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A nullable value type T?
represents all values of its underlying value type T
and an additional null value. For example, you can assign any of the following three values to a bool?
variable: true
, false
, or null
. An underlying value type T
cannot be a nullable value type itself.
Any nullable value type is an instance of the generic System.Nullable<T> structure. You can refer to a nullable value type with an underlying type T
in any of the following interchangeable forms: Nullable<T>
or T?
.
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. For example, a database field may contain true
or false
, or it may contain no value at all, that is, NULL
. You can use the bool?
type in that scenario.
As a value type is implicitly convertible to the corresponding nullable value type, you can assign a value to a variable of a nullable value type as you would do that for its underlying value type. You can also assign the null
value. For example:
double? pi = 3.14;
char? letter = 'a';
int m2 = 10;
int? m = m2;
bool? flag = null;
// An array of a nullable value type:
int?[] arr = new int?[10];
The default value of a nullable value type represents null
, that is, it's an instance whose Nullable<T>.HasValue property returns false
.
You can use the is
operator with a type pattern to both examine an instance of a nullable value type for null
and retrieve a value of an underlying type:
int? a = 42;
if (a is int valueOfA)
{
Console.WriteLine($"a is {valueOfA}");
}
else
{
Console.WriteLine("a does not have a value");
}
// Output:
// a is 42
You always can use the following read-only properties to examine and get a value of a nullable value type variable:
Nullable<T>.HasValue indicates whether an instance of a nullable value type has a value of its underlying type.
Nullable<T>.Value gets the value of an underlying type if HasValue is true
. If HasValue is false
, the Value property throws an InvalidOperationException.
The following example uses the HasValue
property to test whether the variable contains a value before displaying it:
int? b = 10;
if (b.HasValue)
{
Console.WriteLine($"b is {b.Value}");
}
else
{
Console.WriteLine("b does not have a value");
}
// Output:
// b is 10
You can also compare a variable of a nullable value type with null
instead of using the HasValue
property, as the following example shows:
int? c = 7;
if (c != null)
{
Console.WriteLine($"c is {c.Value}");
}
else
{
Console.WriteLine("c does not have a value");
}
// Output:
// c is 7
If you want to assign a value of a nullable value type to a non-nullable value type variable, you might need to specify the value to be assigned in place of null
. Use the null-coalescing operator ??
to do that (you can also use the Nullable<T>.GetValueOrDefault(T) method for the same purpose):
int? a = 28;
int b = a ?? -1;
Console.WriteLine($"b is {b}"); // output: b is 28
int? c = null;
int d = c ?? -1;
Console.WriteLine($"d is {d}"); // output: d is -1
If you want to use the default value of the underlying value type in place of null
, use the Nullable<T>.GetValueOrDefault() method.
You can also explicitly cast a nullable value type to a non-nullable type, as the following example shows:
int? n = null;
//int m1 = n; // Doesn't compile
int n2 = (int)n; // Compiles, but throws an exception if n is null
At run time, if the value of a nullable value type is null
, the explicit cast throws an InvalidOperationException.
A non-nullable value type T
is implicitly convertible to the corresponding nullable value type T?
.
The predefined unary and binary operators or any overloaded operators that are supported by a value type T
are also supported by the corresponding nullable value type T?
. These operators, also known as lifted operators, produce null
if one or both operands are null
; otherwise, the operator uses the contained values of its operands to calculate the result. For example:
int? a = 10;
int? b = null;
int? c = 10;
a++; // a is 11
a = a * c; // a is 110
a = a + b; // a is null
Note
For the bool?
type, the predefined &
and |
operators don't follow the rules described in this section: the result of an operator evaluation can be non-null even if one of the operands is null
. For more information, see the Nullable Boolean logical operators section of the Boolean logical operators article.
For the comparison operators <
, >
, <=
, and >=
, if one or both operands are null
, the result is false
; otherwise, the contained values of operands are compared. Do not assume that because a particular comparison (for example, <=
) returns false
, the opposite comparison (>
) returns true
. The following example shows that 10 is
null
null
int? a = 10;
Console.WriteLine($"{a} >= null is {a >= null}");
Console.WriteLine($"{a} < null is {a < null}");
Console.WriteLine($"{a} == null is {a == null}");
// Output:
// 10 >= null is False
// 10 < null is False
// 10 == null is False
int? b = null;
int? c = null;
Console.WriteLine($"null >= null is {b >= c}");
Console.WriteLine($"null == null is {b == c}");
// Output:
// null >= null is False
// null == null is True
For the equality operator ==
, if both operands are null
, the result is true
, if only one of the operands is null
, the result is false
; otherwise, the contained values of operands are compared.
For the inequality operator !=
, if both operands are null
, the result is false
, if only one of the operands is null
, the result is true
; otherwise, the contained values of operands are compared.
If there exists a user-defined conversion between two value types, the same conversion can also be used between the corresponding nullable value types.
An instance of a nullable value type T?
is boxed as follows:
false
, the null reference is produced.true
, the corresponding value of the underlying value type T
is boxed, not the instance of Nullable<T>.You can unbox a boxed value of a value type T
to the corresponding nullable value type T?
, as the following example shows:
int a = 41;
object aBoxed = a;
int? aNullable = (int?)aBoxed;
Console.WriteLine($"Value of aNullable: {aNullable}");
object aNullableBoxed = aNullable;
if (aNullableBoxed is int valueOfA)
{
Console.WriteLine($"aNullableBoxed is boxed int: {valueOfA}");
}
// Output:
// Value of aNullable: 41
// aNullableBoxed is boxed int: 41
The following example shows how to determine whether a System.Type instance represents a constructed nullable value type, that is, the System.Nullable<T> type with a specified type parameter T
:
Console.WriteLine($"int? is {(IsNullable(typeof(int?)) ? "nullable" : "non nullable")} value type");
Console.WriteLine($"int is {(IsNullable(typeof(int)) ? "nullable" : "non-nullable")} value type");
bool IsNullable(Type type) => Nullable.GetUnderlyingType(type) != null;
// Output:
// int? is nullable value type
// int is non-nullable value type
As the example shows, you use the typeof operator to create a System.Type instance.
If you want to determine whether an instance is of a nullable value type, don't use the Object.GetType method to get a Type instance to be tested with the preceding code. When you call the Object.GetType method on an instance of a nullable value type, the instance is boxed to Object. As boxing of a non-null instance of a nullable value type is equivalent to boxing of a value of the underlying type, GetType returns a Type instance that represents the underlying type of a nullable value type:
int? a = 17;
Type typeOfA = a.GetType();
Console.WriteLine(typeOfA.FullName);
// Output:
// System.Int32
Also, don't use the is operator to determine whether an instance is of a nullable value type. As the following example shows, you cannot distinguish types of a nullable value type instance and its underlying type instance with the is
operator:
int? a = 14;
if (a is int)
{
Console.WriteLine("int? instance is compatible with int");
}
int b = 17;
if (b is int?)
{
Console.WriteLine("int instance is compatible with int?");
}
// Output:
// int? instance is compatible with int
// int instance is compatible with int?
Instead use the Nullable.GetUnderlyingType from the first example and typeof operator to check if an instance is of a nullable value type.
Note
The methods described in this section are not applicable in the case of nullable reference types.
For more information, see the following sections of the C# language specification:
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