How can I extract the regular expression from a RegularExpressionAttribute?

Question

I have a class like

public class Foo
{
    [RegularExpression(@"([A-Za-z0-9\-_ ]+){1,100}")]
    public string Bar { get; set; }
}

and for the purpose of unit testing I want to be able to extract out the "@"([A-Za-z0-9\\-_ ]+){1,100}" .

I know it's something like

string expr = typeof(Foo).GetProperty("Bar").....

but I don't quite know how to finish it.

Answer 1

var property = typeof(Foo).GetProperty("Bar");
var attribute = property.GetCustomAttribute<RegularExpressionAttribute>();
var expr = attribute?.Pattern;

Or single statement:

var expr = typeof(Foo).GetProperty("Bar")
                      .GetCustomAttribute<RegularExpressionAttribute>()?.Pattern;

NOTE: I don't think that you should extract data from property attributes for unit testing. I would either leave property validation for acceptance tests. Or used something like Validator class to actually run validation on your object.