How can I make sure that FirstOrDefault<KeyValuePair> has returned a value
Here's a simplified version of what I'm trying to do:
var days = new Dictionary<int, string>();
days.Add(1, "Monday");
days.Add(2, "Tuesday");
...
days.Add(7, "Sunday");
var sampleText = "My favorite day of the week is 'xyz'";
var day = days.FirstOrDefault(x => sampleText.Contains(x.Value));
Since 'xyz' is not present in the dictionary, the FirstOrDefault method will not return a valid value. I want to be able to check for this situation but I realize that I can't compare the result to "null" because KeyValuePair is a struc. The following code is invalid:
if (day == null) {
System.Diagnotics.Debug.Write("Couldn't find day of week");
}
We you attempt to compile the code, Visual Studio throws the following error:
Operator '==' cannot be applied to operands of type 'System.Collections.Generic.KeyValuePair<int,string>' and '<null>'
How can I check that FirstOrDefault has returned a valid value?
Solution 1:
FirstOrDefault
doesn't return null, it returns default(T)
.
You should check for:
var defaultDay = default(KeyValuePair<int, string>);
bool b = day.Equals(defaultDay);
From MSDN - Enumerable.FirstOrDefault<TSource>
:
default(TSource) if source is empty; otherwise, the first element in source.
Notes:
- If your code is generic it is better to use
EqualityComparer<T>.Default.Equals(day, defaultDay)
, becuase.Equals
may be overridden orday
could be anull
. - In C# 7.1 you will be able to use
KeyValuePair<int, string> defaultDay = default;
, see Target-typed "default" literal. - See also: Reference Source -
FirstOrDefault
Solution 2:
This is the most clear and concise way in my opinion:
var matchedDays = days.Where(x => sampleText.Contains(x.Value));
if (!matchedDays.Any())
{
// Nothing matched
}
else
{
// Get the first match
var day = matchedDays.First();
}
This completely gets around using weird default value stuff for structs.
Solution 3:
You can do this instead :
var days = new Dictionary<int?, string>(); // replace int by int?
days.Add(1, "Monday");
days.Add(2, "Tuesday");
...
days.Add(7, "Sunday");
var sampleText = "My favorite day of the week is 'xyz'";
var day = days.FirstOrDefault(x => sampleText.Contains(x.Value));
and then :
if (day.Key == null) {
System.Diagnotics.Debug.Write("Couldn't find day of week");
}