函数委托与函数

问题描述

有人可以告诉我使用委托而不是调用函数本身的优势，如下所示(或者换句话说，为什么选择选项 A 而不是选项 B)?昨晚我在查看某人的 linq 代码，他们有类似于选项 A 的内容，但它被用于返回已编译的 linq 查询.

Can someone tell me the advantages of using a delegate as opposed to calling the function itself as shown below (or in other words why choose Option A over Option B)? I was looking at someone's linq code last night and they had something similar to Option A but it was being used to return a compiled linq query.

我意识到前者现在可以传递给其他功能......只是不确定它的实用性.顺便说一句，我意识到这不会按原样编译.在发布之前取消注释其中一个功能.TYIA

I realize the former can now be passed around to other functions.. just not sure of its practicality. BTW, I realize this wouldn't compile as-is.. uncommented one of the functions before posting. TYIA

class Program
{
    static void Main(string[] args)
    {   
        Console.WriteLine(SayTwoWords("Hello", "World"));
        Console.ReadKey();
    }

    // Option A
    private static Func<string, string, string>
        SayTwoWords = (a, b) => String.Format("{0} {1}", a, b);

    // Option B
    private static string SayTwoWords(string a, string b)
    {
        return String.Format("{0} {1}", a, b);
    }        
}

************编辑************

************EDIT************

不确定它是否更好地解释了我的问题，但这里是最初让我想到这个的代码类型的示例:

Not sure if it explains my question better but here is an example of the type of code that originally got me thinking about this:

public static class clsCompiledQuery
{
    public static Func<DataContext, string, IQueryable<clsCustomerEntity>>
        getCustomers = CompiledQuery.Compile((DataContext db, string strCustCode)
            => from objCustomer in db.GetTable<clsCustomerEntity>()
            where objCustomer.CustomerCode == strCustCode
            select objCustomer);
}

这样写函数有什么好处吗?

Is there any advantage to writing a function in this way?

推荐答案

你贴的代码没有优势.在您的代码中，使用委托只会增加复杂性以及额外的运行时成本 - 所以您最好直接调用该方法.

There is no advantage in the code you posted. In your code, using the delegate just adds complexity as well as an extra runtime cost - so you're better off just calling the method directly.

但是，委托有很多用途.传递"给其他方法是主要用法，但存储一个函数并在以后使用它也非常有用.

However, delegates have many uses. "Passing around" to other methods is the primary usage, though storing a function and using it later is also very useful.

LINQ 完全建立在这个概念之上.当你这样做时:

LINQ is built on top of this concept entirely. When you do:

var results = myCollection.Where(item => item == "Foo");

您将委托(定义为 lambda:item => item == "Foo")传递给 LINQ 库中的 Where 函数.这就是让它正常工作的原因.

You're passing a delegate (defined as a lambda: item => item == "Foo") to the Where function in the LINQ libraries. This is what makes it work properly.

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