Question

Why is decimal not a primitive type?

Console.WriteLine(typeof(decimal).IsPrimitive);

outputs false.

It is a base type, it’s part of the specifications of the language, but not a primitive. What primitive type(s) do represent a decimal in the framework? An int for example has a field m_value of type int. A double has a field m_value of type double. It’s not the case for decimal. It seems to be represented by a bunch of ints but I’m not sure.

Why does it look like a primitive type, behaves like a primitive type (except in a couple of cases) but is not a primitive type?

Answer 1

Although not a direct answer, the documentation for IsPrimitive lists what the primitive types are:

http://msdn.microsoft.com/en-us/library/system.type.isprimitive.aspx

A similar question was asked here:

http://bytes.com/topic/c-sharp/answers/233001-typeof-decimal-isprimitive-false-bug-feature

Answer quoted from Jon Skeet:

The CLR doesn’t need to have any intrinsic knowledge about the decimal
type – it treats it just as another value type which happens to have
overloaded operators. There are no IL instructions to operate directly
on decimals, for instance.

To me, it seems as though decimal is a type that must exist for a language/runtime wanting to be CLS/CLI-compliant (and is hence termed “primitive” because it is a base type with keyword support), but the actual implementation does not require it to be truly “primitive” (as in the CLR doesn’t think it is a primitive data type).