Master C# Programming From Scratch

Null-Conditional Operator ?.

The null-conditional operator ?. allows you to access a member only when the object is not null.

using System;
 
string? name = null;
 
int? length = name?.Length;
 
Console.WriteLine(length);

Output

The result of name?.Length is null because name is null.

Null-Coalescing Operator ??

The ?? operator provides a default value when the expression on the left side is null.

using System;
 string? name = null;
 
string displayName = name ?? "Guest";
 
Console.WriteLine(displayName);

Output

Guest

Null-Coalescing Assignment ??=

The ??= operator assigns a value only when the variable currently contains null.

using System;
 
string? name = null;
 
name ??= "CIIT Student";
 
Console.WriteLine(name);

Output

CIIT Student

Null-Forgiving Operator !

The null-forgiving operator ! tells the compiler that you believe an expression will not be null at that point.

using System;
 
string? name = GetName();
 
Console.WriteLine(name!.Length);
 
string? GetName()

{

    return "Rahul";

}

Nullable Method Parameters

You can clearly indicate whether a method parameter accepts null.

using System;
 
void PrintName(string? name)
{
    if (name is null)

    {
        Console.WriteLine("Name is missing.");
        return;
    }
    
    Console.WriteLine($"Name: {name}");
}
PrintName(null);

Output

Name is missing.

Nullable Return Values

A method can use string? as its return type when it may return null.

using System;
 
string? FindStudentName(int id)
{
    if (id == 1)

    {
        return "Amit";

    }
       return null;
}
string? name = FindStudentName(10);

Console.WriteLine(name ?? "Student not found.");

Output

Student not found.

Nullable Properties

Model properties can also be marked nullable when a value may legitimately be absent.

using System;
 
namespace n1
{
    public class Student
    {
        public string Name { get; set; } = "";
        public string? Email { get; set; }
    }
    public class Program
    {

        static void Main(string[] args)
        {

            Student student = new Student
            {
                Name = "Priya",
                Email = null
            };

            Console.WriteLine(student.Email ?? "Email not provided.");
        }

    }

}

Output

Email not provided.

Nullable Reference Types in ASP.NET Core

Nullable reference types are particularly useful in ASP.NET Core applications because API models, database queries, request values, and external data can sometimes contain missing values.

public class StudentDto

{

    public string Name { get; set; } = "";

    public string? PhoneNumber { get; set; }

    public string? Email { get; set; }

}

Here, Name is expected to contain a value, while PhoneNumber and Email are allowed to be null.

Example: Database Data 👀🫥

Suppose a database contains a student whose phone number is optional. The application can represent this correctly using a nullable reference.

using System;
 
string? phoneNumber = GetPhoneNumber();
 
if (phoneNumber is null)
{
    Console.WriteLine("Phone number is not available.");
}
else
{
    Console.WriteLine($"Phone: {phoneNumber}");
}
 
string? GetPhoneNumber()
{
    return null;
}

Output

Phone number is not available.

Nullable Reference Types vs Nullable Value Types

Nullable reference types and nullable value types are related to null handling but work with different categories of types.

Best Practices

• Enable nullable reference type analysis for modern projects.
• Use ? when a reference is intentionally allowed to be null.
• Check nullable values before accessing their members.
• Use ?? when a sensible default value is available.
• Use ?. for safe member access when appropriate.
• Avoid using ! simply to suppress warnings without understanding why the value is safe.
• Design method parameters and return types to accurately communicate whether null is allowed.

Common Mistakes

Interview Questions

1. What are Nullable Reference Types in C#?
2. Why were Nullable Reference Types introduced?
3. How do you declare a nullable reference type?
4. What is the difference between string and string??
5. What is the purpose of the null-forgiving operator !?
6. What is the difference between ?. and ???
7. What does ??= do?
8. Do Nullable Reference Types provide runtime protection?
9. How are Nullable Reference Types useful in ASP.NET Core?
10. What is the difference between nullable reference types and nullable value types?

Practice Programs

1. Create a program using string? and safely check a name.
2. Create a student class with nullable Email and PhoneNumber properties.
3. Use the ?? operator to provide a default student name.
4. Use the ?. operator to safely access a nullable object's property.
5. Create a method that may return a nullable student name.
6. Create an ASP.NET Core DTO containing both required and optional properties.