When i define this:

[Xtensive.Orm.Field(DefaultValue=123456789.12M)]
public decimal MyField {get;set;}

Compiler throws error:

An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type

But when i use Double type like this:

[Xtensive.Orm.Field(DefaultValue=123456789.12D)]
public double MyField {get;set;}

Everythink compiled and works.

When you look deeper at first example 123456789.12M is shorthand for decimal value, but cannot be used (by some reason that suffix M for decimal don't work same way like other numeric suffixes D-double,L-long,...) in Attribute as value of property DefaultValue. I found "workaround" for this situation here. Solution is to make something what is made in framework type System.ComponentModel.DefaultValueAttribute, where is constructor System.Type which define type of value.

My suggestion is to add another optional property to class FieldAttribute, maybe with name DefaultValueType of type System.Type. When user do not specify directly this type it will continue to work like now, but when user specify this DefaultValueType then you make type conversion (by TypeConverter?) to get the real typed value. In my case i would write this:

[Xtensive.Orm.Field(DefaultValueType=typeof(Decimal), DefaultValue="123456789.12")]
public decimal MyField {get;set;}

and DO converts string "123456789.12" to type Decimal specified in property DefaultValueType.

Update: Found same thing in attribute Xtensive.Orm.TypeDiscriminatorValue where property Value is of type object. Maybe there could be also new property ValueType of System.Type ?