C# in Depth

Lambda expressions as delegates233Listing 9.1Using an anonymous method to create a delegate instanceFunc returnLength;returnLength = delegate (string text) { return text.Length; };Console.WriteLine (returnLength("Hello"));Listing 9.1 prints “5,” just as we’d expect it to. I’ve separated out the declaration ofreturnLength from the assignment to it so we can keep it on one line—it’s easier tokeep track of that way. The anonymous method expression is the part in bold, andthat’s the part we’re going to convert into a lambda expression.The most long-winded form of a lambda expression is this:(explicitly-typed-parameter-list) => { statements }The => part is new to C# 3 and tells the compiler that we’re using a lambda expression.Most of the time lambda expressions are used with a delegate type that has a nonvoidreturn type—the syntax is slightly less intuitive when there isn’t a result. This isanother indication of the changes in idiom between C# 1 and C# 3. In C# 1, delegateswere usually used for events and rarely returned anything. Although lambda expressionscertainly can be used in this way (and we’ll show an example of this later), muchof their elegance comes from the shortcuts that are available when they need toreturn a value.With the explicit parameters and statements in braces, this version looks very similarto an anonymous method. Listing 9.2 is equivalent to listing 9.1 but uses a lambdaexpression.Listing 9.2A long-winded first lambda expression, similar to an anonymous methodFunc returnLength;returnLength = (string text) => { return text.Length; };Console.WriteLine (returnLength("Hello"));Again, I’ve used bold to indicate the expression used to create the delegate instance.When reading lambda expressions, it helps to think of the => part as “goes to”—so theexample in listing 9.2 could be read as “text goes to text.Length.” As this is the onlypart of the listing that is interesting for a while, I’ll show it alone from now on. You canreplace the bold text from listing 9.2 with any of the lambda expressions listed in thissection and the result will be the same.The same rules that govern return statements in anonymous methods apply tolambdas too: you can’t try to return a value from a lambda expression with a voidreturn type, whereas if there’s a nonvoid return type every code path has to return acompatible value. 2 It’s all pretty intuitive and rarely gets in the way.So far, we haven’t saved much space or made things particularly easy to read. Let’sstart applying the shortcuts.2Code paths throwing exceptions don’t need to return a value, of course, and neither do detectable infiniteloops.Licensed to Rhona Hadida