Using C# LINQ - A Practical Overview

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Methods - Extract a single element or default

The previous two lessons had a note explaining that the First(), Last(), etc. methods would throw an exception if they couldn't return an element. The methods shown here don't throw an exception. They instead return a default value.

What value do they return? That depends on the type (T) of the IEnumerable<T> sequence. If T is a reference type (a class), then the default value is always null. If it is a struct, then the default value is an instance of that struct with all its elements set to their default values.

For all other data types, there is a table of default values available on MSDN.

FirstOrDefault() method

What do you think this call to FirstOrDefault() would return?

List<double> doubles = new List<double> { 2.0, 2.1, 2.2, 2.3 };
double whatsThis = doubles.FirstOrDefault(val => val > 2.3);
What is the value of the whatsThis variable?

LastOrDefault() method

What do you think the LastOrDefault() in this code would return?

List<double> doubles = new List<double> { 2.0, 2.1, 2.2, 2.3 };
double whatsThis = doubles.LastOrDefault(val => val > 2.0 && val < 2.3);
What is the value of the whatsThis variable?

ElementAtOrDefault() method

Try this one. What would the ElementAtOrDefault() call return in this code snippet?

List<double> doubles = new List<double> { 2.0, 2.1, 2.2, 2.3 };
double whatsThis = doubles.ElementAtOrDefault(4);
What is the value of the whatsThis variable?

SingleOrDefault() method

SingleOrDefault() is a bit different from the others. If there is no element in the list that meets the criteria, then a default value is returned. If more than one value meets the criteria, then SingleOrDefault() still throws an exception, just like the Single() method.

With this in mind, what do you think this would do?

List<double> doubles = new List<double> { 2.0, 2.1, 2.2, 2.3 };
double whatsThis = doubles.SingleOrDefault(val => val > 2.1 && val <= 2.3);
What is the value of the whatsThis variable?
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