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Docs-.NET-C#-指南-语言参考-关键字-值类型：可以 null 的值类型

ylbtech-Docs-.NET-C#-指南-语言参考-关键字-值类型：可以 null 的值类型

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Nullable value types (C# reference)

2019/11/04
7 分钟阅读时长

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.

备注

C# 8.0 introduces the nullable reference types feature. For more information, see Nullable reference types. The nullable value types are available beginning with C# 2.

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.

Declaration and assignment

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 also can 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 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.

Examination of an instance of a nullable value type

Beginning with C# 7.0, 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 also can 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

Conversion from a nullable value type to an underlying type

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 also can 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 also can 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?.

Lifted operators

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

备注

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

neither greater than or equal to null
nor less than 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

The preceding example also shows that an equality comparison of two nullable value type instances that are both null evaluates to true.

If there exists a user-defined conversion between two value types, the same conversion can also be used between the corresponding nullable value types.

Boxing and unboxing

An instance of a nullable value type T? is boxed as follows:

If HasValue returns false, the null reference is produced.
If HasValue returns 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

How to identify a nullable value type

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($"int? is {(IsNullable(typeof(int?)) ? "nullable" : "non nullable")} type");
Console.WriteLine($"int is {(IsNullable(typeof(int)) ? "nullable" : "non-nullable")} type");

bool IsNullable(Type type) => Nullable.GetUnderlyingType(type) != null;

// Output:
// int? is nullable type
// int is non-nullable 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?

You can use the code presented in the following example to determine whether an instance is of a nullable value type:

int? a = 14;
Console.WriteLine(IsOfNullableType(a));  // output: True

int b = 17;
Console.WriteLine(IsOfNullableType(b));  // output: False

bool IsOfNullableType<T>(T o)
{
    var type = typeof(T);
    return Nullable.GetUnderlyingType(type) != null;
}

备注

The methods described in this section are not applicable in the case of nullable reference types.

C# language specification

For more information, see the following sections of the C# language specification: