How to get a list of properties with a given attribute?

Question

I have a type, t, and I would like to get a list of the public properties that have the attribute MyAttribute. The attribute is marked with AllowMultiple = false, like this:

[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]

Currently what I have is this, but I'm thinking there is a better way:

foreach (PropertyInfo prop in t.GetProperties())
{
    object[] attributes = prop.GetCustomAttributes(typeof(MyAttribute), true);
    if (attributes.Length == 1)
    {
         //Property with my custom attribute
    }
}

How can I improve this? My apologies if this is a duplicate, there are a ton of reflection threads out there...seems like it's quite a hot topic.

Nope. You need a PropertyInfo before you can find out if the property has an attribute. — Hans Passant
– Hans Passant, Commented Feb 17, 2010 at 15:48

Marc Gravell · Accepted Answer · 2010-02-17 16:09:52Z

481

var props = t.GetProperties().Where(
                prop => Attribute.IsDefined(prop, typeof(MyAttribute)));

This avoids having to materialize any attribute instances (i.e. it is cheaper than GetCustomAttribute[s]().

answered Feb 17, 2010 at 16:09

Marc Gravell

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7 Comments

wsanville Over a year ago

Good suggestion. I will however need the attribute instance, but I like it.

Örjan Jämte Over a year ago

I was just looking for a way to check the existence of an attribute without the side-effect that the property get is called. Thanks Marc, it work!

Marc Gravell Over a year ago

@ÖrjanJämte the property get is not called even when using GetCustomAttributes; however, the attribute is instantiated, which isn't free. If you don't need to check specific values of the attribute, IsDefined is cheaper. And in 4.5, there are ways to check the instantiation data without actually creating any attribute instances (although this is intended for very specific scenarios only)

Marc Gravell Over a year ago

@bjhuffine msdn.microsoft.com/en-us/library/…

Rtype Over a year ago

for dotnet core: var props = t.GetProperties().Where(e => e.IsDefined(typeof(MyAttribute)));

|

wsanville · Accepted Answer · 2011-03-09 17:38:52Z

56

The solution I end up using most is based off of Tomas Petricek's answer. I usually want to do something with both the attribute and property.

var props = from p in this.GetType().GetProperties()
            let attr = p.GetCustomAttributes(typeof(MyAttribute), true)
            where attr.Length == 1
            select new { Property = p, Attribute = attr.First() as MyAttribute};

answered Mar 9, 2011 at 17:38

wsanville

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1 Comment

Yawar Murtaza Over a year ago

+1 - "I usually want to do something with both the attribute and property" is what i was looking for - many thanks for posting your answer!

Black Frog · Accepted Answer · 2017-01-18 21:35:57Z

36

As far as I know, there isn't any better way in terms of working with Reflection library in a smarter way. However, you could use LINQ to make the code a bit nicer:

var props = from p in t.GetProperties()
            let attrs = p.GetCustomAttributes(typeof(MyAttribute), true)
            where attrs.Length != 0 select p;

// Do something with the properties in 'props'

I believe this helps you to structure the code in a more readable fashion.

edited Jan 18, 2017 at 21:35

Black Frog

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answered Feb 17, 2010 at 15:39

Tomas Petricek

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Comments

P Daddy · Accepted Answer · 2010-02-17 15:43:10Z

12

There's always LINQ:

t.GetProperties().Where(
    p=>p.GetCustomAttributes(typeof(MyAttribute), true).Length != 0)

answered Feb 17, 2010 at 15:43

P Daddy

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Comments

flq · Accepted Answer · 2010-02-17 16:00:56Z

If you deal regularly with Attributes in Reflection, it is very, very practical to define some extension methods. You will see that in many projects out there. This one here is one I often have:

public static bool HasAttribute<T>(this ICustomAttributeProvider provider) where T : Attribute
{
  var atts = provider.GetCustomAttributes(typeof(T), true);
  return atts.Length > 0;
}

which you can use like typeof(Foo).HasAttribute<BarAttribute>();

Other projects (e.g. StructureMap) have full-fledged ReflectionHelper classes that use Expression trees to have a fine syntax to identity e.g. PropertyInfos. Usage then looks like that:

ReflectionHelper.GetProperty<Foo>(x => x.MyProperty).HasAttribute<BarAttribute>()

feeeper · Accepted Answer · 2018-09-05 09:58:21Z

4

In addition to previous answers: it's better to use method Any() instead of check length of the collection:

propertiesWithMyAttribute = type.GetProperties()
  .Where(x => x.GetCustomAttributes(typeof(MyAttribute), true).Any());

The example at dotnetfiddle: https://dotnetfiddle.net/96mKep

edited Sep 5, 2018 at 9:58

answered Jan 17, 2018 at 12:56

feeeper

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2 Comments

feeeper Over a year ago

@cogumel0 First of all, sure .Any() does not check the length. But my answer was not about found properties with exactly one attribute. Second, I'm not sure that you read the code correctly - .Any method called on the result of the GetCustomAttrubutes method. So the type of the propertiesWithMyAttribute will be the collection of the properties. Check out the example at dotnetfiddle (I add the link to the answer).

PRMan Over a year ago

You can replace .Where with .Any, since .Any also allows lambdas.

RoJaIt · Accepted Answer · 2022-04-06 21:15:57Z

I made my selfe an extension method for the class Type.

I used LINQ and the Tuples introduced in C# 7.0.

    public static class TypeExtensionMethods
    {
        public static List<(PropertyInfo Info, T Attribute)> GetPropertyWithAttribute<T>(this Type type) where T : Attribute
        {
#pragma warning disable CS8619 // Checked here: .Where(x => x.Value != default)

            return type.GetProperties()
                       .Select(x => (Info: x, Attribute: GetAttribute<T>(x)))
                       .Where(x => x.Attribute != default)
                       .ToList();

#pragma warning restore CS8619 // Checked here: .Where(x => x.Value != default) 
        }

        private static T? GetAttribute<T>(PropertyInfo info) where T : Attribute
        {
            return (T?)info.GetCustomAttributes(typeof(T), true)
                           .FirstOrDefault();
        }
    }

It can be used like this:

var props = typeof(DemoClass).GetPropertyWithAttribute<MyAttribute>();

foreach((PropertyInfo Info, MyAttribute Attribute) in props)
{
    Console.WriteLine($"Property {Info.Name}: {Attribute.DisplayName}");
}

The pragma could be removed, but I don't want any warnings...

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