Confusing warning about a constant decimal field in C#

Question

I was experimenting with the const modifier while exploring a plethora of C# tutorials, and placed a bunch of const modifiers in a class like this without actually using them anywhere:

class ConstTesting
{
    const decimal somedecimal = 1;
    const int someint = 2;
    ...
}

With this class, I get the following warning (using csc) :

ConstTesting.cs(3,19): warning CS0414: The field ‘ConstTesting.somedecimal’ is assigned but its value is never used

What I don't understand is that I only get the warning for the const decimal. The const int doesn't give me any warning, regardless of the order or anything like that.

My question is, why does this happen? Why would my csc compiler be warning me about a const in the first place, and if it is then more importantly why would it only be warning me about const decimal when I'm writing const int in exactly the same way? What on earth would the difference between int and decimal have to do with it?

Please note:

- I do not have ReSharper
- I am using VS 2010
- I am 100% certain neither `const` is being used anywhere in my code.

Answer 1

Int is a simple value type of a fixed size. Decimal is a bit more complicated due to scale. If you decompile your code, you'll find that it looks like this:

[DecimalConstant(0, 0, 0, 0, 1)]
private readonly static decimal somedecimal;

private const int someint = 2;

Where the decimal is not a constant, but has a DecimalConstant attribute courtesy of mscorlib.dll, where the true definition if decimal is:

public struct Decimal : IFormattable, IComparable, IConvertible,
IDeserializationCallback, IComparable<decimal>, IEquatable<decimal>

Much more in-depth exploration of this topic is covered in this blog post.