MSDN Magazine - March 2008 - (Page 24)

Figure 2 Using Contains and Conversion using (Entities entities = new Entities()) { Customers customerBSBEV = (from c in entities.Customers where c.CustomerID == “BSBEV” select c).First(); var customersUK = from c in entities.Customers where c.Country == “UK” select c; bool isCustomerInSequence = customersUK.Contains(customerBSBEV); } Console.WriteLine(isCustomerInSequence? “Yes” : “No”); in Figure 1 could be amended to use the following code to further qualify the ordering rules: foreach (var item in query.OrderByDescending(x => x.Total) .ThenBy(x => x.CustomerID)) { Console.WriteLine(item.CustomerID + “ == “ + item.Total); } In this code snippet, I added the ThenBy operator and a lambda expression to indicate that the sequence should be ordered first in descending order by the Total property and then in ascending order by the CustomerID property of the projection. The Contains operator is similar to the Any operator in that it evaluates whether the sequence contains what you are looking for. The Any operator can determine if a value exists within an item in the sequence, but the Contains operator determines if a specific item instance exists in the sequence. For example, before you add an object to a sequence, you might want to make sure the sequence does not already contain that object. Figure 2 demonstrates how to check. Notice in Figure 2 that a Customers entity is retrieved for the BSBEV customer. Then the sequence of Customers entities where the customers are from the United Kingdom is retrieved. Finally, the Contains operator is used to check whether the sequence of customers contains the instance held by the customerBSBEV variable. The implementation of the Contains operator shown in Figure 2 works in situations where you can confidently compare objects based on their actual instance. However, what if you need the Contains operator to test based on a logical identity? Fortunately, the Contains operator has an overload allowing you to pass in an object implementing the IEqualityComparer interface. To use Contains based on CustomerID, the code in Figure 2 can be rewritten as follows using (Entities entities = new Entities()) { bool isCustomerInSequence = customersUK.Contains(customerBSBEV, new CustomerComparer()); } Console.WriteLine(isCustomerInSequence? “Yes” : “No”); Quantifiers and Conversion When you need to determine if a value exists in a sequence, the standard query operator Any can be used. Quantifiers such as Any, All, and Contains search a sequence of elements and evaluate whether or not the sequence meets a lambda expression’s condition. This can be very helpful when you need to examine a sequence to determine things like whether a customer from a specific address exists, if all customers are from the same country, or any other analytical deterministic question. For example, the following LINQ query checks to see if all customers from the United Kingdom are located in London. This uses the quantifier All and passes it the lambda expression that simply evaluates if the city is London. If every element in the sequence meets this criterion and returns true from the lambda expression, then the All operator will return true: using (Entities entities = new Entities()) { bool allUKCustomerAreFromLondon = (from c in entities.Customers where c.Country == “UK” select c).All( c => c.City.Equals(“London”)); Console.WriteLine(allUKCustomerAreFromLondon ? “Yes” : “No”); } where CustomerComparer is defined as private class CustomerComparer : IEqualityComparer { public bool Equals(Customers x, Customers y) { if (x == null || y == null) return false; return x.CustomerID.Equals(y.CustomerID); } } Wrapping Up There are many standard query operators that are defined as extension methods for the Enumerable and Queryable sequence classes. These operators can help extend the functionality of LINQ, as I’ve show you here. I’ve also demonstrated how many of the new enhancements to the .NET Framework 3.5 (including lambda expressions, LINQ, the Entity Framework, and implicitly typed variables) come together to make writing robust code and logic easier. Another question that might need to be asked in this query is if there are any entities in the sequence from the city Cowes in the United Kingdom. For this question, you can use the Any quantifier to operate on the sequence, as shown here: using (Entities entities = new Entities()) { bool isOneUKCustomerFromCowes = (from c in entities.Customers where c.Country == “UK” select c).Any( c => c.City.Equals(“Cowes”)); Console.WriteLine(isOneUKCustomerFromCowes? “Yes” : “No”); } Code download available at msdn.microsoft.com/msdnmag/code08.aspx. Send your questions and comments for John to mmdata@microsoft.com. John Papa is a senior .NET consultant with ASPSOFT (aspsoft.com) and a baseball fanatic who spends most of his summer nights rooting for the Yankees with his family and his faithful dog, Kadi. John, a C# MVP and INETA speaker, has authored several books on data access technologies. He can often be found speaking at industry conferences or blogging at www.johnpapa.net. 24 msdnmagazine Data Points http://msdn.microsoft.com/msdnmag/code08.aspx http://aspsoft.com http://www.johnpapa.net

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

MSDN Magazine - March 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Introducing ASP.NET MVC Loosen Up CI Server Performance Office Development Test Run Security Briefs Extreme ASP.NET Foundations .NET Matters {End Bracket}

MSDN Magazine - March 2008

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