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So far, all of the variables that we have been using are declared at the start of the Main( ) method However, C# allows a local variable to be declared within any block As explained in 1, a block begins with an opening curly brace and ends with a closing curly brace A block defines a scope Thus, each time you start a new block, you are creating a new scope A scope determines what names are visible to other parts of your program without qualification It also determines the lifetime of local variables The most important scopes in C# are those defined by a class and those defined by a method A discussion of class scope (and variables declared within it) is deferred until later in this book, when classes are described For now, we will examine only the scopes defined by or within a method The scope defined by a method begins with its opening curly brace and ends with its closing curly brace However, if that method has parameters, they too are included within the scope defined by the method As a general rule, local variables declared inside a scope are not visible to code that is defined outside that scope Thus, when you declare a variable within a scope, you are protecting it from access or modification from outside the scope Indeed, the scope rules provide the foundation for encapsulation Scopes can be nested For example, each time you create a block of code, you are creating a new, nested scope When this occurs, the outer scope encloses the inner scope This means that local variables declared in the outer scope will be visible to code within the inner scope

3:

3 + 1 4

However, the reverse is not true Local variables declared within the inner scope will not be visible outside it To understand the effect of nested scopes, consider the following program:

// Demonstrate block scope using System; class ScopeDemo { static void Main() { int x; // known to all code within Main() x = 10; if(x == 10) { // start new scope int y = 20; // known only to this block // x and y both known here ConsoleWriteLine("x and y: " + x + " " + y); x = y * 2; } // y = 100; // Error! y not known here // x is still known here ConsoleWriteLine("x is " + x); } }

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))

In the last example, we have counted positive area as positive and negative area as negative Our calculation shows that the aggregate area is positive---but bear in mind that the calculation entailed counting area above the x-axis as positive and area below the x-axis as negative (so there was some cancellation) We encourage the reader to draw a graph to make this result plausible

Part I: