Mastering NullReferenceException Prevention in C#/

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LINQ XOrDefault Methods

LINQ XOrDefault Methods

Prevent a NullReferenceException when working with LINQ XOrDefault methods.

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We get a NullReferenceException when we use the properties or methods of an uninitialized object reference. We can also get a NullReferenceException when working with the LINQ FirstOrDefault(), LastOrDefault(), and SingleOrDefault() methods.

Finding one element of a collection

LINQ is a set of methods to filter, group, and order collections. Let’s focus on only the XOrDefault methods.

The FirstOrDefault() method finds the first element of a collection or the first element matching a condition. If the collection is empty or doesn’t have matching elements, FirstOrDefault() returns the default value of the collection’s type. For reference types, that’s a null. And we know what happens if we access a method or property on a reference that’s null.

For example, let’s find the first movie with a high and low rating in our catalog.

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main.cs
NRE.csproj
var movies = new List<Movie>
{
    new Movie("Shrek", 2001, 7.95f),
    new Movie("Inside Out", 2015, 8.2f),
    new Movie("Ratatouille", 2007, 8.1f),
    new Movie("Toy Story", 1995, 8.3f),
    new Movie("Cloudy with a Chance of Meatballs", 2009, 6.9f)
};
var withAGoodRating = movies.FirstOrDefault(movie => movie.Rating > 8.0);
Console.WriteLine(withAGoodRating == null); // false
var withLowRating = movies.FirstOrDefault(movie => movie.Rating < 6.0);
Console.WriteLine(withLowRating == null); // true
Console.WriteLine(withLowRating.Name);
// If we forget to check for nulls after using FirstOrDefault,
// it will break with the NullReferenceException
record Movie(string Name, int ReleaseYear, float Rating);

Notice we used FirstOrDefault() to find the first movie with a ...