13.1 Implicit conversions

王朝other·作者佚名  2006-01-10
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13.1 Implicit conversions

The following conversions are classified as implicit conversions:

?Identity conversions

?Implicit numeric conversions

?Implicit enumeration conversions.

?Implicit reference conversions

?Boxing conversions

?Implicit constant expression conversions

?User-defined implicit conversions

Implicit conversions can occur in a variety of situations, including

function member invocations (?4.4.3),

cast expressions (?4.6.6), and assignments (?4.13).

The pre-defined implicit conversions always succeed and never cause

exceptions to be thrown. [Note:

Properly designed user-defined implicit conversions should exhibit these

characteristics as well. end note]

13.1.1 Identity conversion

An identity conversion converts from any type to the same type. This

conversion exists only such that an

entity that already has a required type can be said to be convertible to

that type.

13.1.2 Implicit numeric conversions

The implicit numeric conversions are:

?From sbyte to short, int, long, float, double, or decimal.

?From byte to short, ushort, int, uint, long, ulong, float, double, or

decimal.

?From short to int, long, float, double, or decimal.

?From ushort to int, uint, long, ulong, float, double, or decimal.

?From int to long, float, double, or decimal.

?From uint to long, ulong, float, double, or decimal.

?From long to float, double, or decimal.

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?From ulong to float, double, or decimal.

?From char to ushort, int, uint, long, ulong, float, double, or decimal.

?From float to double.

Conversions from int, uint, long or ulong to float and from long or ulong

to double may cause a

loss of precision, but will never cause a loss of magnitude. The other

implicit numeric conversions never

lose any information.

There are no implicit conversions to the char type, so values of the other

integral types do not automatically

convert to the char type.

13.1.3 Implicit enumeration conversions

An implicit enumeration conversion permits the decimal-integer-literal 0 to

be converted to any enum-type.

13.1.4 Implicit reference conversions

The implicit reference conversions are:

?From any reference-type to object.

?From any class-type S to any class-type T, provided S is derived from T.

?From any class-type S to any interface-type T, provided S implements T.

?From any interface-type S to any interface-type T, provided S is derived

from T.

?From an array-type S with an element type SE to an array-type T with an

element type TE, provided all

of the following are true:

S and T differ only in element type. In other words, S and T have the same

number of dimensions.

Both SE and TE are reference-types.

An implicit reference conversion exists from SE to TE.

?From any array-type to System.Array.

?From any delegate-type to System.Delegate.

?From any array-type or delegate-type to System.ICloneable.

?From the null type to any reference-type.

The implicit reference conversions are those conversions between

reference-types that can be proven to

always succeed, and therefore require no checks at run-time.

Reference conversions, implicit or explicit, never change the referential

identity of the object being

converted. [Note: In other words, while a reference conversion may change

the type of the reference, it never

changes the type or value of the object being referred to. end note]

13.1.5 Boxing conversions

A boxing conversion permits any value-type to be implicitly converted to

the type object or

System.ValueType or to any interface-type implemented by the value-type.

Boxing a value of a valuetype

consists of allocating an object instance and copying the value-type value

into that instance. A struct

can be boxed to the type System.ValueType, since that is a base class for

all structs (?8.3.2).

Boxing conversions are described further in ?1.3.1.

13.1.6 Implicit constant expression conversions

An implicit constant expression conversion permits the following

conversions:

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115

?A constant-expression (?4.15) of type int can be converted to type

sbyte, byte, short, ushort,

uint, or ulong, provided the value of the constant-expression is within the

range of the destination

type.

?A constant-expression of type long can be converted to type ulong,

provided the value of the constantexpression

is not negative.

13.1.7 User-defined implicit conversions

A user-defined implicit conversion consists of an optional standard

implicit conversion, followed by

execution of a user-defined implicit conversion operator, followed by

another optional standard implicit

conversion. The exact rules for evaluating user-defined conversions are

described in ?3.4.3.

 
 
 
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