dll" (ByVal lpszUsername As [String], _
            ByVal lpszDomain As [String], ByVal lpszPassword As [String], _
            ByVal dwLogonType As Integer, ByVal dwLogonProvider As Integer, _
            ByRef phToken As IntPtr) As Boolean

Private Declare Auto Function DuplicateToken Lib "advapi32.dll" (ByVal ExistingTokenHandle As IntPtr, _
                ByVal SECURITY_IMPERSONATION_LEVEL As Integer, _
                ByRef DuplicateTokenHandle As IntPtr) As Boolean

Private Declare Auto Function CloseHandle Lib "kernel32.dll" (ByVal handle As IntPtr) As Boolean

_
    Private Shared Function FormatMessage(ByVal dwFlags As Integer, ByRef lpSource As IntPtr, _
            ByVal dwMessageId As Integer, ByVal dwLanguageId As Integer, ByRef lpBuffer As [String], _
            ByVal nSize As Integer, ByRef Arguments As IntPtr) As Integer
    End Function

Private Function GetErrorMessage(ByVal errorCode As Integer) As String
        Dim FORMAT_MESSAGE_ALLOCATE_BUFFER As Integer = &H100
        Dim FORMAT_MESSAGE_IGNORE_INSERTS As Integer = &H200
        Dim FORMAT_MESSAGE_FROM_SYSTEM As Integer = &H1000

Dim messageSize As Integer = 255
        Dim lpMsgBuf As String
        Dim dwFlags As Integer = FORMAT_MESSAGE_ALLOCATE_BUFFER Or FORMAT_MESSAGE_FROM_SYSTEM Or FORMAT_MESSAGE_IGNORE_INSERTS

Dim ptrlpSource As IntPtr = IntPtr.Zero
        Dim prtArguments As IntPtr = IntPtr.Zero

Dim retVal As Integer = FormatMessage(dwFlags, ptrlpSource, errorCode, 0, lpMsgBuf, messageSize, prtArguments)
        If 0 = retVal Then
            Throw New System.Exception("Failed to format message for error code " + errorCode.ToString() + ". ")
        End If

Return lpMsgBuf
    End Function

End Region

End Class