11. Structs

Structs are similar to classes in that they represent data structures that can contain data members and function members. However, unlike classes, structs are value types and do not require heap allocation. A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data, where the latter is known as an object.

Structs are particularly useful for small data structures that have value semantics. Complex numbers, points in a coordinate system, or key–value pairs in a dictionary are all good examples of structs. Key to these data structures is that they have few data members, they do not require use of inheritance or referential identity, and they can be conveniently implemented using value semantics where assignment copies the value instead of the reference.

As described in §4.1.4, the simple types provided by C#, such as int, double, and bool, are, in fact, all struct types. Just as these predefined types are structs, so it is also possible to use structs and operator overloading to implement new “primitive” types in the C# language. Two examples of such types are given at the end of this chapter (§11.4)).

11.1 Struct Declarations

A struct-declaration is a type-declaration (§9.6) that declares a new struct:

     struct-declaration:
            attributes_opt struct-modifiers_opt partial_opt struct identifier type-parameter-list_opt
                          struct-interfaces_opt type-parameter-constraints-clauses_opt struct-body ;_opt

A struct-declaration consists of an optional set of attributes (§17), followed by an optional set of struct-modifiers (§11.1.1), followed by an optional partial modifier, followed by the keyword struct and an identifier that names the struct, followed by an optional type-parameter-list specification (§10.1.3), followed by an optional struct-interfaces specification (§11.1.2), followed by an optional type-parameters-constraints-clauses specification (§10.1.5), followed by a struct-body (§11.1.4), optionally followed by a semicolon.

11.1.1 Struct Modifiers

A struct-declaration may optionally include a sequence of struct modifiers:

    struct-modifiers:
            struct-modifier
            struct-modifiers struct-modifier

    struct-modifier:

                new
                public
                protected
                internal
                private

It is a compile-time error for the same modifier to appear multiple times in a struct declaration.

The modifiers of a struct declaration have the same meaning as the modifiers of a class declaration (§10.1).

11.1.2 partial Modifier

The partial modifier indicates that this struct-declaration is a partial type declaration. Multiple partial struct declarations with the same name within an enclosing namespace or type declaration combine to form one struct declaration, following the rules specified in §10.2.

11.1.3 Struct Interfaces

A struct declaration may include a struct-interfaces specification, in which case the struct is said to directly implement the given interface types.

    struct-interfaces:
            :  interface-type-list

Interface implementations are discussed further in §13.4.

11.1.4 Struct Body

The struct-body of a struct defines the members of the struct.

    struct-body:
            { struct-member-declarations_opt }

11.2 Struct Members

The members of a struct consist of the members introduced by its struct-member-declarations and the members inherited from the type System.ValueType.

    struct-member-declarations:
            struct-member-declaration
            struct-member-declarations  struct-member-declaration

    struct-member-declaration:
            constant-declaration
            field-declaration
            method-declaration
            property-declaration
            event-declaration
            indexer-declaration
            operator-declaration
            constructor-declaration
            static-constructor-declaration
            type-declaration

Except for the differences noted in §11.3, the descriptions of class members provided in §10.3 through §10.14 apply to struct members as well.

11.3 Class and Struct Differences

Structs differ from classes in several important ways:

•  Structs are value types (§11.3.1).

•  All struct types implicitly inherit from the class System.ValueType (§11.3.2).

•  Assignment to a variable of a struct type creates a copy of the value being assigned (§11.3.3).

•  The default value of a struct is the value produced by setting all value type fields to their default value and all reference type fields to null (§11.3.4).

•  Boxing and unboxing operations are used to convert between a struct type and the type object (§11.3.5).

•  The meaning of this is different for structs (§7.5.7).

•  Instance field declarations for a struct are not permitted to include variable initializers (§11.3.7).

•  A struct is not permitted to declare a parameterless instance constructor (§11.3.8).

•  A struct is not permitted to declare a destructor (§11.3.9).

11.3.1 Value Semantics

Structs are value types (§4.1) and are said to have value semantics. Classes, by contrast, are reference types (§4.2) and are said to have reference semantics.

A variable of a struct type directly contains the data of the struct, whereas a variable of a class type contains a reference to the data, where the latter is known as an object. When a struct B contains an instance field of type A and A is a struct type, it is a compile-time error for A to depend on B. A struct X directly depends on a struct Y if X contains an instance field of type Y. Given this definition, the complete set of structs upon which a struct depends is the transitive closure of the directly depends on relationship. For example,

        struct Node
        {
                int data;
                Node next; // error, Node directly depends on itself
        }

is an error because Node contains an instance field of its own type. Similarly,

        struct A { B b; }
        struct B { C c; }
        struct C { A a; }

is an error because each of the types A, B, and C depends on the others.

With classes, it is possible for two variables to reference the same object and, therefore, for operations on one variable to affect the object referenced by the other variable. With structs, each of the variables has its own copy of the data (except in the case of ref and out parameter variables), and it is not possible for operations on one variable to affect the other. Furthermore, because structs are not reference types, it is not possible for values of a struct type to be null.

Given the declaration

        struct Point
        {
                public int x, y;
                public Point(int x, int y) {
                        this.x = x;
                        this.y = y;
                }
         }

the code fragment

        Point a = new Point(10, 10);
        Point b = a;
        a.x = 100;
        System.Console.WriteLine(b.x);

outputs the value 10. The assignment of a to b creates a copy of the value, so b is unaffected by the assignment to a.x. Had Point instead been declared as a class, the output would be 100 because a and b would reference the same object.

11.3.2 Inheritance

All struct types implicitly inherit from the class System.ValueType, which in turn inherits from class object. A struct declaration may specify a list of implemented interfaces, but it is not possible for a struct declaration to specify a base class.

Struct types are never abstract and are always implicitly sealed. The abstract and sealed modifiers are, therefore, not permitted in a struct declaration.

Because inheritance isn’t supported for structs, the declared accessibility of a struct member cannot be protected or protected internal.

Function members in a struct cannot be abstract or virtual. In addition, the override modifier is allowed to override only methods inherited from System.ValueType.

11.3.3 Assignment

Assignment to a variable of a struct type creates a copy of the value being assigned. This behavior differs from assignment to a variable of a class type, which copies the reference but not the object identified by the reference.

Similar to an assignment, when a struct is passed as a value parameter or returned as the result of a function member, a copy of the struct is created. A struct may be passed by reference to a function member using a ref or out parameter.

When a property or indexer of a struct is the target of an assignment, the instance expression associated with the property or indexer access must be classified as a variable. If the instance expression is classified as a value, a compile-time error occurs. This issue is described in further detail in §7.16.1.

11.3.4 Default Values

As described in §5.2, several kinds of variables are automatically initialized to their default values when they are created. For variables of class types and other reference types, this default value is null. Because structs are value types that cannot be null, however, the default value of a struct is the value produced by setting all value type fields to their default values and all reference type fields to null.

Referring to the Point struct declared earlier in this section, the example

        Point[] a = new Point[100];

initializes each Point in the array to the value produced by setting the x and y fields to zero.

The default value of a struct corresponds to the value returned by the default constructor of the struct (§4.1.2). Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead, every struct implicitly has a parameterless instance constructor, which always returns the value that results from setting all value type fields to their default values and all reference type fields to null.

Structs should be designed to consider the default initialization state a valid state. In the example

        using System;
        struct KeyValuePair
        {
                string key;
                string value;
                public KeyValuePair(string key, string value) {
                        if (key == null || value == null) throw new ArgumentException();
                        this.key = key;
                        this.value = value;
              }
        }

the user-defined instance constructor protects against null values only where it is explicitly called. In cases where a KeyValuePair variable is subject to default value initialization, the key and value fields will be null, and the struct must be prepared to handle this state.

11.3.5 Boxing and Unboxing

A value of a class type can be converted to type object or to an interface type that is implemented by the class simply by treating the reference as another type at compile time. Likewise, a value of type object or a value of an interface type can be converted back to a class type without changing the reference (but of course a runtime type check is required in this case).

Because structs are not reference types, these operations are implemented differently for struct types. When a value of a struct type is converted to type object or to an interface type that is implemented by the struct, a boxing operation takes place. Likewise, when a value of type object or a value of an interface type is converted back to a struct type, an unboxing operation takes place. A key difference from the same operations on class types is that boxing and unboxing copies the struct value either into or out of the boxed instance. Thus, following a boxing or unboxing operation, changes made to the unboxed struct are not reflected in the boxed struct.

When a struct type overrides a virtual method inherited from System.Object (such as Equals, GetHashCode, or ToString), invocation of the virtual method through an instance of the struct type does not cause boxing to occur. This is true even when the struct is used as a type parameter and the invocation occurs through an instance of the type parameter type. For example,

        using System;
        struct Counter
        {
                int value;
                public override string ToString() {
                        value++;
                        return value.ToString();
                }
        }
        class Program
        {
                static void Test<T>() where T: new() {
                      T x = new T();
                      Console.WriteLine(x.ToString());
                      Console.WriteLine(x.ToString());
                      Console.WriteLine(x.ToString());
                }
                static void Main() {
                      Test<Counter>();
                }
        }

produces the following output:

        1
        2
        3

Although it is bad style for ToString to have side effects, this example demonstrates that no boxing occurred for the three invocations of x.ToString().

Similarly, boxing never implicitly occurs when accessing a member on a constrained type parameter. For example, suppose an interface ICounter contains a method Increment that can be used to modify a value. If ICounter is used as a constraint, the implementation of the Increment method is called with a reference to the variable that Increment was called on—but never a boxed copy.

        using System;
        interface ICounter
        {
                void Increment();
        }
        struct Counter: ICounter
        {
                int value;
                public override string ToString() {
                      return value.ToString();
                }
                void ICounter.Increment() {
                      value++;
                }
        }
        class Program
        {
                static void Test<T>() where T: ICounter, new() {
                      T x = new T();
                      Console.WriteLine(x);
                      x.Increment();                                        // Modify x
                      Console.WriteLine(x);
                      ((ICounter)x).Increment();                  // Modify boxed copy of x
                      Console.WriteLine(x);
                }
                static void Main() {
                      Test<Counter>();
                }
        }

The first call to Increment modifies the value in the variable x. This is not equivalent to the second call to Increment, which modifies the value in a boxed copy of x. Thus the output of the program is

        0
        1
        1

For further details on boxing and unboxing, see §4.3.

11.3.6 Meaning of this

Within an instance constructor or instance function member of a class, this is classified as a value. Thus, while this can be used to refer to the instance for which the function member was invoked, it is not possible to make an assignment to this in a function member of a class.

Within an instance constructor of a struct, this corresponds to an out parameter of the struct type; within an instance function member of a struct, this corresponds to a ref parameter of the struct type. In both cases, this is classified as a variable, and it is possible to modify the entire struct for which the function member was invoked by assigning to this or by passing this as a ref or out parameter.

11.3.7 Field Initializers

As described in §11.3.4, the default value of a struct consists of the value that results from setting all value type fields to their default values and all reference type fields to null. For this reason, a struct does not permit instance field declarations to include variable initializers. This restriction applies only to instance fields. Static fields of a struct are permitted to include variable initializers.

The example

        struct Point
        {
                public int x = 1; // Error, initializer not permitted
                public int y = 1; // Error, initializer not permitted
        }

is in error because the instance field declarations include variable initializers.

11.3.8 Constructors

Unlike a class, a struct is not permitted to declare a parameterless instance constructor. Instead, every struct implicitly has a parameterless instance constructor, which always returns the value that results from setting all value type fields to their default values and all reference type fields to null (§4.1.2). A struct can declare instance constructors having parameters. For example.

        struct Point
        {
                int x, y;
                public Point(int x, int y) {
                      this.x = x;
                      this.y = y;
                }
        }

Given the above declaration, the statements

        Point p1 = new Point();

        Point p2 = new Point(0, 0);

both create a Point with x and y initialized to zero.

A struct instance constructor is not permitted to include a constructor initializer of the form base(…).

If the struct instance constructor doesn’t specify a constructor initializer, the this variable corresponds to an out parameter of the struct type; similar to an out parameter, this must be definitely assigned (§5.3) at every location where the constructor returns. If the struct instance constructor specifies a constructor initializer, the this variable corresponds to a ref parameter of the struct type; similar to a ref parameter, this is considered definitely assigned on entry to the constructor body. Consider the following instance constructor implementation:

        struct Point
        {
                int x, y;
                public int X {
                      set { x = value; }
                }
                public int Y {
                      set { y = value; }
                }
                public Point(int x, int y) {
                      X = x;              // Error, this is not yet definitely assigned
                      Y = y;              // Error, this is not yet definitely assigned
                }
            }

No instance member function (including the set accessors for the properties X and Y) can be called until all fields of the struct being constructed have been definitely assigned. Note, however, that if Point were a class instead of a struct, the instance constructor implementation would be permitted.

11.3.9 Destructors

A struct is not permitted to declare a destructor.

11.3.10 Static Constructors

Static constructors for structs follow most of the same rules as for classes. The execution of a static constructor for a struct type is triggered by the first of the following events to occur within an application domain:

•  An instance member of the struct type is referenced.

•  A static member of the struct type is referenced.

•  An explicitly declared constructor of the struct type is called.

The creation of default values (§11.3.4) of struct types does not trigger the static constructor. (An example of this behavior is seen with the initial values of elements in an array.)

11.4 Struct Examples

This section shows two significant examples of using struct types to create types that can be used similarly to the built-in types of the language, albeit with modified semantics.

11.4.1 Database Integer Type

The DBInt struct shown here implements an integer type that can represent the complete set of values of the int type, plus an additional state that indicates an unknown value. A type with these characteristics is commonly used in databases, and is similar in many ways to the type int?.

        using System;
        public struct DBInt
        {
                // The Null member represents an unknown DBInt value.
                public static readonly DBInt Null = new DBInt();
                // When the defined field is true, this DBInt represents a known value
                //  which is stored in the value field. When the defined field is false,
                //  this DBInt represents an unknown value, and the value field is 0.
                int value;
                bool defined;
                //  Private instance constructor. Creates a DBInt with a known value.
                DBInt(int value) {
                          this.value = value;
                          this.defined = true;
                }
                //  The IsNull property is true if this DBInt represents an unknown value.
                public bool IsNull { get { return !defined; } }
                // The Value property is the known value of this DBInt, or 0 if this
                // DBInt represents an unknown value.
                public int Value { get { return value; } }
                // Implicit conversion from int to DBInt.
                public static implicit operator DBInt(int x) {
                    return new DBInt(x);
                }
                // Explicit conversion from DBInt to int. Throws an exception if the
                // given DBInt represents an unknown value.
                public static explicit operator int(DBInt x) {
                        if (!x.defined) throw new InvalidOperationException();
                        return x.value;
                }
                public static DBInt operator +(DBInt x) {
                        return x;
                }
                public static DBInt operator -(DBInt x) {
                        return x.defined ? -x.value : Null;
                }
                public static DBInt operator +(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value + y.value: Null;
                }
                public static DBInt operator -(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value - y.value: Null;
                }
                public static DBInt operator *(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value * y.value: Null;
                }
                public static DBInt operator /(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value / y.value: Null;
                }
                public static DBInt operator %(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value % y.value: Null;
                }
                public static DBBool operator ==(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value == y.value: DBBool.Null;
                }
                public static DBBool operator !=(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value != y.value: DBBool.Null;
                }
                public static DBBool operator >(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value > y.value: DBBool.Null;
                }
                public static DBBool operator <(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value < y.value: DBBool.Null;
                }
                public static DBBool operator >=(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value >= y.value: DBBool.Null;
                }
                public static DBBool operator <=(DBInt x, DBInt y) {
                        return x.defined && y.defined? x.value <= y.value: DBBool.Null;
                }
                public override bool Equals(object obj) {
                        if (!(obj is DBInt)) return false;
                        DBInt x = (DBInt)obj;
                        return value == x.value && defined == x.defined;
                }
                public override int GetHashCode() {
                        return value;
                }
                public override string ToString()
                        { return defined? value.ToString(): "DBInt.Null";
                }
        }

11.4.2 Database Boolean Type

The DBBool struct below implements a three-valued logical type. The possible values of this type are DBBool.True, DBBool.False, and DBBool.Null, where the Null member indicates an unknown value. Such three-valued logical types are commonly used in databases, and are similar to the type bool?.

        using System;
        public struct DBBool
        {
                //  The three possible DBBool values.
                public static readonly DBBool Null = new DBBool(0);
                public static readonly DBBool False = new DBBool(-1);
                public static readonly DBBool True = new DBBool(1);
                //  Private field that stores –1, 0, 1 for False, Null, True.
                sbyte value;
                //  Private instance constructor. The value parameter must be –1, 0, or 1.
                DBBool(int value) {
                           this.value = (sbyte)value;
                }
                //  Properties to examine the value of a DBBool. Return true if this
                //  DBBool has the given value, false otherwise.
                public bool IsNull { get { return value == 0; } }
                public bool IsFalse { get { return value < 0; } }
                public bool IsTrue { get { return value > 0; } }
                //  Implicit conversion from bool to DBBool. Maps true to DBBool.True and
                //  false to DBBool.False.
                public static implicit operator DBBool(bool x) {
                         return x? True: False;
                }
                //  Explicit conversion from DBBool to bool. Throws an exception if the
                //  given DBBool is Null, otherwise returns true or false.
                public static explicit operator bool(DBBool x) {
                           if (x.value == 0) throw new InvalidOperationException();
                           return x.value > 0;
                }
                //  Equality operator. Returns Null if either operand is Null, otherwise
                //  returns True or False.
                public static DBBool operator ==(DBBool x, DBBool y) {
                           if (x.value == 0 || y.value == 0) return Null;
                           return x.value == y.value? True: False;
                }
                //  Inequality operator. Returns Null if either operand is Null, otherwise
                //  returns True or False.
                public static DBBool operator !=(DBBool x, DBBool y) {
                           if (x.value == 0 || y.value == 0) return Null;
                           return x.value != y.value? True: False;
                }
                //  Logical negation operator. Returns True if the operand is False, Null
               //  if the operand is Null, or False if the operand is True.
                public static DBBool operator !(DBBool x) {
                           return new DBBool(-x.value);
                }
                //  Logical AND operator. Returns False if either operand is False,
                //  otherwise Null if either operand is Null, otherwise True.
                public static DBBool operator &(DBBool x, DBBool y) {
                           return new DBBool(x.value < y.value? x.value: y.value);
                }
                //  Logical OR operator. Returns True if either operand is True, otherwise
                //  Null if either operand is Null, otherwise False.
                public static DBBool operator |(DBBool x, DBBool y) {
                           return new DBBool(x.value > y.value? x.value: y.value);
                }
                //  Definitely true operator. Returns true if the operand is True,
                //  otherwise false.
                public static bool operator true(DBBool x) {
                           return x.value > 0;
                }
                //  Definitely false operator. Returns true if the operand is False,
                //  otherwise false.
                public static bool operator false(DBBool x) {
                           return x.value < 0;
                }
                public override bool Equals(object obj) {
                           if (!(obj is DBBool)) return false;
                           return value == ((DBBool)obj).value;
                }
                public override int GetHashCode() {
                           return value;
                }
                public override string ToString() {
                           if (value > 0) return "DBBool.True";
                           if (value < 0) return "DBBool.False";
                           return "DBBool.Null";
                }
        }