虽然在一个开源项目打转转，我尝试的的ToString 的DateTime对象是受阻于编译器。当我跳的定义，我看到这样的：

Whilst playing around in an open source project, my attempt to ToString a DateTime object was thwarted by the compiler. When I jumped to the definition, I saw this:

public DateTime? timestamp;

可能有人请赐教什么这就是所谓的，为什么它可能是有用的？

Might someone please enlighten me on what this is called and why it might be useful?

推荐答案

这是一个的可空类型。可空类型允许值类型（如 INT 和结构，如日期时间）包含空。

This is a nullable type. Nullable types allow value types (e.g. ints and structures like DateTime) to contain null.

在 是语法糖可空＆LT; D​​ateTime的＆GT; ，因为它的使用如此频繁

The ? is syntactic sugar for Nullable<DateTime> since it's used so often.

要叫的ToString（）：

if (timstamp.HasValue) {        // i.e. is not null
    return timestamp.Value.ToString();
}
else {
    return "<unknown>";   // Or do whatever else that makes sense in your context
}