c# - 默认参数值必须是编译时常量错误消息

Question

我似乎收到一条错误消息，如标题中提到的那样。
“'randomizer'的默认参数值必须是编译时常量错误消息。”

对于此代码:

    public Random Randomizer {
            get;
            private set;
        }
        public ShuffleDeck( Random randomizer = new Random(DateTime.Now.Millisecond)) {
            this.Randomizer = randomizer;
        }

我不知道为什么吗？有人有任何线索吗？

Answer 1

and I cant figure out why?

嗯，这就是该语言的 documentation所说的。

A default value must be one of the following types of expressions:

• a constant expression;
• an expression of the form new ValType(), where ValType is a value type, such as an enum or a struct;
• an expression of the form default(ValType), where ValType is a value type.

您可以做(但请参见 Servy's comment):

public ShuffleDeck(Random randomizer = null) {
    if (randomizer == null)
        this.Randomizer = new Random(DateTime.Now.Millisecond);
    else
        this.Randomizer = randomizer;
   //or this.Randomizer = randomizer ?? new Random());
}

或者，您可以具有如下重载构造函数:

public ShuffleDeck()
    : this(new Random(DateTime.Now.Millisecond))
{
}
public ShuffleDeck(Random randomizer)
{
    this.Randomizer = randomizer;
}