.net 奇数返回语法语句

发布于07月26日

我知道这听起来可能很奇怪，但我甚至不知道如何在互联网上搜索这种语法，而且我也不知道确切的意思.

所以我看了一些MoreLINQ代码，然后我注意到了这个方法

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;
        }
    }
}

这张奇怪的退货单是多少？return _();？

推荐答案

这是支持本地功能的C#7.0....

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;
            }
        }
    }

当前版本为Func<T>的C#

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(); 

    }

诀窍是，()在使用后被声明，这是非常好的.

Pratical use of local functions

上面的示例只是演示如何使用内联方法，但如果只调用一次该方法，则很可能没有任何用处.

但在上面的例子中，正如Phoshi和Luaan的 comments 中所提到的，使用局部函数有一个优点.由于除非有人对函数进行迭代，否则不会执行具有yield return的函数，因此在这种情况下，将执行本地函数之外的方法，并执行参数验证，即使没有人对该值进行迭代.

我们在方法中多次重复代码，让我们看一下这个示例.

  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... 
  }