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C# allows the predefined implicit and explicit conversions to be augmented by userdefined conversions. Userdefined conversions are introduced by declaring conversion operators (§10.9.3) in class and struct types.
C# permits only certain userdefined 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 interfacetype.
T is not a base class of S, and S is not a base class of T.
The restrictions that apply to userdefined conversions are discussed further in §10.9.3.
A userdefined conversion converts a value from its type, called the source type, to another type, called the target type. Evaluation of a userdefined conversion centers on finding the most specific userdefined 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 userdefined 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 userdefined operators, and that nonclass types have no base classes).
From that set of types, determining which userdefined conversion operators are applicable. For a conversion operator to be applicable, it must be possible to perform a standard conversion (§6.3) 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 userdefined 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 userdefined conversion operator are defined in the following sections.
Once a most specific userdefined 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 userdefined conversion operator to perform the conversion.
Finally, if required, performing a standard conversion from the result type of the userdefined conversion operator to the target type.
Evaluation of a userdefined conversion never involves more than one userdefined conversion operator. In other words, a conversion from type S to type T will never first execute a userdefined conversion from S to X and then execute a userdefined conversion from X to T.
Exact definitions of evaluation of userdefined implicit or explicit conversions are given in the following sections. The definitions make use of the following terms:
If a standard implicit conversion (§6.3.1) exists from a type A to a type B, and if neither A nor B are interfacetypes, 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.
A userdefined implicit conversion from type S to type T is processed as follows:
Find the set of types, D, from which userdefined 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 userdefined conversion operators, U. This set consists of the userdefined 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 an error occurs.
Find the most specific source type, S_{X}, of the operators in U:
If any of the operators in U convert from S, then S_{X} is S.
Otherwise, S_{X} 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 an error occurs.
Find the most specific target type, T_{X}, of the operators in U:
If any of the operators in U convert to T, then T_{X} is T.
Otherwise, T_{X} 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 an error occurs.
If U contains exactly one userdefined conversion operator that converts from S_{X} to T_{X}, 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 an error occurs. Otherwise, the userdefined conversion is applied:
If S is not S_{X}, then a standard implicit conversion from S to S_{X} is performed.
The most specific userdefined conversion operator is invoked to convert from S_{X} to T_{X}.
If T_{X} is not T, then a standard implicit conversion from T_{X} to T is performed.
A userdefined explicit conversion from type S to type T is processed as follows:
Find the set of types, D, from which userdefined 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 userdefined conversion operators, U. This set consists of the userdefined 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 an error occurs.
Find the most specific source type, S_{X}, of the operators in U:
If any of the operators in U convert from S, then S_{X} is S.
Otherwise, if any of the operators in U convert from types that encompass S, then S_{X} 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 an error occurs.
Otherwise, S_{X} 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 an error occurs.
Find the most specific target type, T_{X}, of the operators in U:
If any of the operators in U convert to T, then T_{X} is T.
Otherwise, if any of the operators in U convert to types that are encompassed by T, then T_{X} is the most encompassing type in the combined set of source types of those operators. If no most encompassing type can be found, then the conversion is ambiguous and an error occurs.
Otherwise, T_{X} 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 an error occurs.
If U contains exactly one userdefined conversion operator that converts from S_{X} to T_{X}, 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 an error occurs. Otherwise, the userdefined conversion is applied:
If S is not S_{X}, then a standard explicit conversion from S to S_{X} is performed.
The most specific userdefined conversion operator is invoked to convert from S_{X} to T_{X}.
If T_{X} is not T, then a standard explicit conversion from T_{X} to T is performed.
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