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 255 possible values.

In C# 1 you had to choose between adding a "missing value" flag or reserving a special value to indicating 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, provided a shortcut ? operator to be placed after existing type references to indicate they may be null and allows default(T) to allow a type to be initialized to either null, 0 or 0.0 depending on the type it references

Code

Now

int? quantity;
if (int.TryParse(txtQuantity.Value, out quantity)) {
    updateOrderLine(quantity);
}

Before

bool quantityProvided = txtUser.Value != "";
int quantity = 0;
if (quantityProvided && int.TryParse(txtQuantity.Value, out quantity)) {
    updateOrderLine(quantity);
}

Notes

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