4. Types

The types of the C# language are divided into two main categories: Value types and reference types.

type:
value-type
reference-type

A third category of types, pointers, is available only in unsafe code. This is discussed further in §‎18.2.

Value types differ from reference types in that variables of the value types directly contain their data, whereas variables of the reference types store references to their data, the latter being known as objects. With reference types, it is possible for two variables to reference the same object, and thus possible for operations on one variable to affect the object referenced by the other variable. With value types, the variables each have their own copy of the data, and it is not possible for operations on one to affect the other.

C#’s type system is unified such that a value of any type can be treated as an object. Every type in C# directly or indirectly derives from the object class type, and object is the ultimate base class of all types. Values of reference types are treated as objects simply by viewing the values as type object. Values of value types are treated as objects by performing boxing and unboxing operations (§‎4.3).

4.1 Value types

A value type is either a struct type or an enumeration type. C# provides a set of predefined struct types called the simple types. The simple types are identified through reserved words.

value-type:
struct-type
enum-type

struct-type:
type-name
simple-type

simple-type:
numeric-type
bool

numeric-type:
integral-type
floating-point-type
decimal

integral-type:
sbyte
byte
short
ushort
int
uint
long
ulong
char

floating-point-type:
float
double

enum-type:
type-name

A variable of a value type always contains a value of that type. Unlike reference types, it is not possible for a value of a value type to be null, or to reference an object of a more derived type.

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

4.1.1 The System.ValueType type

All value types implicitly inherit from the class System.ValueType, which, in turn, inherits from class object. It is not possible for any type to derive from a value type, and value types are thus implicitly sealed (§‎10.1.1.2).

Note that System.ValueType is not itself a value-type. Rather, it is a class-type from which all value-types are automatically derived.

4.1.2 Default constructors

All value types implicitly declare a public parameterless instance constructor called the default constructor. The default constructor returns a zero-initialized instance known as the default value for the value type:

·         For all simple-types, the default value is the value produced by a bit pattern of all zeros:

o        For sbyte, byte, short, ushort, int, uint, long, and ulong, the default value is 0.

o        For char, the default value is '\x0000'.

o        For float, the default value is 0.0f.

o        For double, the default value is 0.0d.

o        For decimal, the default value is 0.0m.

o        For bool, the default value is false.

·         For an enum-type E, the default value is 0.

·         For a struct-type, the default value is the value produced by setting all value type fields to their default value and all reference type fields to null.

Like any other instance constructor, the default constructor of a value type is invoked using the new operator. For efficiency reasons, this requirement is not intended to actually have the implementation generate a constructor call. In the example below, variables i and j are both initialized to zero.

class A
{
   void F() {
      int i = 0;
      int j = new int();
   }
}

Because every value type implicitly has a public parameterless instance constructor, it is not possible for a struct type to contain an explicit declaration of a parameterless constructor. A struct type is however permitted to declare parameterized instance constructors (§‎11.3.8).

4.1.3 Struct types

A struct type is a value type that can declare constants, fields, methods, properties, indexers, operators, instance constructors, static constructors, and nested types. Struct types are described in §‎11.

4.1.4 Simple types

C# provides a set of predefined struct types called the simple types. The simple types are identified through reserved words, but these reserved words are simply aliases for predefined struct types in the System namespace, as described in the table below.

 

Reserved word	Aliased type
sbyte	System.SByte
byte	System.Byte
short	System.Int16
ushort	System.UInt16
int	System.Int32
uint	System.UInt32
long	System.Int64
ulong	System.UInt64
char	System.Char
float	System.Single
double	System.Double
bool	System.Boolean
decimal	System.Decimal

 

Because a simple type aliases a struct type, every simple type has members. For example, int has the members declared in System.Int32 and the members inherited from System.Object, and the following statements are permitted:

int i = int.MaxValue;         // System.Int32.MaxValue constant
string s = i.ToString();      // System.Int32.ToString() instance method
string t = 123.ToString();    // System.Int32.ToString() instance method

The simple types differ from other struct types in that they permit certain additional operations:

·         Most simple types permit values to be created by writing literals (§‎2.4.4). For example, 123 is a literal of type int and 'a' is a literal of type char. C# makes no provision for literals of struct types in general, and non-default values of other struct types are ultimately always created through instance constructors of those struct types.

·         When the operands of an expression are all simple type constants, it is possible for the compiler to evaluate the expression at compile-time. Such an expression is known as a constant-expression (§‎7.15). Expressions involving operators defined by other struct types are not considered to be constant expressions.

·         Through const declarations it is possible to declare constants of the simple types (§‎10.3). It is not possible to have constants of other struct types, but a similar effect is provided by static readonly fields.

·         Conversions involving simple types can participate in evaluation of conversion operators defined by other struct types, but a user-defined conversion operator can never participate in evaluation of another user-defined operator (§‎6.4.2).

4.1.5 Integral types

C# supports nine integral types: sbyte, byte, short, ushort, int, uint, long, ulong, and char. The integral types have the following sizes and ranges of values:

·         The sbyte type represents signed 8-bit integers with values between –128 and 127.

·         The byte type represents unsigned 8-bit integers with values between 0 and 255.

·         The short type represents signed 16-bit integers with values between –32768 and 32767.

·         The ushort type represents unsigned 16-bit integers with values between 0 and 65535.

·         The int type represents signed 32-bit integers with values between –2147483648 and 2147483647.

·         The uint type represents unsigned 32-bit integers with values between 0 and 4294967295.

·         The long type represents signed 64-bit integers with values between –9223372036854775808 and 9223372036854775807.

·         The ulong type represents unsigned 64-bit integers with values between 0 and 18446744073709551615.

·         The char type represents unsigned 16-bit integers with values between 0 and 65535. The set of possible values for the char type corresponds to the Unicode character set. Although char has the same representation as ushort, not all operations permitted on one type are permitted on the other.

The integral-type unary and binary operators always operate with signed 32-bit precision, unsigned 32-bit precision, signed 64-bit precision, or unsigned 64-bit precision:

·         For the unary + and ~ operators, the operand is converted to type T, where T is the first of int, uint, long, and ulong that can fully represent all possible values of the operand. The operation is then performed using the precision of type T, and the type of the result is T.

·         For the unary – operator, the operand is converted to type T, where T is the first of int and long that can fully represent all possible values of the operand. The operation is then performed using the precision of type T, and the type of the result is T. The unary – operator cannot be applied to operands of type ulong.

·         For the binary +, –, *, /, %, &, ^, |, ==, !=, >, <, >=, and <= operators, the operands are converted to type T, where T is the first of int, uint, long, and ulong that can fully represent all possible values of both operands. The operation is then performed using the precision of type T, and the type of the result is T (or bool for the relational operators). It is not permitted for one operand to be of type long and the other to be of type ulong with the binary operators.

·         For the binary << and >> operators, the left operand is converted to type T, where T is the first of int, uint, long, and ulong that can fully represent all possible values of the operand. The operation is then performed using the precision of type T, and the type of the result is T.

The char type is classified as an integral type, but it differs from the other integral types in two ways:

·         There are no implicit conversions from other types to the char type. In particular, even though the sbyte, byte, and ushort types have ranges of values that are fully representable using the char type, implicit conversions from sbyte, byte, or ushort to char do not exist.

·         Constants of the char type must be written as character-literals or as integer-literals in combination with a cast to type char. For example, (char)10 is the same as '\x000A'.

The checked and unchecked operators and statements are used to control overflow checking for integral-type arithmetic operations and conversions (§‎7.5.12). In a checked context, an overflow produces a compile-time error or causes a System.OverflowException to be thrown. In an unchecked context, overflows are ignored and any high-order bits that do not fit in the destination type are discarded.

4.1.6 Floating point types

C# supports two floating point types: float and double. The float and double types are represented using the 32-bit single-precision and 64-bit double-precision IEEE 754 formats, which provide the following sets of values:

·         Positive zero and negative zero. In most situations, positive zero and negative zero behave identically as the simple value zero, but certain operations distinguish between the two (§‎7.7.2).

·         Positive infinity and negative infinity. Infinities are produced by such operations as dividing a non-zero number by zero. For example, 1.0 / 0.0 yields positive infinity, and –1.0 / 0.0 yields negative infinity.

·         The Not-a-Number value, often abbreviated NaN. NaNs are produced by invalid floating-point operations, such as dividing zero by zero.

·         The finite set of non-zero values of the form s × m × 2^e, where s is 1 or −1, and m and e are determined by the particular floating-point type: For float, 0 < m < 2²⁴ and −149 ≤ e ≤ 104, and for double, 0 < m < 2⁵³ and −1075 ≤ e ≤ 970. Denormalized floating-point numbers are considered valid non-zero values.

The float type can represent values ranging from approximately 1.5 × 10⁻⁴⁵ to 3.4 × 10³⁸ with a precision of 7 digits.

The double type can represent values ranging from approximately 5.0 × 10⁻³²⁴ to 1.7 × 10³⁰⁸ with a precision of 15-16 digits.

If one of the operands of a binary operator is of a floating-point type, then the other operand must be of an integral type or a floating-point type, and the operation is evaluated as follows:

·         If one of the operands is of an integral type, then that operand is converted to the floating-point type of the other operand.

·         Then, if either of the operands is of type double, the other operand is converted to double, the operation is performed using at least double range and precision, and the type of the result is double (or bool for the relational operators).

·         Otherwise, the operation is performed using at least float range and precision, and the type of the result is float (or bool for the relational operators).

The floating-point operators, including the assignment operators, never produce exceptions. Instead, in exceptional situations, floating-point operations produce zero, infinity, or NaN, as described below:

·         If the result of a floating-point operation is too small for the destination format, the result of the operation becomes positive zero or negative zero.

·         If the result of a floating-point operation is too large for the destination format, the result of the operation becomes positive infinity or negative infinity.

·         If a floating-point operation is invalid, the result of the operation becomes NaN.

·         If one or both operands of a floating-point operation is NaN, the result of the operation becomes NaN.

Floating-point operations may be performed with higher precision than the result type of the operation. For example, some hardware architectures support an “extended” or “long double” floating-point type with greater range and precision than the double type, and implicitly perform all floating-point operations using this higher precision type. Only at excessive cost in performance can such hardware architectures be made to perform floating-point operations with less precision, and rather than require an implementation to forfeit both performance and precision, C# allows a higher precision type to be used for all floating-point operations. Other than delivering more precise results, this rarely has any measurable effects. However, in expressions of the form x * y / z, where the multiplication produces a result that is outside the double range, but the subsequent division brings the temporary result back into the double range, the fact that the expression is evaluated in a higher range format may cause a finite result to be produced instead of an infinity.

4.1.7 The decimal type

The decimal type is a 128-bit data type suitable for financial and monetary calculations. The decimal type can represent values ranging from 1.0 × 10⁻²⁸ to approximately 7.9 × 10²⁸ with 28-29 significant digits.

The finite set of values of type decimal are of the form (–1)^s × c × 10^-e, where the sign s is 0 or 1, the coefficient c is given by 0 ≤ c < 2⁹⁶, and the scale e is such that 0 ≤ e ≤ 28.The decimal type does not support signed zeros, infinities, or NaN's. A decimal is represented as a 96-bit integer scaled by a power of ten. For decimals with an absolute value less than 1.0m, the value is exact to the 28^th decimal place, but no further. For decimals with an absolute value greater than or equal to 1.0m, the value is exact to 28 or 29 digits. Contrary to the float and double data types, decimal fractional numbers such as 0.1 can be represented exactly in the decimal representation. In the float and double representations, such numbers are often infinite fractions, making those representations more prone to round-off errors.

If one of the operands of a binary operator is of type decimal, then the other operand must be of an integral type or of type decimal. If an integral type operand is present, it is converted to decimal before the operation is performed.

The result of an operation on values of type decimal is that which would result from calculating an exact result (preserving scale, as defined for each operator) and then rounding to fit the representation. Results are rounded to the nearest representable value, and, when a result is equally close to two representable values, to the value that has an even number in the least significant digit position (this is known as “banker’s rounding”). A zero result always has a sign of 0 and a scale of 0.

If a decimal arithmetic operation produces a value less than or equal to 5 × 10^-29 in absolute value, the result of the operation becomes zero. If a decimal arithmetic operation produces a result that is too large for the decimal format, a System.OverflowException is thrown.

The decimal type has greater precision but smaller range than the floating-point types. Thus, conversions from the floating-point types to decimal might produce overflow exceptions, and conversions from decimal to the floating-point types might cause loss of precision. For these reasons, no implicit conversions exist between the floating-point types and decimal, and without explicit casts, it is not possible to mix floating-point and decimal operands in the same expression.

4.1.8 The bool type

The bool type represents boolean logical quantities. The possible values of type bool are true and false.

No standard conversions exist between bool and other types. In particular, the bool type is distinct and separate from the integral types, and a bool value cannot be used in place of an integral value, and vice versa.

In the C and C++ languages, a zero integral or floating-point value, or a null pointer can be converted to the boolean value false, and a non-zero integral or floating-point value, or a non-null pointer can be converted to the boolean value true. In C#, such conversions are accomplished by explicitly comparing an integral or floating-point value to zero, or by explicitly comparing an object reference to null.

4.1.9 Enumeration types

An enumeration type is a distinct type with named constants. Every enumeration type has an underlying type, which must be byte, sbyte, short, ushort, int, uint, long or ulong. The set of values of the enumeration type is the same as the set of values of the underlying type. Values of the enumeration type are not restricted to the values of the named constants. Enumeration types are defined through enumeration declarations (§‎14.1).

4.2 Reference types

A reference type is a class type, an interface type, an array type, or a delegate type.

reference-type:
class-type
interface-type
array-type
delegate-type

class-type:
type-name
object
string

interface-type:
type-name

array-type:
non-array-type   rank-specifiers

non-array-type:
type

rank-specifiers:
rank-specifier
rank-specifiers   rank-specifier

rank-specifier:
[   dim-separators_opt   ]

 dim-separators:
,
dim-separators   ,

delegate-type:
type-name

A reference type value is a reference to an instance of the type, the latter known as an object. The special value null is compatible with all reference types and indicates the absence of an instance.

4.2.1 Class types

A class type defines a data structure that contains data members (constants and fields), function members (methods, properties, events, indexers, operators, instance constructors, destructors and static constructors), and nested types. Class types support inheritance, a mechanism whereby derived classes can extend and specialize base classes. Instances of class types are created using object-creation-expressions (§‎7.5.10.1).

Class types are described in §‎10.

Certain predefined class types have special meaning in the C# language, as described in the table below.

 

Class type	Description
System.Object	The ultimate base class of all other types. See §‎4.2.2.
System.String	The string type of the C# language. See §‎4.2.3.
System.ValueType	The base class of all value types. See §‎4.1.1.
System.Enum	The base class of all enum types. See §‎14.
System.Array	The base class of all array types. See §‎12.
System.Delegate	The base class of all delegate types. See §‎15.
System.Exception	The base class of all exception types. See §‎16.

 

4.2.2 The object type

The object class type is the ultimate base class of all other types. Every type in C# directly or indirectly derives from the object class type.

The keyword object is simply an alias for the predefined class System.Object.

4.2.3 The string type

The string type is a sealed class type that inherits directly from object. Instances of the string class represent Unicode character strings.

Values of the string type can be written as string literals (§‎2.4.4).

The keyword string is simply an alias for the predefined class System.String.

4.2.4 Interface types

An interface defines a contract. A class or struct that implements an interface must adhere to its contract. An interface may inherit from multiple base interfaces, and a class or struct may implement multiple interfaces.

Interface types are described in §‎13.

4.2.5 Array types

An array is a data structure that contains zero or more variables which are accessed through computed indices. The variables contained in an array, also called the elements of the array, are all of the same type, and this type is called the element type of the array.

Array types are described in §‎12.

4.2.6 Delegate types

A delegate is a data structure that refers to one or more methods, and for instance methods, it also refers to their corresponding object instances.

The closest equivalent of a delegate in C or C++ is a function pointer, but whereas a function pointer can only reference static functions, a delegate can reference both static and instance methods. In the latter case, the delegate stores not only a reference to the method’s entry point, but also a reference to the object instance on which to invoke the method.

Delegate types are described in §‎15.

4.3 Boxing and unboxing

The concept of boxing and unboxing is central to C#’s type system. It provides a bridge between value-types and reference-types by permitting any value of a value-type to be converted to and from type object. Boxing and unboxing enables a unified view of the type system wherein a value of any type can ultimately be treated as an object.

4.3.1 Boxing conversions

A boxing conversion permits a value-type to be implicitly converted to a reference-type. The following boxing conversions exist:

·         From any value-type (including any enum-type) to the type object.

·         From any value-type (including any enum-type) to the type System.ValueType.

·         From any value-type to any interface-type implemented by the value-type.

·         From any enum-type to the type System.Enum.

Boxing a value of a value-type consists of allocating an object instance and copying the value-type value into that instance.

The actual process of boxing a value of a value-type is best explained by imagining the existence of a boxing class for that type. For any value-type T, the boxing class behaves as if it were declared as follows:

sealed class T_Box: System.ValueType
{
   T value;

   public T_Box(T t) {
      value = t;
   }
}

Boxing of a value v of type T now consists of executing the expression new T_Box(v), and returning the resulting instance as a value of type object. Thus, the statements

int i = 123;
object box = i;

conceptually correspond to

int i = 123;
object box = new int_Box(i);

Boxing classes like T_Box and int_Box above don’t actually exist and the dynamic type of a boxed value isn’t actually a class type. Instead, a boxed value of type T has the dynamic type T, and a dynamic type check using the is operator can simply reference type T. For example,

int i = 123;
object box = i;
if (box is int) {
   Console.Write("Box contains an int");
}

will output the string “Box contains an int” on the console.

A boxing conversion implies making a copy of the value being boxed. This is different from a conversion of a reference-type to type object, in which the value continues to reference the same instance and simply is regarded as the less derived type object. For example, given the declaration

struct Point
{
   public int x, y;

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

the following statements

Point p = new Point(10, 10);
object box = p;
p.x = 20;
Console.Write(((Point)box).x);

will output the value 10 on the console because the implicit boxing operation that occurs in the assignment of p to box causes the value of p to be copied. Had Point been declared a class instead, the value 20 would be output because p and box would reference the same instance.

4.3.2 Unboxing conversions

An unboxing conversion permits a reference-type to be explicitly converted to a value-type. The following unboxing conversions exist:

·         From the type object to any value-type (including any enum-type).

·         From the type System.ValueType to any value-type (including any enum-type).

·         From any interface-type to any value-type that implements the interface-type.

·         From the type System.Enum to any enum-type.

An unboxing operation consists of first checking that the object instance is a boxed value of the given value-type, and then copying the value out of the instance.

Referring to the imaginary boxing class described in the previous section, an unboxing conversion of an object box to a value-type T consists of executing the expression ((T_Box)box).value. Thus, the statements

object box = 123;
int i = (int)box;

conceptually correspond to

object box = new int_Box(123);
int i = ((int_Box)box).value;

For an unboxing conversion to a given value-type to succeed at run-time, the value of the source operand must be a reference to an object that was previously created by boxing a value of that value-type. If the source operand is null, a System.NullReferenceException is thrown. If the source operand is a reference to an incompatible object, a System.InvalidCastException is thrown.