IEnumerable - 更新foreach循环内的对象

Question

我有一个非常简单的程序,它创建了一堆对象并迭代它们来设置每个对象的Priority属性.

static void Main(string[] args)
{
    foreach (var obj in ObjectCreator.CreateObjectsWithPriorities())
        Console.WriteLine(String.Format("Object #{0} has priority {1}",
                                         obj.Id, obj.Priority));
}

class ObjectCreator
{
    public static IEnumerable<ObjectWithPriority> CreateObjectsWithPriorities()
    {
        var objs = new[] { 1, 2, 3 }.Select(i => new ObjectWithPriority() { Id = i });
        ApplyPriorities(objs);
        return objs;
    }

    static void ApplyPriorities(IEnumerable<ObjectWithPriority> objs)
    {
        foreach (var obj in objs)
        {
            obj.Priority = obj.Id * 10;
            Console.WriteLine(String.Format("Set priority of object #{0} to {1}", obj.Id, obj.Priority));
        }
    }
}

class ObjectWithPriority
{
    public int Id { get; set; }
    public int Priority { get; set; }
}

我期望IEnumerable在Main方法中包含具有修改优先级的对象.但是,所有这些都具有默认值0.这是日志:

Set priority of object #1 to 10 
Set priority of object #2 to 20 
Set priority of object #3 to 30 
Object #1 has priority 0 
Object #2 has priority 0 
Object #3 has priority 0

suche行为的原因是什么？为了让我的优先事项有效,我应该在这里改变什么？

Answer 1

当你这样做:

var objs = new[] { 1, 2, 3 }.Select(i => new ObjectWithPriority() { Id = i });

您只是创建一个延迟评估的迭代器,这不会分配一个数组/列表来存储ObjectWithPriorty您的项目.每次枚举迭代器时,它将再次迭代值并ObjectWithPriority为每次迭代投影一次,但会丢弃它们.

您要做的是在传递查询之前实现查询,因此稍后您将实际修改已分配的列表/数组.这可以使用Enumerable.ToList或实现Enumerable.ToArray:

public static IEnumerable<ObjectWithPriority> CreateObjectsWithPriorities()
{
    var objs = new[] { 1, 2, 3 }.Select(i => new ObjectWithPriority() { Id = i })
                                .ToList();
    ApplyPriorities(objs);
    return objs;
}

您可以额外使用Enumerable.Range而不是分配固定大小的数组,这将根据请求懒惰地投射数字:

var objs = Enumerable.Range(1, 3).Select(i => new ObjectWithPriority { Id = i })
                                 .ToList();