linq .Value Nullable Object必须具有值。 怎么跳？

Question

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() });

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

Answer 1

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:

1. 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() ... 

2. If you want to use a replacement value, eg 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() ... 

   or, alternatively,

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

3. 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()) }); 

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

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

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

Answer 2

用where

from b in CompleteData where b.TourOpID != null select b.TourOpID

Answer 3

Why don't You just use ValueString = a.ToString() instead of ValueString = a.Value.ToString() . If a has a value it will return this value to string, if not - a.ToString() will return empty string.

IEnumerable<decimal?> arr = new List<decimal?>
                                            {
                                                1m, 4m, null, 10m, 6m
                                            };

foreach (var @decimal in arr)
{
       Console.WriteLine(@decimal.ToString());
}

The output is:

1
4

10
6