using System.Runtime.InteropServices; // DllImport
using System.Security.Principal; // WindowsImpersonationContext
using System.Security.Permissions; // PermissionSetAttribute
...
 
public WindowsImpersonationContext 
    ImpersonateUser(string sUsername, string sDomain, string sPassword)
{
    // initialize tokens
    IntPtr pExistingTokenHandle = new IntPtr(0);
    IntPtr pDuplicateTokenHandle = new IntPtr(0);
    pExistingTokenHandle = IntPtr.Zero;
    pDuplicateTokenHandle = IntPtr.Zero;
 
    // if domain name was blank, assume local machine
    if (sDomain == "")
        sDomain = System.Environment.MachineName;
 
    try
    {
        string sResult = null;
 
        const int LOGON32_PROVIDER_DEFAULT = 0;
 
        // create token
        const int LOGON32_LOGON_INTERACTIVE = 2;
        //const int SecurityImpersonation = 2;
 
        // get handle to token
        bool bImpersonated = LogonUser(sUsername, sDomain, sPassword, 
            LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, 
                ref pExistingTokenHandle);
 
        // did impersonation fail?
        if (false == bImpersonated)
        {
            int nErrorCode = Marshal.GetLastWin32Error();
            sResult = "LogonUser() failed with error code: " + 
                nErrorCode + "\r\n";
 
            // show the reason why LogonUser failed
            MessageBox.Show(this, sResult, "Error", 
                MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
 
        // Get identity before impersonation
        sResult += "Before impersonation: " + 
            WindowsIdentity.GetCurrent().Name + "\r\n";
 
        bool bRetVal = DuplicateToken(pExistingTokenHandle, 
            (int)SECURITY_IMPERSONATION_LEVEL.SecurityImpersonation, 
                ref pDuplicateTokenHandle);
 
        // did DuplicateToken fail?
        if (false == bRetVal)
        {
            int nErrorCode = Marshal.GetLastWin32Error();
            // close existing handle
            CloseHandle(pExistingTokenHandle); 
            sResult += "DuplicateToken() failed with error code: " 
                + nErrorCode + "\r\n";
 
            // show the reason why DuplicateToken failed
            MessageBox.Show(this, sResult, "Error", 
                MessageBoxButtons.OK, MessageBoxIcon.Error);
            return null;
        }
        else
        {
            // create new identity using new primary token
            WindowsIdentity newId = new WindowsIdentity
                                        (pDuplicateTokenHandle);
            WindowsImpersonationContext impersonatedUser = 
                                        newId.Impersonate();
 
            // check the identity after impersonation
            sResult += "After impersonation: " + 
                WindowsIdentity.GetCurrent().Name + "\r\n";
 
            MessageBox.Show(this, sResult, "Success", 
                MessageBoxButtons.OK, MessageBoxIcon.Information);
            return impersonatedUser;
        }
    }
    catch (Exception ex)
    {
        throw ex;
    }
    finally
    {
        // close handle(s)
        if (pExistingTokenHandle != IntPtr.Zero)
            CloseHandle(pExistingTokenHandle);
        if (pDuplicateTokenHandle != IntPtr.Zero) 
            CloseHandle(pDuplicateTokenHandle);
    }
}