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How can I make sure that FirstOrDefault<KeyValuePair> has returned a value

Posted by: admin November 27, 2017 Leave a comment

Questions:

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 KeyValuePair variable, 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?

Answers:

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.

Questions:
Answers:

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: