What is Impersonation?

Impersonation is the security feature that enables control of the Identity under which code is executed. Impersonation gives the following advantages:

• Run a high privilege code through a low privilege user
• Record changes in the account of another user
  
  

What are the Impersonation methods in SharePoint 2010?

SharePoint 2010 provides the following methods of Impersonation:

1. RunWithElevatedPrivileges to impersonate as System Account user
2. Passing User Token inside SPSite to impersonate as a particular user
3. Using Windows API

Note: System Account (SHAREPOINT\system) is the application pool user of SharePoint. If you are using Developer Installations on client operating systems (Windows 7 / Vista) the account name will be different.

Now let us see how to use the above methods.

1. RunWithElevatedPrivileges
   This is the most commonly used method to impersonate. 
   SPSecurity.RunWithElevatedPrivileges(() =>
   {
        // Your code here
   });
   Note: In the case of RunWithElevatedPrivileges the System Account is used to perform the activity.
    
2. Passing User Token
   SPUserToken is the server model which we use for the purpose. Each user's token can be represented by this class. The User Token is actually a byte array. 
   
   The SPUser class contains the property named UserToken. Passing SPUserToken instance into the SPSite constructor impersonates the particular user. 
   
   Eg: new SPSite(UrlText.Text, user.UserToken);
   
   For enumerating all the users of a site the web.Users property can be used.
   
   Eg: web.Users

Running the Code
The attached source contains the following samples:

1. Enumerate Users
   For enumerating users for a given website the following code can be used:
   using (SPSite site = new SPSite(UrlText.Text)){
       using (SPWeb web = site.OpenWeb())
       {
           SPContext context = SPContext.GetContext(web);
           var users = context.Web.Users; 
           // Display to grid        usersGrid.DataSource = users.Cast<SPUser>().ToList<SPUser>();
       }
   }
   
   On clicking the button we will see the following users as shown below:
   
   
   • Please note that there are only 2 users for the site I use
   • The current user is logged in as Admin
     
     
      
2. Create Data Impersonating each User
   Now we can try creating list items impersonating each user. The created item will have the system property > Created By set to various users:
   
   The following code does that:
   int count = 1;foreach (SPUser user in web.Users)
   {
       SPSite newSite = new SPSite(UrlText.Text, user.UserToken); // Impersonate    SPWeb newWeb = newSite.OpenWeb();
       SPListItem item = newWeb.Lists[ListName].AddItem();
       item["Title"] = "Item " + count++.ToString();
       item.Update(); 
       newSite.Dispose();
       newWeb.Dispose();
   }
   
   On running the code the above we will see the items created as shown below:
   
   
   • Please note that the Created By property is different for each row
     
     
   
   Note: An exception will be thrown if any of the users above do not have write permission.
   
3. Create Data using RunWithElevatedPrivileges
   
   Now we can try creating the list items using a RunWithElevatedPrivileges block. In this case the user is impersonated to be the System Account.
   
   The code for the same is shown below:
   SPSecurity.RunWithElevatedPrivileges(() =>
   {
         using (SPSite site = new SPSite(UrlText.Text))
         {           using (SPWeb web = site.OpenWeb())
              {
                   SPListItem item = web.Lists[ListName].AddItem();
                   item["Title"] = "Item created with RunWithElevatedPriveleges";
                   item.Update();
                   // Item will be created with System Account 
                   ShowData(web);
              }
         } });

          We can see that the new item is created with the System Account as shown below: