当前上下文错误中是否存在if/else变量声明和对象 [英] if/else variable declaration and object does not exist in current context error

问题描述

我有以下代码声明两个变量，然后执行查询.无论如何都将创建变量，所以令我惊讶的是，当要执行查询时，当前上下文错误中没有对象不存在.我该怎么做?它已经发生过好几次了.我也尝试在变量的声明中使用if/else语句，但这没有用.(然后我得到错误的无效表达式，如果)我需要说些什么才能使它起作用?

I have the following code which declares two variables and then executes a query. The variables will be created no matter what, so I am surprised to be getting an object does not exist in current context error when the query is about to execute. How can I do something like this? It has happened several times. I also tries to use an if/else statement in the declaration of the variable but that has not worked. (Then I get the error invalid expression if) What do I need to say for this to work?

        if (from_date == null) {
           var from_date_choose = DateTime.Today.AddDays(-30);
        } else { 
            var from_date_choose = from_date;
        }

        if (to_date == null) {
           var to_date_choose = DateTime.Today;
        } else {
            var to_date_choose = to_date;
        }

        var voyages = db.Voyages
       .Where(v => v.ArrivalDatetime >= from_date_choose)
       .Where(v => v.ArrivalDatetime <= to_date_choose);

推荐答案

像这样修改代码，否则您的变量仅存在于本地范围内.您还需要使用 from_date.Value 和 to_date.Value (我假设它们的类型为 Nullable< DateTime> ):

Modify the code like this, otherwise your variables exist only in local scope. You also need to use from_date.Value and to_date.Value (I assume these are of type Nullable<DateTime>):

DateTime from_date_choose;
if (from_date == null) {
   from_date_choose = DateTime.Today.AddDays(-30);
} else { 
    from_date_choose = from_date.Value;
}

DateTime to_date_choose;
if (to_date == null) {
   to_date_choose = DateTime.Today;
} else {
    to_date_choose = to_date.Value;
}

var voyages = db.Voyages
.Where(v => v.ArrivalDatetime >= from_date_choose)
.Where(v => v.ArrivalDatetime <= to_date_choose);

您也可以使用三元运算符，例如:

You may also use the ternary operator, eg.:

DateTime from_date_choose = from_date == null ? DateTime.Today.AddDays(-30) : from_date.Value;

或者正如Jeppe Stig Nielsen建议使用合并运算符，例如:

Or as Jeppe Stig Nielsen suggested use coalesce operator, eg:

DateTime from_date_choose = from_date ?? DateTime.Today.AddDays(-30);

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