问题
I have 2 classes which looks like this:
class Widget
{
string Selected { get; set; }
List<Option> Options { get; set; }
}
class Option
{
string InternalCode { get; set; }
string ExternalCode { get; set; }
}
Options
gets populated dynamically with different data per client for showing ExternalCode
as options
Selected
gets populated with ExternalCode
.
I then need to access the InternalCode
which matches.
At present I am doing this:
var option = widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected);
var internalCode = option == null ? string.Empty : option.InternalCode;
Is this possible using a single line using Null Coalesce?
回答1:
Yes, you can use the null propagation and null coalescing operator, which suits your needs if you can use C# 6:
var option = widget.Options
.SingleOrDefault(o => o.ExternalCode == widget.Selected)?.InternalCode
?? string.Empty;
The ?.
will translate to your use of the option == null ?
part.
回答2:
Sure, with a small change:
var option = widget.Options
.Where(o => o.ExternalCode == widget.Selected)
.Select(o => o.InternalCode)
.FirstOrDefault() ?? "";
In other words, project the sequence of matching options to a sequence of internal codes, and then take the first of those, defaulting to null
... which allows you to use the null-coalescing operator on the result.
You can use the null-conditional operator as per Patrick's answer instead, but personally I'd prefer the code in this answer - I think it's simpler to understand.
回答3:
You can do this if you wrote extension method:
public static class MonadExtensions
{
public static TResult With<TSource, TResult>(TSource source, Func<TSource, TResult> action) where TSource : class
{
if (source != default(TSource))
return action(source);
return default(TResult);
}
}
And use it:
var internalCode = widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected).With(o=>o.InternalCode)??"";
回答4:
Is this possible using a single line using Null Coalesce?
Yes, here is how:
var option = (widget.Options.SingleOrDefault(o => o.ExternalCode == widget.Selected) ?? new Option() { InternalCode = string.Empty }).InternalCode;
The other answers using a single statement I have seen so far yield wrong results in cases where there is a matching instance of Option
with InternalCode == null
来源:https://stackoverflow.com/questions/34630920/c-sharp-null-coalesce-with-linq