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Written like this, it looks recognizably like a function definition, except that we use the delegate keyword to let the compiler know we are providing a delegate. There s no need to specify the return type that is inferred from the context. (In this case, the delegate type is Predicate<T>, so the compiler knows the return type is bool.) Any parameters in our parameter list are accessible only inside the body of the anonymous method itself. Why do we call it an anonymous method Because it doesn t have a name that can be referenced elsewhere! The variable that references the delegate to the anonymous method has a name, but not the anonymous delegate type, or the anonymous method itself. If you compile and run the code you ll see the new output:

The production team is happy; but is the job done Not quite; although this inline syntax for an anonymous method is a lot more compact than a static class/function declaration, we can get more compact and expressive still, using lambda expression syntax, which was added in C# 3.0 (anonymous methods having been around since C# 2.0).

In the 1930s (a fertile time for computing theory!) two mathematicians named Church and Kleene devised a formal system for investigating the properties of functions. This was called lambda calculus, and (as further developed by Curry and others) it is still a staple part of computational theory for computer scientists. Fast-forward 70 or so years, and we see just a hint of this theory peeking through in C# s lambda expressions only a hint, though, so bear with it. As we saw before, you can think of a function as an expression that maps a set of inputs (the parameters) to an output (the return value). Mathematicians sometimes use a notation similar to this to define a function: (x,y,z) x + y + z You can read this as defining a function that operates on three parameters (x, y, and z). The result of the function is just the sum of the three parameters, and, by definition, it can have no side effects on the system. The parameters themselves aren t modified by the function; we just map from the input parameters to a result. Lambda expressions in C# use syntax very similar to this to define functional expressions. Here s the C# equivalent of that mathematical expression we used earlier:

(x,y,z) => x + y + z;

Class Members Instance Class Members Static Fields Static Function Members Other Static Class Member Types Constants Properties Instance Constructors Static Constructors Accessibility of Constructors Finalizers Comparison of Constructors and Finalizers The readonly Modifier The this Keyword Indexers Access Modifiers on Accessors Partial Classes

Notice how it rather cutely uses => as the programming language equivalent of . C++ users should not mistake this for the -> operator it is quite different!

var dateTime = Date.now();

This defines a lambda expression that takes three parameters and returns the sum of those three parameters.

Some languages enforce the no side effects constraint; but in C# there is nothing to stop you from writing a lambda expression such as this one:

(x,y,z) => { SomeStaticClass.CrashAndBurnAndMessWithEverything(); x.ModifyInternalState(); return x + y + z; }

The previous two chapters covered two of the nine types of class members: fields and methods. In this chapter, I will introduce more class member types, and discuss the lifetimes of class members. Table 6-1 shows a list of the class member types. Those that have already been introduced are marked with diamonds. Those that will be covered in this chapter are marked with a check. Those that will be covered later in the text are marked with empty check boxes. Table 6-1. Types of Class Members

(Incidentally, this form of lambda expression, using braces to help define its body, is called a statement-form lambda.) In C#, a lambda is really just a concise way to write an anonymous method. We re just writing normal code, so we can include operations that have side effects. So, although C# brings along some functional techniques with lambda syntax, it is not a pure functional language like ML or F#. Nor does it intend to be. So, what use is a lambda, then We ll see some very powerful techniques in 8 and 14, where lambdas play an important role in LINQ. Some of the data access features of the .NET Framework use the fact that we can convert lambdas into data structures called expression trees, which can be composed to create complex query-like expressions over various types of data. For now, we re merely going to take advantage of the fact that we can implicitly create a delegate from a lambda, resulting in less cluttered code. How do we write our anonymous delegate as a lambda Here s the original:

Predicate<Document> predicate = delegate(Document doc ) { return !doc.Text.Contains(" "); }

And here it is rewritten using a lambda expression:

Predicate<Document> predicate = doc => !doc.Text.Contains(" ");

Data Members (Store Data)

Compact, isn t it! For a lot of developers, this syntax takes some getting used to, because it is completely unlike anything they ve ever seen before. Where are the type declarations Is this taking advantage of some of these dynamic programming techniques we ve heard so much about The short answer is no (but we ll get to dynamic typing in 18, don t worry). One of the nicer features of lambda expression syntax is that it takes care of working out what types the various parameters need to be, based on the context. In this case, the compiler knows that it needs to produce a Predicate<Document>, so it can infer that

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