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Have you ever tried to use the Convert.ChangeType() method to convert a value to a Nullable<T> type? Awkwardly, it will throw an InvalidCastException saying "Null object cannot be converted to a value type".
Try running this on your immediate window: ?System.Convert.ChangeType(null, typeof(int?))
For some obscure reason, Nullables are considered value types. For example, typeof(int?).IsValueType returns true.
For me, since Nullable<T> accept null, it's a class type, not a value type. Does anyone know why it would implemented differently?
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You typically use a nullable value type when you need to represent the undefined value of an underlying value type. For example, a Boolean, or bool , variable can only be either true or false . However, in some applications a variable value can be undefined or missing.
The Nullable type allows you to assign a null value to a variable. Nullable types introduced in C#2.0 can only work with Value Type, not with Reference Type. The nullable types for Reference Type is introduced later in C# 8.0 in 2019 so that we can explicitly define if a reference type can or can not hold a null value.
Nullable variables may either contain a valid value or they may not — in the latter case they are considered to be nil . Non-nullable variables must always contain a value and cannot be nil . In Oxygene (as in C# and Java), the default nullability of a variable is determined by its type.
Nullable reference types are a compile time feature. That means it's possible for callers to ignore warnings, intentionally use null as an argument to a method expecting a non nullable reference. Library authors should include runtime checks against null argument values.
System.Nullable<T> is technically a structure, so it's a value type (the null value for a Nullable<T> is not exactly the same thing as a null reference. It's a boolean flag that denotes the lack of a value.) However, it's treated specially by the runtime and this makes it an awkward value type. First, it doesn't satisfy where T : struct type constraint (it doesn't satisfy where T : class either, for what it's worth). Second, it exhibits an interesting boxing behavior. Boxing a Nullable<T> will result in:
null reference, if the value is null.A boxed value of the underlying type if it actually contains a value. That is, in the statements:
int? x = 2;
object y = x;
y is not a boxed Nullable<int>. It's simply a boxed int. You can unbox any boxed value of type T to Nullable<T> (and T, of course). Casting a null reference to Nullable<T> results in a null value (as you might expect).
This special treatment by the runtime makes nullable types work more similar to the way null works in reference types.
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