C# in Depth

232 CHAPTER 9 Lambda expressions and expression trees9.1 Lambda expressions as delegatesIn many ways, lambda expressions can be seen as an evolution of anonymous methodsfrom C# 2. There’s almost nothing that an anonymous method can do that can’t be doneusing a lambda expression, and it’s almost always more readable and compact usinglambdas. In particular, the behavior of captured variables is exactly the same in lambdaexpressions as in anonymous methods. In their most explicit form, not much differenceexists between the two—but lambda expressions have a lot of shortcuts available to makethem compact in common situations. Like anonymous methods, lambda expressionshave special conversion rules—the type of the expression isn’t a delegate type in itself,but it can be converted into a delegate instance in various ways, both implicitly andexplicitly. The term anonymous function covers anonymous methods and lambda expressions—inmany cases the same conversion rules apply to both of them.We’re going to start with a very simple example, initially expressed as an anonymousmethod. We’ll create a delegate instance that takes a string parameter and returns anint (which is the length of the string). First we need to choose a delegate type to use;fortunately, .NET 3.5 comes with a whole family of generic delegate types to help us out.9.1.1 Preliminaries: introducing the Func delegate typesThere are five generic Func delegate types in the System namespace of .NET 3.5.There’s nothing special about Func—it’s just handy to have some predefined generictypes that are capable of handling many situations. Each delegate signature takesbetween zero and four parameters, the types of which are specified as type parameters.The last type parameter is used for the return type in each case. Here are the signaturesof all the Func delegate types:public delegate TResult Func()public delegate TResult Func(T arg)public delegate TResult Func(T1 arg1, T2 arg2)public delegate TResult Func(T1 arg1, T2 arg2, T3 arg3)public delegate TResult Func(T1 arg1, T2 arg2, T3 arg3, T4 arg4)For example, Func is equivalent to a delegate type of the formdelegate int SomeDelegate(string arg1, double arg2)The Action set of delegates provide the equivalent functionality when you want avoid return type. The single parameter form of Action existed in .NET 2.0, but the restare new to .NET 3.5. For our example we need a type that takes a string parameterand returns an int, so we’ll use Func.9.1.2 First transformation to a lambda expressionNow that we know the delegate type, we can use an anonymous method to create ourdelegate instance. Listing 9.1 shows this, along with executing the delegate instanceafterward so we can see it working.Licensed to Rhona Hadida