C# allows the pre-defined implicit and explicit conversions to be augmented
by user-defined conversions.
User-defined conversions are introduced by declaring conversion operators (?
7.9.3) in class and struct
types.
13.4.1 Permitted user-defined conversions
C# permits only certain user-defined conversions to be declared. In
particular, it is not possible to redefine
an already existing implicit or explicit conversion. A class or struct is
permitted to declare a conversion from
a source type S to a target type T only if all of the following are true:
?S and T are different types.
?Either S or T is the class or struct type in which the operator
declaration takes place.
?Neither S nor T is object or an interface-type.
?T is not a base class of S, and S is not a base class of T.
The restrictions that apply to user-defined conversions are discussed
further in ?7.9.3.
13.4.2 Evaluation of user-defined conversions
A user-defined conversion converts a value from its type, called the source
type, to another type, called the
target type. Evaluation of a user-defined conversion centers on finding the
most specific user-defined
conversion operator for the particular source and target types. This
determination is broken into several
steps:
?Finding the set of classes and structs from which user-defined conversion
operators will be considered.
This set consists of the source type and its base classes and the target
type and its base classes (with the
implicit assumptions that only classes and structs can declare user-defined
operators, and that non-class
types have no base classes).
?From that set of types, determining which user-defined conversion
operators are applicable. For a
conversion operator to be applicable, it must be possible to perform a
standard conversion (?3.3) from
the source type to the operand type of the operator, and it must be
possible to perform a standard
conversion from the result type of the operator to the target type.
?From the set of applicable user-defined operators, determining which
operator is unambiguously the
most specific. In general terms, the most specific operator is the operator
whose operand type is
?closest? to the source type and whose result type is ?closest? to the
target type. The exact rules for
establishing the most specific user-defined conversion operator are defined
in the following sections.
Once a most specific user-defined conversion operator has been identified,
the actual execution of the userdefined
conversion involves up to three steps:
?First, if required, performing a standard conversion from the source type
to the operand type of the userdefined
conversion operator.
?Next, invoking the user-defined conversion operator to perform the
conversion.
?Finally, if required, performing a standard conversion from the result
type of the user-defined
conversion operator to the target type.
Evaluation of a user-defined conversion never involves more than one
user-defined conversion operator. In
other words, a conversion from type S to type T will never first execute a
user-defined conversion from S to
X and then execute a user-defined conversion from X to T.
Exact definitions of evaluation of user-defined implicit or explicit
conversions are given in the following
sections. The definitions make use of the following terms:
?If a standard implicit conversion (?3.3.1) exists from a type A to a
type B, and if neither A nor B are
interface-types, then A is said to be encompassed by B, and B is said to
encompass A.
?The most encompassing type in a set of types is the one type that
encompasses all other types in the set.
If no single type encompasses all other types, then the set has no most
encompassing type. In more
intuitive terms, the most encompassing type is the ?largest? type in the
set?the one type to which each
of the other types can be implicitly converted.
?The most encompassed type in a set of types is the one type that is
encompassed by all other types in the
set. If no single type is encompassed by all other types, then the set has
no most encompassed type. In
more intuitive terms, the most encompassed type is the ?smallest? type in
the set?the one type that can
be implicitly converted to each of the other types.
13.4.3 User-defined implicit conversions
A user-defined implicit conversion from type S to type T is processed as
follows:
?Find the set of types, D, from which user-defined conversion operators
will be considered. This set
consists of S (if S is a class or struct), the base classes of S (if S is a
class), T (if T is a class or struct),
and the base classes of T (if T is a class).
?Find the set of applicable user-defined conversion operators, U. This set
consists of the user-defined
implicit conversion operators declared by the classes or structs in D that
convert from a type
encompassing S to a type encompassed by T. If U is empty, the conversion is
undefined and a compiletime
error occurs.
?Find the most specific source type, SX, of the operators in U:
If any of the operators in U convert from S, then SX is S.
Otherwise, SX is the most encompassed type in the combined set of source
types of the operators in U. If no
most encompassed type can be found, then the conversion is ambiguous and a
compile-time error occurs.
?Find the most specific target type, TX, of the operators in U:
If any of the operators in U convert to T, then TX is T.
Otherwise, TX is the most encompassing type in the combined set of target
types of the operators in U. If no
most encompassing type can be found, then the conversion is ambiguous and a
compile-time error occurs.
?If U contains exactly one user-defined conversion operator that converts
from SX to TX, then this is the
most specific conversion operator. If no such operator exists, or if more
than one such operator exists,
then the conversion is ambiguous and a compile-time error occurs.
Otherwise, the user-defined
conversion is applied:
If S is not SX, then a standard implicit conversion from S to SX is
performed.
The most specific user-defined conversion operator is invoked to convert
from SX to TX.
If TX is not T, then a standard implicit conversion from TX to T is
performed.
13.4.4 User-defined explicit conversions
A user-defined explicit conversion from type S to type T is processed as
follows:
?Find the set of types, D, from which user-defined conversion operators
will be considered. This set
consists of S (if S is a class or struct), the base classes of S (if S is a
class), T (if T is a class or struct),
and the base classes of T (if T is a class).
?Find the set of applicable user-defined conversion operators, U. This set
consists of the user-defined
implicit or explicit conversion operators declared by the classes or
structs in D that convert from a type
encompassing or encompassed by S to a type encompassing or encompassed by
T. If U is empty, the
conversion is undefined and a compile-time error occurs.
?Find the most specific source type, SX, of the operators in U:
If any of the operators in U convert from S, then SX is S.
Otherwise, if any of the operators in U convert from types that encompass
S, then SX is the most
encompassed type in the combined set of source types of those operators. If
no most encompassed type can
be found, then the conversion is ambiguous and a compile-time error occurs.
Otherwise, SX is the most encompassing type in the combined set of source
types of the operators in U. If no
most encompassing type can be found, then the conversion is ambiguous and a
compile-time error occurs.
?Find the most specific target type, TX, of the operators in U:
If any of the operators in U convert to T, then TX is T.
Otherwise, if any of the operators in U convert to types that are
encompassed by T, then TX is the most
encompassing type in the combined set of target types of those operators.
If no most encompassing type can
be found, then the conversion is ambiguous and a compile-time error occurs.
Otherwise, TX is the most encompassed type in the combined set of target
types of the operators in U. If no
most encompassed type can be found, then the conversion is ambiguous and a
compile-time error occurs.
?If U contains exactly one user-defined conversion operator that converts
from SX to TX, then this is the
most specific conversion operator. If no such operator exists, or if more
than one such operator exists,
then the conversion is ambiguous and a compile-time error occurs.
Otherwise, the user-defined
conversion is applied:
If S is not SX, then a standard explicit conversion from S to SX is
performed.
The most specific user-defined conversion operator is invoked to convert
from SX to TX.
If TX is not T, then a standard explicit conversion from TX to T is
performed.
