linq .Value可为空的对象必须具有一个值.如何跳过?

问题描述：

我有一些linq代码，有时是null:

I have some linq code that is sometimes null:

        cbo3.ItemsSource = empty.Union(from a in
                                           (from b in CompleteData
                                            select b.TourOpID).Distinct()
                                       select new ComboBoxItemString() { ValueString = a.Value.ToString() });

但是TourOpID有时null在a.Value.ToString()上引发错误.我该如何解决?

But TourOpID is sometimes null throwing an error on a.Value.ToString() . How do I solve this?

答

发生此问题是因为您访问Nullable类型的Nullable类型的="noreferrer"> Value属性(或更准确地说，其HasValue属性为false).如何解决此问题取决于您要执行的操作:

The problem occurs because you access the Value property of a Nullable type which is null (or, more precisely, whose HasValue property is false). How to fix this depends on what you want to do:

如果要过滤出TourOpID为null的项目，只需添加where子句:

If you want to filter out items where TourOpID is null, just add a where clause:

...
(from b in CompleteData
 where b.TourOpID != null         // filter
 select b.TourOpID).Distinct()
...

如果要使用替换值，例如0，如果TourOpID为空，请使用空合并运算符 ??，它将您的int?转换为int:

If you want to use a replacement value, e.g. 0, if TourOpID is null, use the null coalescing operator ??, which converts your int? into an int:

...
(from b in CompleteData
 select b.TourOpID ?? 0).Distinct()
...

或者，或者，

...
select new ComboBoxItemString() { ValueString = a.GetValueOrDefault().ToString() });

如果在TourOpID为null的情况下只想显示另一个ComboBox条目，请使用三元运算符?::

If you just want to show a different ComboBox entry if TourOpID is null, use the ternary operator ?::

...
select new ComboBoxItemString() { 
    ValueString = (a == null ? "no tour operator" : a.Value.ToString())
});

如果要在a为null时显示空字符串，则解决方案甚至更简单:

If you want to show the empty string if a is null, the solution is even simpler:

...
select new ComboBoxItemString() { ValueString = a.ToString() });

因为 Nullable.ToString 返回空字符串(如果没有)有一个值.

since Nullable.ToString returns an empty string if it does not have a value.

linq .Value可为空的对象必须具有一个值.如何跳过?

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