Lambda表达式是表示匿名方法的一种简洁方式。匿名方法和Lambda表达式都允许您内联定义方法实现，但是，匿名方法显式地要求您定义方法的参数类型和返回类型。Lambda表达式使用c# 3.0的类型推断特性，该特性允许编译器根据上下文推断变量的类型。这是非常方便的，因为这节省了我们大量的输入!

Microsoft has given us a cleaner, more convenient way of creating anonymous delegates called Lambda expressions. However, there is not a lot of attention being paid to the expressions portion of this statement. Microsoft released a entire namespace, System.Linq.Expressions, which contains classes to create expression trees based on lambda expressions. Expression trees are made up of objects that represent logic. For example, x = y + z is an expression that might be part of an expression tree in .Net. Consider the following (simple) example:

using System;
using System.Linq;
using System.Linq.Expressions;


namespace ExpressionTreeThingy
{
    class Program
    {
        static void Main(string[] args)
        {
            Expression<Func<int, int>> expr = (x) => x + 1; //this is not a delegate, but an object
            var del = expr.Compile(); //compiles the object to a CLR delegate, at runtime
            Console.WriteLine(del(5)); //we are just invoking a delegate at this point
            Console.ReadKey();
        }
    }
}

This example is trivial. And I am sure you are thinking, "This is useless as I could have directly created the delegate instead of creating an expression and compiling it at runtime". And you would be right. But this provides the foundation for expression trees. There are a number of expressions available in the Expressions namespaces, and you can build your own. I think you can see that this might be useful when you don't know exactly what the algorithm should be at design or compile time. I saw an example somewhere for using this to write a scientific calculator. You could also use it for Bayesian systems, or for genetic programming (AI). A few times in my career I have had to write Excel-like functionality that allowed users to enter simple expressions (addition, subtrations, etc) to operate on available data. In pre-.Net 3.5 I have had to resort to some scripting language external to C#, or had to use the code-emitting functionality in reflection to create .Net code on the fly. Now I would use expression trees.

Lambda表达式是表示匿名方法的一种简洁方式。匿名方法和Lambda表达式都允许您内联定义方法实现，但是，匿名方法显式地要求您定义方法的参数类型和返回类型。Lambda表达式使用c# 3.0的类型推断特性，该特性允许编译器根据上下文推断变量的类型。这是非常方便的，因为这节省了我们大量的输入!

Lambda表达式对于匿名委托来说是一种更简单的语法，可以在任何可以使用匿名委托的地方使用。然而，反过来就不对了;lambda表达式可以转换为表达式树，这允许很多魔术，如LINQ到SQL。

下面是一个使用匿名委托的LINQ to Objects表达式的例子，然后是lambda表达式，以显示它们是多么容易。

// anonymous delegate
var evens = Enumerable
                .Range(1, 100)
                .Where(delegate(int x) { return (x % 2) == 0; })
                .ToList();

// lambda expression
var evens = Enumerable
                .Range(1, 100)
                .Where(x => (x % 2) == 0)
                .ToList();

Lambda表达式和匿名委托相对于编写单独的函数有一个优势:它们实现了闭包，允许您将局部状态传递给函数，而无需向函数添加参数或创建一次性对象。

表达式树是c# 3.0的一个非常强大的新特性，它允许API查看表达式的结构，而不仅仅是获取一个可以执行的方法的引用。API只需要将委托参数转换为Expression<T>参数，编译器就会从lambda而不是匿名委托生成表达式树:

void Example(Predicate<int> aDelegate);

被称为:

Example(x => x > 5);

就变成:

void Example(Expression<Predicate<int>> expressionTree);

后者将获得描述表达式x > 5的抽象语法树的表示。LINQ to SQL依赖于此行为，能够将c#表达式转换为服务器端过滤/排序等所需的SQL表达式。

这是一种将小操作放在非常接近使用位置的地方的方法(与在接近使用点的地方声明变量没有什么不同)。这将使您的代码更具可读性。通过匿名化表达式，如果在其他地方使用了该函数并对其进行了修改以“增强”它，那么其他人就很难破坏您的客户端代码。

同样，为什么需要使用foreach?你可以用一个简单的for循环来完成foreach中的所有事情，或者直接使用IEnumerable。答案:你不需要它，但它使你的代码更可读。

当我想使用另一个控件为某个控件的事件声明一个处理程序时，我发现它们很有用。 要做到这一点，你必须将控件的引用存储在类的字段中，这样你就可以在不同的方法中使用它们。

private ComboBox combo;
private Label label;

public CreateControls()
{
    combo = new ComboBox();
    label = new Label();
    //some initializing code
    combo.SelectedIndexChanged += new EventHandler(combo_SelectedIndexChanged);
}

void combo_SelectedIndexChanged(object sender, EventArgs e)
{
    label.Text = combo.SelectedValue;
}

多亏了lambda表达式，你可以这样使用它:

public CreateControls()
{
    ComboBox combo = new ComboBox();
    Label label = new Label();
    //some initializing code
    combo.SelectedIndexChanged += (s, e) => {label.Text = combo.SelectedValue;};
}

容易得多。