Tuesday, 6 September 2011

Lambda Expressions in C# 3.0


A lambda expression is an anonymous function that can contain expressions and statements, and can be used to create delegates or expression tree types.
All lambda expressions use the lambda operator =>, which is read as "goes to". The left side of the lambda operator specifies the input parameters (if any) and the right side hold the expression or statement block. The lambda expression x => x * x is read "x goes to x times x".

Expression Lambda:
A lambda expression with an expression on the right side is called an expression lambda. Expression lambdas are used extensively in the construction of Expression Tree (C# and Visual Basic).  An expression lambda returns the result of the expression and takes the following basic form:
            (input parameters) => expression
The parentheses are optional only if the lambda has one input parameter; otherwise they are required. Two or more input parameters are separated by commas enclosed in parentheses:
            (x, y) => x == y
Sometimes it is difficult or impossible for the compiler to infer the input types. When this occurs, you can specify the types explicitly as shown in the following example:
            (int x, string s) => s.Length > x
Specify zero input parameters with empty parentheses:
            () => SomeMethod()
Note in the previous example that the body of an expression lambda can consist of a method call. However, if you are creating expression trees that will be consumed in another domain, such as SQL Server, you should not use method calls in lambda expressions. The methods will have no meaning outside the context of the .NET common language runtime.

Statement Lambda:
A statement lambda resembles an expression lambda except that the statement(s) is enclosed in braces:
            (input parameters) => {statement;}
The body of a statement lambda can consist of any number of statements; however, in practice there are typically no more than two or three.
delegate void TestDelegate(string s);
TestDelegate myDel = n => { string s = n + " " + "World"; Console.WriteLine(s); };
myDel("Hello");
Statement lambdas, like anonymous methods, cannot be used to create expression trees.

Lambdas with the Standard Query Operators:
Many Standard query operators have an input parameter whose type is one of the Func<T, TResult> family of generic delegates. The Func<T, TResult> delegates use type parameters to define the number and type of input parameters, and the return type of the delegate. Func delegates are very useful for encapsulating user-defined expressions that are applied to each element in a set of source data. For example, consider the following delegate type:
            public delegate TResult Func<TArg0, TResult>(TArg0 arg0)
The delegate can be instantiated as Func<int,bool> myFunc where int is an input parameter and bool is the return value. The return value is always specified in the last type parameter. Func<int, string, bool> defines a delegate with two input parameters, int and string, and a return type of bool. The following Func delegate, when it is invoked, will return true or false to indicate whether the input parameter is equal to 5:
            Func<int, bool> myFunc = x => x == 5;
            bool result = myFunc(4); // returns false of course
You can also supply a lambda expression when the argument type is an Expression<Func>, for example in the standard query operators that are defined in System.Linq.Queryable. When you specify an Expression<Func> argument, the lambda will be compiled to an expression tree.
A standard query operator, the Count method, is shown here:
            int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
            int oddNumbers = numbers.Count(n => n % 2 == 1);
The compiler can infer the type of the input parameter, or you can also specify it explicitly. This particular lambda expression counts those integers (n) which when divided by two have a remainder of 1.
The following method will produce a sequence that contains all the elements in the numbers array that are to the left of the 9, because that is the first number in the sequence that does not meet the condition:
var firstNumbersLessThan6 = numbers.TakeWhile(n => n < 6);
This example shows how to specify multiple input parameters by enclosing them in parentheses. The method returns all the elements in the numbers array until a number is encountered whose value is less than its position. Do not confuse the lambda operator (=>) with the greater than or equal operator (>=).
var firstSmallNumbers = numbers.TakeWhile((n, index) => n >= index);

Type Inference in Lambdas:
When writing lambdas, you often do not have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the underlying delegate type, and other factors as described in the C# Language Specification.
For most of the standard query operators, the first input is the type of the elements in the source sequence. So if you are querying an IEnumerable<Customer>, then the input variable is inferred to be a Customer object, which means you have access to its methods and properties:
customers.Where(c => c.City == "London");
The general rules for lambdas are as follows:
·         The lambda must contain the same number of parameters as the delegate type.
·         Each input parameter in the lambda must be implicitly convertible to its corresponding delegate parameter.
·         The return value of the lambda (if any) must be implicitly convertible to the delegate's return type.
Note that lambda expressions in themselves do not have a type because the common type system has no intrinsic concept of "lambda expression." However, it is sometimes convenient to speak informally of the "type" of a lambda expression. In these cases the type refers to the delegate type or Expression type to which the lambda expression is converted.

Variable Scope in Lambda Expressions:
Lambdas can refer to outer variables that are in scope in the enclosing method or type in which the lambda is defined. Variables that are captured in this manner are stored for use in the lambda expression even if variables would otherwise go out of scope and be garbage collected. An outer variable must be definitely assigned before it can be consumed in a lambda expression.
The following rules apply to variable scope in lambda expressions:
·         A variable that is captured will not be garbage-collected until the delegate that references it goes out of scope.
·         Variables introduced within a lambda expression are not visible in the outer method.
·         A lambda expression cannot directly capture a ref or out parameter from an enclosing method.
·         A return statement in a lambda expression does not cause the enclosing method to return.
·         A lambda expression cannot contain a goto statement, break statement, or continue statement whose target is outside the body or in the body of a contained anonymous function.

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