查找谓词DayOfWeek问题c#

Question

最近，当我试图在一个DayOfWeek列表中找到一个特定的一天时，我遇到了一个问题--它在周日不是很好，但是在其他的日子里都有效。

我做了一个谓词，但是当我在一个空列表或不包含星期日的列表中寻找星期日时，它就会找到它。

例子：

namespace testing1
{
    public partial class Timer : Form
    {
        public static List<dayOfWeek> day = new List<dayOfWeek>();//I can set things in it

        public Timer()
        {
            InitializeComponent();
        }

        private void Timer_Load(object sender, EventArgs e)
        {
                Console.WriteLine(day.Count); //return 0 when I instantiate the form without inserting from an another form.

                checkBox1.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Monday; }) == dayOfWeek.Monday; // return false OK
                checkBox2.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Tuesday; }) == dayOfWeek.Tuesday; // return false OK
                checkBox3.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Wednesday; }) == dayOfWeek.Wednesday; // return false OK
                checkBox4.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Thursday; }) == dayOfWeek.Thursday; // return false OK
                checkBox5.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Friday; }) == dayOfWeek.Friday; // return false OK
                checkBox6.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Saturday; }) == dayOfWeek.Saturday; // return false OK
                checkBox7.Checked = day.Find((dayOfWeek d) => { return d == dayOfWeek.Sunday; }) == dayOfWeek.Sunday; // return true WTF ????
        }
    }
}

我能帮忙吗？

Answer 1

DayOfWeek是一个enum，因此它的值类型、值类型变量不能有空值，除非标记为Nullable<>，这意味着DayOfWeek enum中的第一个值很可能是Sunday，这将是默认值，因此每当Find无法找到该值时，它就返回默认值为Sunday。

您应该使用Contains：

day.Contains(DayOfWeek.Sunday);
//..

Answer 2

Enum是值类型，Find扩展方法在处理这种类型的列表(引用自MSDN)时具有以下行为：

搜索包含值类型的列表时，请确保该类型的默认值不满足搜索谓词。否则，无法区分表示未找到匹配的默认值和碰巧具有该类型默认值的list元素。如果默认值满足搜索谓词，则使用FindIndex方法。