Access users and contacts in active directory sites

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C#5.0

active-directory

Good Morning
I have a list of sites in my domain and i want to access the user and contacts inside the sites of the active directory because I want to copy all contact details to users details. But I am only getting to the user objects of the domain not the user objects inside the sites.

Below I have tried to describe the tree structure of the domain the best I can

Domain
Sites
Site1
Site2
Users--this is the object I want to access
Contacts--this is na object I want to access
Site3
Users

What I have tried:

public List<string> GetAllUsers(String Domain)
      {
          try
          {
              List<string> users = new List<string>();

              PrincipalContext oPrincipal = GetPrincipalContext(Domain);
              GroupPrincipal gPrincipal = GroupPrincipal.FindByIdentity(oPrincipal, IdentityType.SamAccountName, "Domain Users");

              if (gPrincipal != null)
              {
                  foreach (Principal p in gPrincipal.GetMembers(false))
                  {
                      users.Add(p.DisplayName);
                  }
              }

              return users;
          }
          catch (Exception ex)
          {

              throw ex;
          }
      }