DataAnnotation with a configurable value?

Question

I'd like to set a DataAnnotation on a view model to a dynamic value that is configurable via the web.config. In the following example I get this error "An attribute argument must be a constant expression, typeof expression or array creation expression of an attribute parameter type". Anyone know if this is possible? Thanks

[DataType(DataType.Password)]
[RegularExpression(Properties.Settings.Default.PasswordExpression)]
public string Password { get; set; }

Answer 1

Attribute parameters must be constants, i.e. something whose value can be resolved at compile time. But you could write your own simple Attribute class that took the name of the the item in the appSettings, got the underlying value, and passed that on to the normal RegularExpression processing. Then your attribute would look like this:

[ConfigedRegularExpression("PasswordExpression")]

where PasswordExpression was the name of the app setting containing the actual regular expression string.

and, after writing this and doing a search (I should have done that first), I see someone's worked it out for you here:

How to write custom RegularExpressionValidator which takes the values from the config file?