'Decimal' source code from Microsoft - will it build?

Question

I was recently attempting to answer a question that a user posted about why the decimal struct does not declare its Min/Max values as const like ev

Answer 1

There are a few aspects of mscorlib and the like which wouldn't compile as-written, without some interesting hacks. In particular, there are some cyclic dependencies. This is another case, but I think it's reasonable to consider MaxValue and MinValue as being const as far as the C# compiler is concerned.

In particular, it's valid to use them within other const calculations:

const decimal Sum = decimal.MaxValue + decimal.MinValue;

The fields have the DecimalConstantAttribute applied to them, which is effectively a hack to get around an impedance mismatch between C# and the CLR: you can't have a constant field of type decimal in the CLR in the same way that you can have a constant field of type int or string, with an IL declaration using static literal ....

(This is also why you can't use decimal values in attribute constructors - there, the "const-ness" requirement is true IL-level constness.)

Instead, any const decimal declaration in C# code is compiled to a static initonly field with DecimalConstantAttribute applied to it specifying the appropriate data. The C# compiler uses that information to treat such a field as a constant expression elsewhere.

Basically, decimal in the CLR isn't a "known primitive" type in the way that int, float etc are. There are no decimal-specific IL instructions.

Now, in terms of the specific C# code you're referring to, I suspect there are two possibilities:

• No, this isn't the exact source code used.
• The C# compiler used to compile mscorlib and other core aspects of the framework may have special flags applied to allow such code, converting it directly to DecimalConstantAttribute

To a large extent you can ignore this - it won't affect you. It's a shame that MSDN documents the fields as being static readonly rather than const though, as that gives the mistaken impression that one can't use them in const expressions :(