MSDN Magazine - July 2008 - (Page 39)

items.Add("Monday") items.Add(3) items.Add(5) items.Add("September") Dim numbers = items.OfType(Of Integer)() Dim strings = items.OfType(Of String)() Figure 3 IEqualityComparer Public Class CustomerCountryComparer Implements IEqualityComparer(Of Customer) Public Function Equals1( _ ByVal x As Customer, ByVal y As Customer) As Boolean _ Implements IEqualityComparer(Of Customer).Equals Return x.Country.Equals(y.Country) End Function Public Function GetHashCode1( _ ByVal obj As Customer) As Integer _ Implements IEqualityComparer(Of Customer).GetHashCode Return obj.Country.GetHashCode End Function End Class After running the code, the two output lists contain the follow ing data: 0, 3, 5 January, Monday, September The Enumerable.Where method allows you to specify a condi tion that filters the input sequence. A second overloaded version provides access to the index of each item in the collection so you can filter based on the index as well. The following example uses both the Where and Select methods, first filtering and then pro jecting the results, retrieving a sequence of files in the C:\Windows folder smaller than 100 bytes: ' From WhereDemo in the sample: Dim files As IEnumerable(Of FileInfo) = _ New DirectoryInfo("C:\Windows").GetFiles() Dim fileResults = _ files _ .Where(Function(file) file.Length < 100) _ .Select(Function(file) _ String.Format("{0} ({1})", file.Name, file.Length)) Figure 4 Using DefaultIfEmpty ' From DefaultIfEmptyDemo in the sample: Dim db As New SimpleDataContext Dim sw As New StringWriter Dim noCustomers = _ From cust In db.Customers Where cust.Country = "XXX" Dim noCustomers1 = _ noCustomers.DefaultIfEmpty() Dim results = _ String.Format( _ "noCustomers contains {0} element(s). " & _ "noCustomers1 contains {1} element(s).", _ noCustomers.Count, noCustomers1.Count) sw.WriteLine(results) ' You can specify the exact value to use if the ' collection is empty: noCustomers1 = noCustomers.DefaultIfEmpty( _ New Customer With {.CustomerID = "XXXXX"}) For Each cust In noCustomers1 sw.WriteLine(cust.CustomerID) Next Because arrays implement the IEnumerable interface, you can use the Where method to filter array contents just as with any other collection. In this case, the code filters the array of FileInfo objects returned by the GetFiles method. Also note that the example cas cades calls to the Where method and then the Select method. On my computer, the sample returned the following results: Addrfixr.ini (62) bthservsdp.dat (12) iltwain.ini (36) S86D5A060.tmp (48) setuperr.log (0) You can use the second overload of the Where method to ac cess the index of each input item that is processed. For example, the following Where method call returns files less than 100 bytes in size that also appear within the first 20 input files: ' From WhereDemo in the sample: fileResults = _ files _ .Where(Function(file, index) _ (file.Length < 100) And (index < 20)) _ .Select(Function(file) _ String.Format("{0} ({1})", file.Name, file.Length)) In this case, on my computer, the results looked like this (the miss ing files fell outside the first 20 files processed): Addrfixr.ini (62) bthservsdp.dat (12) iltwain.ini (36) The Enumerable.Distinct method allows you to filter a list so that the output sequence contains only the unique items from the input list. For simple values (strings, numbers, and so on), this process is easy. For lists of complex objects, how can the runtime engine per form the comparisons to determine whether or not two objects are duplicates? It can’t simply compare the instance variables, as this would, in practice, simply compare the memory addresses of the objects. Instead, for complex objects, you must supply a compar er—an instance of a class that implements IEqualityComparer(Of msdnmagazine.com T) to perform the comparison. (This same issue applies to several methods of the Enumerable class, and it will show up again later in this column.) Suppose you had a sequence containing customer information and you wanted a unique list of countries from where those cus tomers came. If you had a simple list of countries, you could use the default comparer to compare the strings. In this case, you have a list of customers instead. (Yes, you could use the Select method to project the list so that you only had a list of countries, but then you’d lose all the remaining data for other calculations using the list.) The sample project includes the CustomerCountryComparer class, which implements the IEqualityComparer(Of Customer) interface, as shown in Figure 3. The sample code demonstrates two different ways to use the Distinct method. The first projects a list of customers to a list of country names first, and then uses the default string comparer to reduce the list so that it’s unique. The second uses the specific CustomerCountry Comparer class to perform the comparisons and create the unique list, and then projects the resulting distinct list of customers into a list of strings: July 2008 39 http://msdnmagazine.com

Table of Contents Feed for the Digital Edition of MSDN Magazine - July 2008

MSDN Magazine - July 2008 Contents Toolbox CLR Inside Out Flex Your Data Data Points Advanced Basics Office Space Cutting Edge Data Services ADO.NET Data and WPF Transactions WCF P2P Test Run Security Briefs Foundations .NET Matters {End Bracket}

MSDN Magazine - July 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.