PART I in Visual C#.NET

Creation QR Code 2d barcode in Visual C#.NET PART I

Another LINQ-related feature is the expression tree An expression tree is a representation of a lambda expression as data Thus, an expression tree, itself, cannot be executed It can, however, be converted into an executable form Expression trees are encapsulated by the SystemLinqExpressionsExpression<TDelegate> class Expression trees are useful in situations in which a query will be executed by something outside the program, such as a database that uses SQL By representing the query as data, the query can be converted into a format understood by the database This process is used by the LINQ to SQL feature provided by Visual C#, for example Thus, expression trees help C# support a variety of data sources You can obtain an executable form of an expression tree by calling the Compile( ) method defined by Expression It returns a reference that can be assigned to a delegate and then executed You can declare your own delegate type or use one of the predefined Func delegate types defined within the System namespace Two forms of the Func delegate were mentioned earlier, when the query methods were described, but there are several others Expression trees have one key restriction: Only expression lambdas can be represented by expression trees They cannot be used to represent statement lambdas Here is a simple example of an expression tree in action It creates an expression tree whose data represents a method that determines if one integer is a factor of another It then compiles the expression tree into executable code Finally, it demonstrates the compiled code

// A simple expression tree using System; using SystemLinq; using SystemLinqExpressions; class SimpleExpTree { static void Main() { // Represent a lambda expression as data Expression<Func<int, int, bool>> IsFactorExp = (n, d) => (d != 0) (n % d) == 0 : false; // Compile the expression data into executable code Func<int, int, bool> IsFactor = IsFactorExpCompile(); // Execute the expression if(IsFactor(10, 5)) ConsoleWriteLine("5 is a factor of 10"); if(!IsFactor(10, 7))

Part I:

ConsoleWriteLine("7 is not a factor of 10"); ConsoleWriteLine(); } }

The output is shown here:

The program illustrates the two key steps in using an expression tree First, it creates an expression tree by using this statement:

Expression<Func<int, int, bool>> IsFactorExp = (n, d) => (d != 0) (n % d) == 0 : false;

This constructs a representation of a lambda expression in memory As explained, this representation is data, not code This representation is referred to by IsFactorExp The following statement converts the expression data into executable code:

Func<int, int, bool> IsFactor = IsFactorExpCompile();

After this statement executes, the IsFactor delegate can be called to determine if one value is a factor of another One other point: Notice that Func<int, int, bool> indicates the delegate type This form of Func specifies two parameters of type int and a return type of bool This is the form of Func that is compatible with the lambda expression used in the program because that expression requires two parameters Other lambda expressions may require different forms of Func, based on the number of parameters they require In general, the specific form of Func must match the requirements of the lambda expression

As mentioned earlier, extension methods provide a means by which functionality can be added to a class without using the normal inheritance mechanism Although you won t often create your own extension methods (because the inheritance mechanism offers a better solution in many cases), it is still important that you understand how they work because of their integral importance to LINQ An extension method is a static method that must be contained within a static, non-generic class The type of its first parameter determines the type of objects on which the extension method can be called Furthermore, the first parameter must be modified by this The object on which the method is invoked is passed automatically to the first parameter It is not explicitly passed in the argument list A key point is that even though an extension method is declared static, it can still be called on an object, just as if it were an instance method Here is the general form of an extension method: static ret-type name(this invoked-on-type ob, param-list)

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