Include old values

In order to get the old value too, simply set includeOldValues = true and retrieve the old value in OnChanged event handler as shown below:

    string ConnectionString = "data source=.;initial catalog=myDB;integrated security=True";

    string tableName = "Clients";
    string schemaName = "dbo";

    using(var tableDependency = new SqlTableDependency<Customers>(
        ConnectionString, 
        schemaName: schemaName, 
        tableName: tableName,
        includeOldValues: true))
    {
        tableDependency.OnChanged += TableDependency_Changed;
        tableDependency.Start();

        Console.WriteLine("Waiting for receiving notifications...");
        Console.WriteLine("Press a key to stop");
        Console.ReadKey();
    }
  }
  ....
  private void TableDependency_Changed2(object sender, RecordChangedEventArgs<Customers> 
  {
      switch (e.ChangeType)
      {
          case ChangeType.Insert:
              CheckValues2[ChangeType.Insert.ToString()].Add(e.Entity);
              break;

          case ChangeType.Update:
              CheckValues2[ChangeType.Update.ToString()].Add(e.Entity);
              CheckValuesOld2[ChangeType.Update.ToString()].Add(e.EntityOldValues);
              break;

          case ChangeType.Delete:
              CheckValues2[ChangeType.Delete.ToString()].Add(e.Entity);
              break;
        }
    }