Get custom attributes from an object

Question

When I try to get the custom attributes from an object the function returns null. Why?

class Person
{
    [ColumnName("first_name")]
    string FirstName { get; set; }

    Person()
    {
        FirstName = "not important";
        var attrs = AttributeReader.Read(FirstName);
    }
}

static class AttributeReader
{
    static object[] Read(object column)
    {
        return column.GetType().GetCustomAttributes(typeof(ColumnNameAttribute), false);
    }
}

Answer 1

You are passing a string, "not important" to that method. The Type is therefore typeof(string). Which does not have those attributes. Further, even Person doesn't have that attribute: only the MemberInfo (FirstName) has them.

There are ways of doing that by passing an Expression:

public static ColumnNameAttribute[] Read<T>(Expression<Func<T>> func)
{
    var member = func.Body as MemberExpression;
    if(member == null) throw new ArgumentException(
         "Lambda must resolve to a member");
    return (ColumnNameAttribute[])Attribute.GetCustomAttributes(
         member.Member, typeof(ColumnNameAttribute), false);
}

with

var attrs = AttributeReader.Read(() => FirstName);

However! I should advise that I'm not sure that the Person constructor is an appropriate place for this. Probably needs caching.

If you don't want to use lambdas, then passing a Type and the member-name would work too, i.e.

var attrs = AttributeReader.Read(typeof(Person), "FirstName");

(and do reflection from there) - or mixing with generics (for no real reason):

var attrs = Attribute.Read<Person>("FirstName");