单行if语句生成强制转换错误 [英] Single line if statement generates cast error

问题描述

C＃的新手，所以我遇到了一个简短陈述的问题。我想转换...

new to C# so I'm having an issue with a short-hand statement. I want to convert...

if (m_dtLastLogin == null)
    drow["LastLogin"] = DBNull.Value;
else
    drow["LastLogin"] = m_dtLastLogin;

到

drow["LastLogin"] = (m_dtLastLogin == null) ? System.DBNull.Value : m_dtLastLogin;

长版本效果很好，但是，短版本会生成错误Type of因为System.DBNull和System.DateTime？之间没有隐式转换，所以无法确定条件。我的支持代码基本上是......

The long-hand version works great, however, the short-hand version generates the error "Type of conditional cannot be determined because there is no implicit conversion between 'System.DBNull' and 'System.DateTime?'". My supporting code is basically...

private DateTime? m_dtLastLogin;
m_dtLastLogin = null;
DataRow drow;
drow = m_oDS.Tables["Users"].Rows[0];

有人可以帮忙我这里的短手吗？

Can someone help me with the short-hand here?

推荐答案

嗯，错误信息很清楚。编译器需要确定整个x？y：z表达式的类型。如果y和z具有相同的类型，则很容易。如果y可转换为z，则表达式的类型为z的类型，同样，如果z可转换为y，则类型为y的类型。

Well, the error message is pretty clear. The compiler needs to determine the type of the whole x?y:z expression. If y and z have the same type, it's easy. If y is convertible to z, the type of the expression is that of z, likewise if z is convertible to y the type is that of y.

在你的情况下，y的类型是DBNull，y的类型是m_dtLastLogin的类型（可能是日期时间）。这两种类型不能相互转换，并且没有公共基类型（Object除外），因此编译器不知道该怎么做。

In your case the type of y is DBNull, the type of y is the type of m_dtLastLogin (probably a datetime). Those two types cannot be converted into each other and have no common base type (except for Object), so the compiler doesn't know what to do.

但是，您可以通过将y或z强制转换为对象来帮助编译器：

You can help the compiler, however, by casting either y or z to object:

drow["LastLogin"] = (m_dtLastLogin == null) ? (object)System.DBNull.Value : m_dtLastLogin;

或

drow["LastLogin"] = (m_dtLastLogin == null) ? System.DBNull.Value : (object)m_dtLastLogin;

这样整个表达式都有类型对象，然后可以将其赋值给卓尔[ LastLogin] 。

This way the whole expression has the type object, which can then be assigned to drow["LastLogin"].

参考：
C＃语言规范 - http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-334.pdf

第14.13节，条件运算符

Section 14.13, Conditional operator

引用

第二和第三个操作数？：运算符控制条件表达式的类型。设X和Y是第二个和第三个操作数的类型。然后，

The second and third operands of the ?: operator control the type of the conditional expression. Let X and Y be the types of the second and third operands. Then,

• 如果X和Y是相同的类型，那么这就是条件表达式的类型。

• If X and Y are the same type, then this is the type of the conditional expression.

否则，如果从X到Y存在隐式转换（第13.1节），而不是从Y到X，则Y是条件表达式的类型。

Otherwise, if an implicit conversion (§13.1) exists from X to Y, but not from Y to X, then Y is the type of the conditional expression.

否则，如果从Y到X存在隐式转换（第13.1节），而不是从X到Y，则X是条件表达式的类型。

Otherwise, if an implicit conversion (§13.1) exists from Y to X, but not from X to Y, then X is the type of the conditional expression.

否则，无法确定表达式类型，并发生编译时错误。

Otherwise, no expression type can be determined, and a compile-time error occurs.

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