private Category CreateOrFindCategory(string categoryName, PLEntities context)
        {
            Category category;
            using (TransactionScope tran = new TransactionScope(TransactionScopeOption.RequiresNew,
                new TransactionOptions() { IsolationLevel = System.Transactions.IsolationLevel.Serializable }))
            {
                category = context.Categories.FirstOrDefault(cat => cat.Name == categoryName);
                if (category == null)
                {
                    category = new Category() { Name = categoryName };
                    context.Categories.AddObject(category);
                    context.SaveChanges();
                }
                tran.Complete();
            }
            return category;
        }

public void CreateClient(string clientName, string categoryName)
{
  using (PLEntities context = new PLEntities ())
  {
     Category cat = CreateOrFindCategory(categoryName, context);
     Client client = new Client();
     client.Name = clientName;
     clietn.Category = Category;
     context.Clients.AddObject(client);
     context.SaveChanges();
  }
}