The following conversions are classified as explicit conversions: ?All implicit conversions. ?Explicit numeric conversions. ?Explicit enumeration conversions. ?Explicit reference conversions. ?Explicit interface conversions. ?Unboxing conversions. ?User-defined explicit conversions. Explicit conversions can occur in cast expressions (?4.6.6). The set of explicit conversions includes all implicit conversions. [Note: This means that redundant cast expressions are allowed. end note] The explicit conversions that are not implicit conversions are conversions that cannot be proven to always succeed, conversions that are known to possibly lose information, and conversions across domains of types sufficiently different to merit explicit notation. 13.2.1 Explicit numeric conversions The explicit numeric conversions are the conversions from a numeric-type to another numeric-type for which an implicit numeric conversion (?3.1.2) does not already exist: ?From sbyte to byte, ushort, uint, ulong, or char. ?From byte to sbyte and char. ?From short to sbyte, byte, ushort, uint, ulong, or char. ?From ushort to sbyte, byte, short, or char. ?From int to sbyte, byte, short, ushort, uint, ulong, or char. ?From uint to sbyte, byte, short, ushort, int, or char. ?From long to sbyte, byte, short, ushort, int, uint, ulong, or char. ?From ulong to sbyte, byte, short, ushort, int, uint, long, or char. ?From char to sbyte, byte, or short. ?From float to sbyte, byte, short, ushort, int, uint, long, ulong, char, or decimal. ?From double to sbyte, byte, short, ushort, int, uint, long, ulong, char, float, or decimal. C# LANGUAGE SPECIFICATION 116 ?From decimal to sbyte, byte, short, ushort, int, uint, long, ulong, char, float, or double. Because the explicit conversions include all implicit and explicit numeric conversions, it is always possible to convert from any numeric-type to any other numeric-type using a cast expression (?4.6.6). The explicit numeric conversions possibly lose information or possibly cause exceptions to be thrown. An explicit numeric conversion is processed as follows: ?For a conversion from an integral type to another integral type, the processing depends on the overflow checking context (?4.5.12) in which the conversion takes place: In a checked context, the conversion succeeds if the value of the source operand is within the range of the destination type, but throws a System.OverflowException if the value of the source operand is outside the range of the destination type. In an unchecked context, the conversion always succeeds, and proceeds as follows. ?If the source type is larger than the destination type, then the source value is truncated by discarding its ?extra? most significant bits. The result is then treated as a value of the destination type. ?If the source type is smaller than the destination type, then the source value is either signextended or zero-extended so that it is the same size as the destination type. Sign-extension is used if the source type is signed; zero-extension is used if the source type is unsigned. The result is then treated as a value of the destination type. ?If the source type is the same size as the destination type, then the source value is treated as a value of the destination type ?For a conversion from decimal to an integral type, the source value is rounded towards zero to the nearest integral value, and this integral value becomes the result of the conversion. If the resulting integral value is outside the range of the destination type, a System.OverflowException is thrown. ?For a conversion from float or double to an integral type, the processing depends on the overflowchecking context (?4.5.12) in which the conversion takes place: In a checked context, the conversion proceeds as follows: ?The value is rounded towards zero to the nearest integral value. If this
integral value is within the range of the destination type, then this value is the result of the conversion. ?Otherwise, a System.OverflowException is thrown. In an unchecked context, the conversion always succeeds, and proceeds as follows. ?The value is rounded towards zero to the nearest integral value. If this integral value is within the range of the destination type, then this value is the result of the conversion. ?Otherwise, the result of the conversion is an unspecified value of the destination type. ?For a conversion from double to float, the double value is rounded to the nearest float value. If the double value is too small to represent as a float, the result becomes positive zero or negative zero. If the double value is too large to represent as a float, the result becomes positive infinity or negative infinity. If the double value is NaN, the result is also NaN. ?For a conversion from float or double to decimal, the source value is converted to decimal representation and rounded to the nearest number after the 28th decimal place if required (?1.1.6). If the source value is too small to represent as a decimal, the result becomes zero. If the source value is NaN, infinity, or too large to represent as a decimal, a System.OverflowException is thrown. ?For a conversion from decimal to float or double, the decimal value is rounded to the nearest double or float value. While this conversion may lose precision, it never causes an exception to be thrown. 13.2.2 Explicit enumeration conversions The explicit enumeration conversions are: ?From sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, or decimal to any enum-type. ?From any enum-type to sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, or decimal. ?From any enum-type to any other enum-type. An explicit enumeration conversion between two types is processed by treating any participating enum-type as the underlying type of that enum-type, and then performing an implicit or explicit numeric conversion between the resulting types. [Example: For example, given an enum-type E with and underlying type of int, a conversion from E to byte is processed as an explicit numeric conversion ( ?3.2.1) from int to byte, and a conversion from byte to E is processed as an implicit numeric conversion (?3.1.2) from byte to int. end example] 13.2.3 Explicit reference conversions The explicit reference conversions are: ?From object to any reference-type. ?From any class-type S to any class-type T, provided S is a base class of T. ?From any class-type S to any interface-type T, provided S is not sealed and provided S does not implement T. ?From any interface-type S to any class-type T, provided T is not sealed or provided T implements S. ?From any interface-type S to any interface-type T, provided S is not 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 explicit reference conversion exists from SE to TE. ?From System.Array and the interfaces it implements, to any array-type. ?From System.Delegate and the interfaces it implements, to any delegate-type. The explicit reference conversions are those conversions between reference-types that require run-time
注：本站部分文章源于互联网，版权归原作者所有！如有侵权，请原作者与本站联系，本站将立即删除！ 本站文章除特别注明外均可转载，但需注明出处！ [MinTao学以致用网] 
