c# MoreLinq 之 AssertCount

前言

本系列是对MoreLinq库的学习与总结，分析各个Api的实现方式及用法，也为能写出更高效的Linq打下基础。

AssertCount断言个数，如果列表个数与断言个数不同则抛异常

• 应用

void AssertCountTest()
{
    var nums = Enumerable.Range(0, 2);
    nums.AssertCount(10, (cmp, count) => new Exception($"出现异常：cmp={cmp},count={count}")).Dump();//出现异常：cmp=-1,count=10
}

断言列表个数与给定值是否一样, 不一样则执行自定义异常。自定义抛异常方法也可省略。

• 源码：

public static class MoreEnumerable
{
    static readonly Func<int, int, Exception> DefaultErrorSelector = OnAssertCountFailure;

    static Exception OnAssertCountFailure(int cmp, int count)
    {
        var message = cmp < 0
           ? "Sequence contains too few elements when exactly {0} were expected."
           : "Sequence contains too many elements when exactly {0} were expected.";
        return new SequenceException(string.Format(message, count.ToString("N0")));
        //ToString("N0")  数字的千分位分隔符，如100,000,000
    }

    public static IEnumerable<TSource> AssertCount<TSource>(this IEnumerable<TSource> source, int count, Func<int, int, Exception> errorSelector) =>
            AssertCountImpl(source, count, errorSelector);

    public static IEnumerable<TSource> AssertCount<TSource>(this IEnumerable<TSource> source, int count) =>
          AssertCountImpl(source, count, DefaultErrorSelector);

    public static IEnumerable<TSource> AssertCountImpl<TSource>(IEnumerable<TSource> source, int count, Func<int, int, Exception> errorSelector)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (count < 0) throw new ArgumentOutOfRangeException(nameof(count));
        if (errorSelector == null) throw new ArgumentNullException(nameof(errorSelector));
        return source.TryGetCollectionCount() is { } collectionCount
        ? collectionCount == count
            ? source
            : From<TSource>(() => throw errorSelector(collectionCount.CompareTo(count), count))
        : _();
        IEnumerable<TSource> _()
        {
            var iterations = 0;
            foreach (var element in source)
            {
                iterations++;
                if (iterations > count)
                    throw errorSelector(1, count);
                yield return element;
            }
            if (iterations != count)
                throw errorSelector(-1, count);
        }
    }

    public static int? TryGetCollectionCount<T>(this IEnumerable<T> source) => source switch
    {
        null => throw new ArgumentNullException(nameof(source)),
        ICollection<T> collection => collection.Count,
        IReadOnlyCollection<T> collection => collection.Count,
        _ => null
    };

    public static IEnumerable<T> Evaluate<T>(this IEnumerable<Func<T>> functions) =>
            from f in functions ?? throw new ArgumentNullException(nameof(functions))
            select f();

    public static IEnumerable<T> From<T>(params Func<T>[] functions)
    {
        if (functions == null) throw new ArgumentNullException(nameof(functions));
        return Evaluate(functions);
    }
}

public class SequenceException : Exception
{
    const string DefaultMessage = "Error in sequence.";
    public SequenceException() :
         this(null)
    { }
    public SequenceException(string? message) :
          this(message, null)
    { }

    public SequenceException(string? message, Exception? innerException) :
         base(string.IsNullOrEmpty(message) ? DefaultMessage : message, innerException)
    { }
}

分析：
源码调用的量有些大，不过没关系，核心部分很简单

source.TryGetCollectionCount() is { } collectionCount
        ? collectionCount == count
            ? source
            : From<TSource>(() => throw errorSelector(collectionCount.CompareTo(count), count))
        : _();

首先判断是否是ICollection<T>,从方法TryGetCollectionCount<T>可知如果是则直接返回个数，如果不是，则返回null。
情况1： 返回个数之后再判断collectionCount == count是否相等，相等则直接返回列表，不相等则执行异常， From<TSource>(() => throw errorSelector(collectionCount.CompareTo(count), count))。
注意执行抛异常的方法是From，最终执行的是from f in functions select f()，即分别执行给定的action
情况2：返回null，说明不是ICollection<T>，则需要遍历执行列表，当遍历的个数大于断言的值时，执行异常操作，cmp=1。遍历结束之后遍历个数小于断言的值时，执行异常操作，cmp=-1。
查看默认的断言异常方法

var message = cmp < 0
           ? "Sequence contains too few elements when exactly {0} were expected."
           : "Sequence contains too many elements when exactly {0} were expected.";

cmp 1和-1 的判断返回值，正是我们希望的断言语句。

另外： ToString("N0") 是将数字转为带有千分位分隔符的字符串，如100,000,000

作者：wwmin
微信公众号: DotNet技术说
本文链接：https://www.jianshu.com/p/ca4e75ab25c3
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