C# 0 The Complete Reference

incr

 is passed to 

count

.

The result is returned. The second declaration assigns to 

isEven

 an expression that returns 

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true if the argument is even and false otherwise. Thus, it is compatible with the 

IsEven

delegate declaration.

At this point, you might be wondering how the compiler knows the type of the data 

used in a lambda expression. For example, in the lambda expression assigned to 

incr

, how 

does the compiler know that 

count

 is an 

int

? The answer is that the compiler infers the type 

of the parameter and the expression’s result type from the delegate type. Thus, the lambda 

parameters and return value must be compatible with the parameter type(s) and return type 

of the delegate.

Although type inference is quite useful, in some cases, you might need to explicitly 

specify the type of a lambda parameter. To do so, simply include the type name. For 

example, here is another way to declare the 

incr

 delegate instance:

Incr incr = (int count) => count + 2;

Notice now that 

count

 is explicitly declared as an 

int

. Also notice the use of parentheses. 

They are now necessary. (Parentheses can be omitted only when exactly one parameter is 

specified and no type specifier is used.)

Although the preceding two lambda expressions each used one parameter, lambda 

expressions can use any number, including zero. When using more than one parameter you 

must

 enclose them within parentheses. Here is an example that uses a lambda expression to 

determine if a value is within a specified range:

(low, high, val) => val >= low && val <= high;

Here is a delegate type that is compatible with this lambda expression:

delegate bool InRange(int lower, int upper, int v);

Thus, you could create an 

InRange

 delegate instance like this:

InRange rangeOK = (low, high, val) => val >= low && val <= high;

After doing so, the lambda expression can be executed as shown here:

if(rangeOK(1, 5, 3)) Console.WriteLine("3 is within 1 to 5.");

One other point: Lambda expressions can use outer variables in the same way as 

anonymous methods, and they are captured in the same way.

Statement Lambdas

As mentioned, there are two basic flavors of the lambda expression. The first is the expression 

lambda, which was discussed in the preceding section. As explained, the body of an 

expression lambda consists solely of a single expression. The second type of lambda 

expression is the 

statement lambda.

 A statement lambda expands the types of operations that 

can be handled within a lambda expression because it allows the body of lambda to contain 

multiple statements. For example, using a statement lambda you can use loops, 

if

 statements, 

declare variables, and so on. A statement lambda is easy to create. Simply enclose the body 

within braces. Aside from allowing multiple statements, it works much like the expression 

lambdas just discussed.

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