A Complete Guide - Nullable Types in C#

Step 4: Write Sample Code Using Nullable Types

We will create a Student class that includes a score, which is sometimes not available. For this purpose, we will use a nullable double type.

1. Open Program.cs.
2. Replace the existing code with the following:

using System; namespace NullableTypesDemo
{ class Program { static void Main(string[] args) { // Create students Student student1 = new Student("John Doe", 87.5); Student student2 = new Student("Jane Smith"); Student student3 = new Student("Mike Johnson", 92.7); // Store students in an array Student[] students = { student1, student2, student3 }; // Display scores of each student foreach (Student student in students) { if (student.Score.HasValue) { Console.WriteLine($"{student.Name}'s score is: {student.Score.Value}"); } else { Console.WriteLine($"{student.Name} did not submit the assignment."); } } // Try to get a score even if it doesn't exist try { double johnsScore = student1.Score.Value; Console.WriteLine($"John Doe's score is: {johnsScore}"); // This will throw InvalidOperationException because Score does not exist double janesScore = student2.Score.Value; } catch (InvalidOperationException ex) { Console.WriteLine(ex.Message); } // Use ?? operator to provide default value foreach (Student student in students) { // Provide a default score of 0 if Score is null double finalScore = student.Score ?? 0; Console.WriteLine($"{student.Name}'s final score is: {finalScore}"); } // Use GetValueOrDefault() method to provide default value foreach (Student student in students) { // Provide a default score of 0 if Score is null double finalScore = student.Score.GetValueOrDefault(0); Console.WriteLine($"{student.Name}'s final score is: {finalScore}"); } } } public class Student { public string Name { get; set; } public double? Score { get; set; } // Constructor when Score is provided public Student(string name, double score) { Name = name; Score = score; } // Constructor when Score is not provided public Student(string name) { Name = name; Score = null; // Alternatively, you could omit this line because the default is null } }
}

Step 5: Run the Program

1. Press F5 or click the Start button in Visual Studio to run your program.
2. Observe the output in the console.

Expected Output:

John Doe's score is: 87.5
Jane Smith did not submit the assignment.
Mike Johnson's score is: 92.7
John Doe's score is: 87.5
Nullable object must have a value before using it.
John Doe's final score is: 87.5
Jane Smith's final score is: 0
Mike Johnson's final score is: 92.7
John Doe's final score is: 87.5
Jane Smith's final score is: 0
Mike Johnson's final score is: 92.7

Explanation of the Code

• Nullable Double Declaration: The Score property of the Student class is declared as double? which means it can hold either a double value or null.

public double? Score { get; set; }

• Checking Values: We use the HasValue property to see if a student submitted a score. If they did, we access the score using the Value property.

if (student.Score.HasValue)
{ Console.WriteLine($"{student.Name}'s score is: {student.Score.Value}");
}
else
{ Console.WriteLine($"{student.Name} did not submit the assignment.");
}

• Handling Null: If you try to access the .Value property on a nullable type that is null, it will throw an InvalidOperationException. We wrapped this in a try-catch block to handle such exceptions gracefully.

try
{ double johnsScore = student1.Score.Value; Console.WriteLine($"John Doe's score is: {johnsScore}"); double janesScore = student2.Score.Value;
}
catch (InvalidOperationException ex)
{ Console.WriteLine(ex.Message); // Display exception message if Score is null
}

• Using the Null Coalescing Operator (??): The null coalescing operator allows us to provide a default value if the variable is null.

double finalScore = student.Score ?? 0;
Console.WriteLine($"{student.Name}'s final score is: {finalScore}");

• Using GetValueOrDefault() Method: This is another way to provide a default value if the nullable variable is null.

double finalScore = student.Score.GetValueOrDefault(0);
Console.WriteLine($"{student.Name}'s final score is: {finalScore}");

Conclusion

Nullable types in C# are essential for scenarios where data might be missing. They help maintain clean and robust code. In our example, we demonstrated how to use nullable doubles in a Student class, checking for null, handling exceptions, and providing default values.