Unfortunately you can’t use decimals in attribute values, as the CLR itself doesn’t really know about System.Decimal type – it’s not a primitive type like int, double etc. The C# compiler basically fakes it for const fields of type decimal, but it can’t achieve the same effect with attributes.
From the C# 3 spec, section 17.1.3:
The types of positional and named
parameters for an attribute class are
limited to the attribute parameter
types, which are:
- One of the following
types: bool, byte, char, double,
float, int, long, sbyte, short,
string, uint, ulong, ushort.- The type object.
- The type System.Type.
- An enum
type, provided it has public
accessibility and the types in which
it is nested (if any) also have public
accessibility (§17.2).- Single-dimensional arrays of the above
types.
Then later in section 17.2:
An expression E is an attribute-argument-expression if all of the following statements are > true:
- The type of E is an attribute
parameter type (§17.1.3).- At
compile-time, the value of E can be
resolved to one of the following:
- A
constant value.- A System.Type object.
- A one-dimensional array of
attribute-argument-expressions.