Explicit conversions

Explicit conversions

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 (‎7.6.8).

The set of explicit conversions includes all implicit conversions. In particular, this means that redundant cast expressions are allowed.

The explicit conversions are conversions that cannot be proved to always succeed, conversions that are known to possibly lose information, and conversions across domains of types sufficiently different to merit explicit notation.

Explicit numeric conversions

The explicit numeric conversions are the conversions from a numeric-type to another numeric-type for which an implicit numeric conversion (‎6.1.2) 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.

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 (‎7.6.8).

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 (‎7.5.12) 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 an 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 sign-extended 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 float, double, or 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 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 28^th decimal place if required (‎4.1.6). 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.InvalidCastException 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.

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. 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 (‎6.2.1) from int to byte, and a conversion from byte to E is processed as an implicit numeric conversion (‎6.1.2) from byte to int.

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 S_E to an array-type T with an element type T_E, 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 S_E and T_E are reference-types.

An explicit reference conversion exists from S_E to T_E.

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 checks to ensure they are correct.

For an explicit reference conversion to succeed at run-time, the value of the source operand must be null or the actual type of the object referenced by the source operand must be a type that can be converted to the destination type by an implicit reference conversion (‎6.1.4). If an explicit reference conversion fails, a System.InvalidCastException is thrown.

Reference conversions, implicit or explicit, never change the referential identity of the object being converted. In other words, while a reference conversion may change the type of a value, it never changes the value itself.

Unboxing conversions

An unboxing conversion permits an explicit conversion from type object to any value-type or from any interface-type to any value-type that implements the interface-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.

Unboxing conversions are described further in ‎4.3.2.

User-defined explicit conversions

A user-defined explicit conversion consists of an optional standard explicit conversion, followed by execution of a user-defined implicit or explicit conversion operator, followed by another optional standard explicit conversion. The exact rules for evaluating user-defined conversions are described in ‎6.4.4.