Nullable value types

Some value types (like int, float, bool, etc.) cannot have a null value by default. However, sometimes you might want to represent the absence of a value, especially when working with databases or optional fields.

To allow value types to hold null, C# provides nullable value types using ?:

int? age = null;  // Nullable int
double? price = 99.99;  // Nullable double
bool? isAvailable = null;  // Nullable bool

How it works internally

A nullable value type (T?) is actually a struct called Nullable<T>. This struct has two properties:

HasValue – Returns true if the variable has a value. Value – Retrieves the actual value if HasValue is true.

[Serializable]
public struct Nullable<T> where T : struct
{        
    public bool HasValue { get; }

    public T Value { get; }

    // other implementation
}

Think about this: int? i = null; is the same as nullable<int> i = null;.

Null-Coalescing Operator (??)

Instead of checking HasValue, you can provide a default value using ??:

int? count = null;
int result = count ?? 0;  // If count is null, assign 0
Console.WriteLine(result); // Output: 0

Null-Conditional Operator (?.)

You can also use null-conditional access (?.) to safely handle nullable types:

int? length = null;
Console.WriteLine(length?.ToString()); // Doesn't throw error, just prints empty

Explicitly Converting Nullable to Non-Nullable

If you directly assign a nullable type to a non-nullable type, it will cause a compilation error unless you explicitly handle null cases.

int? nullableNumber = null;
int nonNullableNumber = nullableNumber ?? -1; // Assign -1 if null