奇数返回语法语句 [英] Odd return syntax statement

问题描述

我知道这听起来很奇怪，但我什至不知道如何在互联网上搜索此语法，我也不知道确切的含义。

I know this may sound strange but I don't know even how to search this syntax in internet and also I am not sure what exactly means.

所以我已经查看了MoreLINQ代码，然后我注意到了这种方法

So I've watched over some MoreLINQ code and then I noticed this method

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (keySelector == null) throw new ArgumentNullException(nameof(keySelector));

    return _(); IEnumerable<TSource> _()
    {
        var knownKeys = new HashSet<TKey>(comparer);
        foreach (var element in source)
        {
            if (knownKeys.Add(keySelector(element)))
                yield return element;
        }
    }
}

这个奇怪的回报声明是什么？ return _（）; ？

What is this odd return statement? return _(); ?

推荐答案

这是C＃7.0，它支持本地函数...。

This is C# 7.0 which supports local functions....

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
       this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
    {
        if (source == null) throw new 
           ArgumentNullException(nameof(source));
        if (keySelector == null) throw 
             new ArgumentNullException(nameof(keySelector));

        // This is basically executing _LocalFunction()
        return _LocalFunction(); 

        // This is a new inline method, 
        // return within this is only within scope of
        // this method
        IEnumerable<TSource> _LocalFunction()
        {
            var knownKeys = new HashSet<TKey>(comparer);
            foreach (var element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                    yield return element;
            }
        }
    }

当前C＃中 Func< T>

public static IEnumerable<TSource> DistinctBy<TSource, TKey>(
       this IEnumerable<TSource> source,
        Func<TSource, TKey> keySelector, IEqualityComparer<TKey> comparer)
    {
        if (source == null) throw new 
           ArgumentNullException(nameof(source));
        if (keySelector == null) throw 
             new ArgumentNullException(nameof(keySelector));

        Func<IEnumerable<TSource>> func = () => {
            var knownKeys = new HashSet<TKey>(comparer);
            foreach (var element in source)
            {
                if (knownKeys.Add(keySelector(element)))
                    yield return element;
            }
       };

        // This is basically executing func
        return func(); 

    }

窍门是，_（）在

实际使用局部函数

以上该示例只是一个演示如何使用内联方法的示例，但是最有可能的是，如果您只打算调用一次方法，那么它就没有用了。

Above example is just a demonstration of how inline method can be used, but most likely if you are going to invoke method just once, then it is of no use.

但是在上面的示例中，如 Phoshi 和 Luaan 的评论中所述，使用本地功能。由于除非有人进行迭代，否则不会执行带有yield return的函数，因此在这种情况下，即使没有人迭代该值，也将执行局部函数之外的方法并执行参数验证。

But in example above, as mentioned in comments by Phoshi and Luaan, there is an advantage of using local function. Since function with yield return will not be executed unless someone iterates it, in this case method outside local function will be executed and parameter validation will be performed even if no one will iterate the value.

很多时候我们在方法中重复了代码，让我们来看这个例子。

Many times we have repeated code in method, lets look at this example..

  public void ValidateCustomer(Customer customer){

      if( string.IsNullOrEmpty( customer.FirstName )){
           string error = "Firstname cannot be empty";
           customer.ValidationErrors.Add(error);
           ErrorLogger.Log(error);
           throw new ValidationError(error);
      }

      if( string.IsNullOrEmpty( customer.LastName )){
           string error = "Lastname cannot be empty";
           customer.ValidationErrors.Add(error);
           ErrorLogger.Log(error);
           throw new ValidationError(error);
      }

      ... on  and on... 
  }

我可以使用...

  public void ValidateCustomer(Customer customer){

      void _validate(string value, string error){
           if(!string.IsNullOrWhitespace(value)){

              // i can easily reference customer here
              customer.ValidationErrors.Add(error);

              ErrorLogger.Log(error);
              throw new ValidationError(error);                   
           }
      }

      _validate(customer.FirstName, "Firstname cannot be empty");
      _validate(customer.LastName, "Lastname cannot be empty");
      ... on  and on... 
  }

这篇关于奇数返回语法语句的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！