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

Question

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?

Answer 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 or day could be a null.
• 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

Answer 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.

Answer 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");
}