Sometimes it is necessary to change the user under whose account asp.net runs to execute a special method. This could be the case e.g. if you want to access an ADS with a local asp.net user and you get an error message "invalid username or password" (there is the possibility to pass a username and passwort, but that didn't work for me).

The usage of the class below is very simple:

...
Impersonation imp = new Impersonation();
imp.ImpersonateValidUser("username", "domain", "password");
//execute your work here
imp.UndoImpersonation();
....




using System;
using System.Security.Principal;
using System.Runtime.InteropServices;

namespace Security
{

   public class Impersonation
   {

      public const int LOGON32_LOGON_INTERACTIVE = 2;
      public const int LOGON32_PROVIDER_DEFAULT = 0;
      WindowsImpersonationContext impersonationContext;

      [DllImport("advapi32.dll")]
      public static extern int LogonUserA(String lpszUserName, 
         String lpszDomain,
         String lpszPassword,
         int dwLogonType, 
         int dwLogonProvider,
         ref IntPtr phToken);
      
      [DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
      public static extern int DuplicateToken(IntPtr hToken, 
         int impersonationLevel, 
         ref IntPtr hNewToken);

      [DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
      public static extern bool RevertToSelf();

      [DllImport("kernel32.dll", CharSet=CharSet.Auto)]
      public static extern bool CloseHandle(IntPtr handle);

      public bool ImpersonateValidUser(String userName, String domain, String password)
      {
         WindowsIdentity tempWindowsIdentity;
         IntPtr token = IntPtr.Zero;
         IntPtr tokenDuplicate = IntPtr.Zero;
         
         if(RevertToSelf())
         {
            if(LogonUserA(userName, domain, password, LOGON32_LOGON_INTERACTIVE, 
               LOGON32_PROVIDER_DEFAULT, ref token) != 0)
            {
               if(DuplicateToken(token, 2, ref tokenDuplicate) != 0) 
               {
                  tempWindowsIdentity = new WindowsIdentity(tokenDuplicate);
                  impersonationContext = tempWindowsIdentity.Impersonate();
                  if (impersonationContext != null)
                  {
                     CloseHandle(token);
                     CloseHandle(tokenDuplicate);
                     return true;
                  }
               }
            } 
         }

         if(token!= IntPtr.Zero) CloseHandle(token);
         if(tokenDuplicate!=IntPtr.Zero) CloseHandle(tokenDuplicate);
         return false;
      }

      public void UndoImpersonation()
      {
         impersonationContext.Undo();
      }

   }
}

Anwendungsfall:

• Es soll nur eine Instanz der Klasse existieren bzw. es soll nicht möglich sein, mehrere Instanzen der Klasse innerhalb eines Anwendungskontexts zu erzeugen.

Beispiele für die Anwendung:

• Loadbalancer
• ConnectionPools

Grafische Darstellung:

Beispielcode:

public class Singleton
{

   private static Singleton instance;

   private Singleton()
   {
   }

   public static Singleton GetInstance()
   {
      if (Singleton.instance == null)
      {
         Singleton.instance = new Singleton();
      }
      return Singleton.instance;
   }

}

Problem:
You created a WebSite in Visual Studio 2005 and added a reference to another project or a .dll.
Now, if you want to remove this reference, you can't simply delete the .dll in the "Bin" folder of the WebSite. After a rebuild, the deleted .dll will be recreated from VS automatically.

Solution:
To remove the reference, open the Proptery Pages of the WebSite (right click on your WebSite in Solution Explorer and select "Property Pages"). The first time after a restart of VS, you will get the following "dialog":

Just try it once again, and it will work. Here you can remove your reference permanently.

Search for the file setup.sdb on your cd or dvd.

There, normally on the last line, you will find an entry with the headline "[Product Key]".

This is the case for e.g. Visual Studio Team Foundation Server 2005 or for Visual Studio Team Suite 2005.

Get all tables including the system ones:
SELECT table_name FROM all_tables;

That will give you only the tables owned by the user that you are connecting as:
SELECT table_name FROM user_tables;

Query table columns:
SELECT * FROM all_tab_cols WHERE table_name = 'whatever table name you are looking for'

Looking for comments on the tables:
SELECT * FROM all_tab_comments WHERE table_name = 'whatever table name you are looking for'

...
Page page = new Page();
Control c = page.LoadControl("YourControl.ascx");
// do Something with your control, DataBind, ...
string html = RenderControl(c);

public static string RenderControl(Control ctrl)
{
   System.Text.StringBuilder sb = new System.Text.StringBuilder();
   System.IO.StringWriter tw = new System.IO.StringWriter(sb);
   System.Web.UI.HtmlTextWriter hw = new System.Web.UI.HtmlTextWriter(tw);
   ctrl.RenderControl(hw);
   return sb.ToString();
}

You can save your data during a HttpRequest by passing it to System.Web.HttpContext.Current.Items[itemKey].
Objects stored in this Collection will be available during the whole lifecycle of the HttpRequest.

Example of usage:

System.Web.HttpContext.Current.Items["reloadCount"] = 5;

int reloadCount = (int) System.Web.HttpContext.Current.Items["reloadCount"];

private DataSet ReadDataSet()
{
   DataSet ds = new DataSet();
   ds.ReadXml(dataPath, XmlReadMode.ReadSchema);
   return ds;
}

private void WriteDataSet(DataSet ds, string path)
{
   ds.WriteXml(path, XmlWriteMode.WriteSchema);
}