While reference types can be null (when they refer to no object) value types can not.

This is because a reference type variable is a "pointer" to a memory address and so one such address is reserved to be "null" but in the case of a value type there is no space available to signify the lack of a value - somebody declaring byte for example will want all 256 possible values.

In C# 1 you had to choose between adding a "missing value" flag or reserving a special value to indicate the absence of a value.

A Nullable<T> type was added that wraps the value type with a boolean to flag the presence or absence of a value. The compiler allows the result to be treated as having null semantics, and provides the shorthand T? (a ? suffix on the type name) as an alias for Nullable<T>.

Code

Now

int? quantity = int.TryParse(txtQuantity.Value, out var parsed)
    ? parsed
    : null;
updateOrderLine(quantity);

Before

int quantity = 0;
bool quantityProvided = txtQuantity.Value != ""
    && int.TryParse(txtQuantity.Value, out quantity);
updateOrderLine(quantityProvided ? quantity : -1); // -1 = missing

Notes

• Operators are "lifted" so a + b on two int? values gives null if either is null
• Use HasValue/Value, GetValueOrDefault() or the null-coalescing operator ^{C# 2.0} to get the underlying value
• Writing : null in the "Now" conditional relies on Target-typed conditional expression ^{C# 9.0}; earlier versions need (int?)null
• An alternative way to achieve this in C# 1 was to use the NullableTypes open-source project that provided hand-crafted wrapper types for each of the built-in .NET value types

More information

• Reference types & value types
• C# Specification
• C# Language Reference