Explain the purpose of nullable value types in C#. How are they declared and used?

In C#, nullable value types allow a value type (like int, bool, or DateTime) to hold either a valid value or null. This is useful in scenarios where you need to represent the absence of a value, such as in databases where a field might be optional or unassigned.

Normally, value types cannot be null because they always hold a value. For example, an int always defaults to 0 if not explicitly set. However, by making it nullable (using the ? syntax), it can also represent a null value, providing more flexibility.

How to Declare nullable value Types

You declare a nullable value type by appending a ? to the type. For example:

int? nullableInt = null;
bool? nullableBool = true;

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How to Use Nullable Value Types

To check if a nullable type contains a value, use the .HasValue property:

if (nullableInt.HasValue)
{
    Console.WriteLine(nullableInt.Value); // Access the value
}
else
{
    Console.WriteLine("No value assigned.");
}

Alternatively, you can use the null-coalescing operator (??) to provide a fallback value:

int result = nullableInt ?? 0; // If nullableInt is null, use 0

Why Use Them?

Nullable value types are commonly used:

1. In databases: To represent fields that can be empty (e.g., int? Age).
2. For optional parameters or flags.
3. In scenarios where a default value like 0 or false isn’t meaningful.

This feature improves code clarity and reduces errors when dealing with optional or missing values.