可选参数＆QUOT;必须是一个编译时常＆QUOT; [英] Optional parameters "must be a compile-time constant"
问题描述
我有一类两部分文件划分，如：
I have a class divided in two partial files, like this:
public partial class PersonRepository : BaseRepository<Person>
{
public static readonly string ColumnID = "ID";
...
和
public partial class PersonRepository : BaseRepository<Person>
{
public List<Person> GetByCompany(int companyID, string sortExpression = ColumnID)
{
...
但是编译器口口声声说 SORTEX pression 必须是一个编译时间常数。对我来说，似乎是一个完美的编译时间常数，所以我不明白问题出在哪里。
But the compiler keeps saying that sortExpression "must be a compile-time constant". To me it seems a perfect compile-time constant, so I don't understand where the problem is.
感谢您的帮助！
亚历山德罗
推荐答案
没有，前pression PersonRespository.ColumnID 不属于编译时间常数。这位前pressionID是，但是这不是你用什么作为默认参数。
No, the expression PersonRespository.ColumnID is not classified as a compile-time constant. The expression "ID" is, but that's not what you're using as the default parameter.
特别是，如果 ColumnID的是只是一个正常的领域，那么任何对它的引用将得到解决的为的一个领域 - 所以，如果你编译它指的是现场组装，然后更改值，并重建包含的组件 PersonRepository ，引用组件的将会的看到这种变化。
In particular, if ColumnID is "just a normal field" then any references to it will be resolved as a field - so if you compile an assembly which refers to the field, then change the value and rebuild the assembly containing PersonRepository, the referring assembly will see that change.
如果你改变你的声明：
public const string ColumnID = "ID";
那么它的是的编译时间常数EX pression。这意味着在我们的previous情况下，常量的值被烤成任何code，指的是 - 后来更改值而无需重新编译，当提到code的不会更改使用由参考code的值。
then it is a compile-time constant expression. That means in our previous scenario, the value of the constant is baked into any code that refers to it - and changing the value later without recompiling that referring code won't change the value used by that referring code.
有关什么算是一个恒定的前pression更多详细信息，请参见C＃4语言规范的第7.19。
See section 7.19 of the C# 4 language specification for more details about what counts as a constant expression.
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