.net Nullable Enum 可空类型问题
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Nullable Enum nullable type question
提问by Michael Kniskern
I get the following compilation error with the following source code:
我使用以下源代码收到以下编译错误：
Compilation Error:
编译错误：
Type of conditional expression cannot be determined because there is no implicit conversion between '' and 'MyEnum'
无法确定条件表达式的类型，因为 '' 和 'MyEnum' 之间没有隐式转换
Source Code
源代码
public enum MyEnum
{
Value1, Value2, Value3
}
public class MyClass
{
public MyClass() {}
public MyEnum? MyClassEnum { get; set; }
}
public class Main()
{
object x = new object();
MyClass mc = new MyClass()
{
MyClassEnum = Convert.IsDBNull(x) : null ?
(MyEnum) Enum.Parse(typeof(MyEnum), x.ToString(), true)
};
}
How can I resolve this error?
我该如何解决这个错误？
回答by Luke Quinane
I think you just need to cast the result of Enum.Parseto MyEnum?. This is the case with nullable ints at least. E.g.:
我认为您只需要将结果转换为Enum.Parseto MyEnum?。至少可空整数就是这种情况。例如：
int? i;
i = shouldBeNull ? null : (int?) 123;
So:
所以：
MyClassEnum = Convert.IsDBNull(x)
? null
: (MyEnum?) Enum.Parse(typeof(MyEnum), x.ToString(), true)
回答by M4N
There is a syntax error in your code: the position of ':' and '?' must be exchanged:
您的代码中存在语法错误：':' 和 '?' 的位置 必须交换：
MyClassEnum = Convert.IsDBNull(x) ? null :
(MyEnum) Enum.Parse(typeof(MyEnum), x.ToString(), true)
BTW:
顺便提一句：
as far as I know, the recommended way is to use a enum-element named 'None' instead of a Nullable enum, e.g:
据我所知，推荐的方法是使用名为“None”的枚举元素而不是 Nullable 枚举，例如：
public enum MyEnum
{
None, Value1, Value2, Value3
}
and
和
MyClassEnum = Convert.IsDBNull(x) ? MyEnum.None :
(MyEnum) Enum.Parse(typeof(MyEnum), x.ToString(), true);
回答by Michael Kniskern
I think you will just need to cast the result to (MyEnum?) rather than (MyEnum)?
我认为您只需要将结果转换为 (MyEnum?) 而不是 (MyEnum)？
