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);
    }
}

The code is only a generic example of what I am trying to do. — Anton
– Anton, Commented Jun 4, 2012 at 10:29
Don't you mean to call var attrs = AttributeReader.Read(Person);, not var attrs = AttributeReader.Read(FirstName); ? — ProfK
– ProfK, Commented Mar 28, 2013 at 13:27

Marc Gravell · Accepted Answer · 2012-06-04 10:31:58Z

14

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");

edited Jun 4, 2012 at 10:31

answered Jun 4, 2012 at 10:26

Marc Gravell

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3 Comments

Marc Gravell Over a year ago

@Segfault my caveat still applies though: I personally wouldn't let that code anywhere near an object constructor - it is the wrong place for it.

Anton Over a year ago

Yeah, I was just trying to show what I was trying to do while making the code as clear as possible

b.g Over a year ago

How can I check if a particular attribute is required or not? I am creating an instance of a class as object obj = Activator.CreateInstance("myModel") and I need to check if a particular property is required or not: obj.GetType().GetProperty("prop1")........

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