<MSDN笔记>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);