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.

很多时候，您只在一个地方使用功能，所以创建方法只会使类变得混乱。

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。

在c#中，我们不能像在JavaScript中那样将函数作为参数传递。解决方法是使用委托。

当我们想参数化行为而不是值时，我们使用委托。Lambdas是编写委托的实用语法，可以很容易地将行为作为函数传递。

The biggest benefit of lambda expressions and anonymous functions is the fact that they allow the client (programmer) of a library/framework to inject functionality by means of code in the given library/framework ( as it is the LINQ, ASP.NET Core and many others ) in a way that the regular methods cannot. However, their strength is not obvious for a single application programmer but to the one that creates libraries that will be later used by others who will want to configure the behaviour of the library code or the one that uses libraries. So the context of effectively using a lambda expression is the usage/creation of a library/framework.

此外，由于它们描述的是一次性使用的代码，所以它们不必是类的成员，这样会导致代码更加复杂。想象一下，每当我们想要配置类对象的操作时，都必须声明一个焦点不明确的类。

您还可以在编写作用于方法的泛型代码时使用lambda表达式。

例如:计算方法调用所花费的时间的泛型函数。(即这里的动作)

public static long Measure(Action action)
{
    Stopwatch sw = new Stopwatch();
    sw.Start();
    action();
    sw.Stop();
    return sw.ElapsedMilliseconds;
}

你可以使用lambda表达式调用上述方法，如下所示，

var timeTaken = Measure(() => yourMethod(param));

表达式允许您从方法和out参数中获取返回值

var timeTaken = Measure(() => returnValue = yourMethod(param, out outParam));