c# - 具有默认 0（零）值的属性索引器

Question

我在使用属性索引器 (C#) 时发现了一个奇怪的行为。

考虑以下程序：

public class Program
{
    public static void Main(string[] args)
    {
        CustomMessageList cml = new CustomMessageList
        {
            new CustomMessage(), // Type1 
            new CustomMessage(), // Type1
            new CustomMessage(), // Type1
            new CustomMessage(), // Type1
            new CustomMessage(), // Type1
            new CustomMessage()  // Type1
        };

        // Empty
        IEnumerable<CustomMessage> x1 = cml[MessageType.Type2];

        // Contains all elements (6)
        IEnumerable<CustomMessage> x2 = cml[0]; // MessageType.Type1 ????

        // Does not compile!
        IEnumerable<CustomMessage> x3 = cml[1]; // NOT MessageType.Type2 ????
    }
}

public class CustomMessageList : List<CustomMessage>
{
    public IEnumerable<CustomMessage> this[MessageType type]
    {
        get { return this.Where(msg => msg.MessageType == type); }
    }
}

public class CustomMessage
{
    public MessageType MessageType { get; set; }
}

public enum MessageType
{
    Type1,
    Type2,
    Type3
}

为什么我在使用默认索引器（x2 变量）时会返回所有结果？

看来int参数（0）自动转换为枚举类型（Type1）。这不是我所期待的......

提前感谢您的解释！

Answer 1

The C# specification states that 0 (and only zero) is implicitly convertible to any enum type:

6.1.3 An implicit enumeration conversion permits the decimal-integer-literal 0 to be converted to any enum-type and to any nullable-type whose underlying type is an enum-type.

If you don't explicitly assign your enumerations with values, they are 0..n, so Type1 is 0 in your case. This behavior has nothing to do with property indexers.

Since zero is the only implicit conversion that exists, this will compile fine:

IEnumerable<CustomMessage> x2 = cml[0];

But this will not compile because there is no implicit conversion for 1:

IEnumerable<CustomMessage> x2 = cml[1];

In that case, you would need to explicitly convert the value:

IEnumerable<CustomMessage> x2 = cml[(MessageType)1]; //Compiles fine

Answer 2

That is because MessageType enum is in fact integer and Type1 enumeration value is zero. Try this:

public enum MessageType
{
    Type1 = 1,
    Type2,
    Type3
}