Question

It is really unbelievable but real. This code will not work:

[AttributeUsage(AttributeTargets.Property|AttributeTargets.Field)]
public class Range : Attribute
{
    public decimal Max { get; set; }
    public decimal Min { get; set; }
}

public class Item
{
    [Range(Min=0m,Max=1000m)]  //compile error:'Min' is not a valid named attribute argument because it is not a valid attribute parameter type 
    public decimal Total { get; set; }  
}

While this works:

[AttributeUsage(AttributeTargets.Property|AttributeTargets.Field)]
public class Range : Attribute
{
    public double Max { get; set; }
    public double Min { get; set; }
}

public class Item
{
    [Range(Min=0d,Max=1000d)]
    public decimal Total { get; set; }  
}

Who can tell me why double is OK while decimal is not.

Answer 1

This is a CLR restriction. Only
primitive constants or arrays of
primitives can be used as attribute
parameters. The reason why is that an
attribute must be encoded entirely in
metadata. This is different than a
method body which is coded in IL.
Using MetaData only severely restricts
the scope of values that can be used.
In the current version of the CLR,
metadata values are limited to
primitives, null, types and arrays of
primitives (may have missed a minor
one).

Taken from this answer by JaredPar.

Decimals while a basic type are not a
primitive type and hence cannot be
represented in metadata which prevents
it from being an attribute parameter.