C#种Lambda奇技淫巧之Aggregate

回字的4种写法：

Enumerable.Aggregate 扩展方法在System.Linq命名空间中，是Enumerable类的第一个方法（按字母顺序排名），但确是Enumerable里面相对复杂的方法。

int Sum = listItem.Aggregate((i, j) => i + j);
Console.WriteLine($"Sum:{Sum}");
Console.WriteLine($"值列表:{string.Join(",", listItem)}");

int Sum = listItem.Aggregate((i, j) => i + j);
Console.WriteLine($"Sum:{Sum}");
Console.WriteLine($"值列表:{string.Join(",", listItem)}");

//引用问题
int SumAge = listStudentItem.Aggregate((i, j) =>
{
    i.Age += j.Age;
    return i;
}).Age;
Console.WriteLine($"SumAge:{SumAge}");
Console.WriteLine($"引用列表Error使用:{string.Join(",", listStudentItem)}");

public static partial class Enumerable
{
    public static TSource Aggregate(this IEnumerable source, Func func)
    {
        if (source == null)
        {
            ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
        }

        if (func == null)
        {
            ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
        }

        using (IEnumerator e = source.GetEnumerator())
        {

            if (!e.MoveNext())
            {
                ThrowHelper.ThrowNoElementsException();
            }

            TSource result = e.Current;
            while (e.MoveNext())
            {
                result = func(result, e.Current);
            }
            return result;
        }
    }
}

主要由于“TSource result = e.Current;” 如果是引用类型时将会修改引用类型的值。

int Sum = listItem.Aggregate((i, j) => i + j);
Console.WriteLine($"Sum:{Sum}");
Console.WriteLine($"值列表:{string.Join(",", listItem)}");

//引用问题
int SumAge = listStudentItem.Aggregate((i, j) =>
{
    i.Age += j.Age;
    return i;
}).Age;
Console.WriteLine($"SumAge:{SumAge}");
Console.WriteLine($"引用列表Error使用:{string.Join(",", listStudentItem)}");

InitAggregate();
//没有引用问题
SumAge = listStudentItem.Aggregate(0, (i, j) =>
{
    i += j.Age;
    return i;
});
Console.WriteLine($"SumAge:{SumAge}");
Console.WriteLine($"引用列表:{string.Join(",", listStudentItem)}");

源代码如下：

public static partial class Enumerable
{
    public static TAccumulate Aggregate(this IEnumerable source, TAccumulate seed, Func func)
    {
        if (source == null)
        {
            ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
        }

        if (func == null)
        {
            ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
        }

        TAccumulate result = seed;
        foreach (TSource element in source)
        {
            result = func(result, element);
        }
        return result;
    }
}

private void btnAggregateEqWhereSum_Click(object sender, EventArgs e)
{
    int SumAge = listStudentItem.Where((item,index)=>index%2==0).Sum(a => a.Age);
    Console.WriteLine($"SumAge:{SumAge}");
    Console.WriteLine($"引用列表:{string.Join(",", listStudentItem)}");

    int Sum = listStudentItem.Aggregate(0,(inObj,item,index) => 
    {
        if (index % 2 == 0)
        {
            inObj += item.Age;
        }
        return inObj;
    });
    Console.WriteLine($"Sum:{Sum}");
    Console.WriteLine($"引用列表:{string.Join(",", listStudentItem)}");
}

public static class AggregateExpaned
{
    public static TAccumulate Aggregate(this IEnumerable source, TAccumulate seed, Func func)
    {
        int index = 0;
        using (IEnumerator enumerator = source.GetEnumerator())
        {
            TAccumulate result = seed;
            while (enumerator.MoveNext())
                result = func(result, enumerator.Current, index++);
            return result;
        }
    }
}

public static IEnumerable Where(this IEnumerable source, Func predicate)
{
    if (source == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
    }
    if (predicate == null)
    {
        ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
    }
    return WhereIterator(source, predicate);
}

private static IEnumerable WhereIterator(IEnumerable source, Func predicate)
{
    int index = -1;
    foreach (TSource element in source)
    {
        checked
        {
            index++;
        }
        if (predicate(element, index))
        {
            yield return element;
        }
    }
}