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表达式就像代替委托实例编写的匿名方法。

delegate int MyDelagate (int i);
MyDelagate delSquareFunction = x => x * x;

考虑lambda表达式x => x * x;

输入参数值为x(在=>的左边) 函数逻辑为x * x(在=>的右侧)

lambda表达式的代码可以是语句块，而不是表达式。

x => {return x * x;};

例子

注意:Func是一个预定义的泛型委托。

    Console.WriteLine(MyMethod(x => "Hi " + x));

    public static string MyMethod(Func<string, string> strategy)
    {
        return strategy("Lijo").ToString();
    }

参考文献

委托和接口如何可以互换使用?

匿名函数和表达式对于一次性方法非常有用，这些方法不需要从创建完整方法所需的额外工作中获益。

想想这个例子:

 List<string> people = new List<string> { "name1", "name2", "joe", "another name", "etc" };
 string person = people.Find(person => person.Contains("Joe"));

与

 public string FindPerson(string nameContains, List<string> persons)
 {
     foreach (string person in persons)
         if (person.Contains(nameContains))
             return person;
     return null;
 }

它们在功能上是等价的。

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));

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。

这可能是关于为什么使用lambda表达式的最好解释-> https://youtu.be/j9nj5dTo54Q

总之，这是为了提高代码的可读性，通过重用而不是复制代码来减少错误的机会，并利用发生在幕后的优化。