Why can't Double be implicitly cast to Decimal
I don't understand the casting rules when it comes to decimal and double.
It is legal to do this
decimal dec = 10;
double doub = (double) dec;
What confuses me however is that decimal is a 16 byte datatype and double is 8 bytes so isn't casting a double to a decimal a widening conversation and should therefore be allowed implicitly; with the example above disallowed?
double doub = 3.2;
decimal dec = doub; // CS0029: Cannot implicitly convert type 'double' to 'decimal'
If you convert from double
to decimal
, you can lose information - the number may be completely out of range, as the range of a double
is much larger than the range of a decimal
.
If you convert from decimal
to double
, you can lose information - for example, 0.1 is exactly representable in decimal
but not in double
, and decimal
actually uses a lot more bits for precision than double
does.
Implicit conversions shouldn't lose information (the conversion from long
to double
might, but that's a different argument). If you're going to lose information, you should have to tell the compiler that you're aware of that, via an explicit cast.
That's why there aren't implicit conversions either way.
Decimal is more precise, so you would lose information. That's why you can only do it explicitely. It's to protect you from losing information. See MSDN
http://msdn.microsoft.com/en-us/library/678hzkk9%28v=VS.100%29.aspx
http://msdn.microsoft.com/en-us/library/364x0z75.aspx
You can explicitly cast in both directions: from double
to decimal
and from decimal
to double
.
You can't implicitly convert in either direction for a very good reason: the conversion may not be loss-less.
For example, the decimal number 1234567890123456789 can not be exactly represented as a double. Likewise, the double number 10^32
cannot be exactly represented as a decimal number.
To avoid losing information unintentionally, the implicit conversion is disallowed.