How to get `Type` of subclass from base class

Question

I have an abstract base class where I would like to implement a method that would retrieve an attribute property of the inheriting class. Something like this...

public abstract class MongoEntityBase : IMongoEntity {

    public virtual object GetAttributeValue<T>(string propertyName) where T : Attribute {
        var attribute = (T)typeof(this).GetCustomAttribute(typeof(T));
        return attribute != null ? attribute.GetType().GetProperty(propertyName).GetValue(attribute, null) : null;
    }
}

And Implemented like so...

[MongoDatabaseName("robotdog")]
[MongoCollectionName("users")]
public class User : MonogoEntityBase {
    public ObjectId Id { get; set; }

    [Required]
    [DataType(DataType.EmailAddress)]
    public string email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    public string password { get; set; }

    public IEnumerable<Movie> movies { get; set; }
}

But of course with the above code the GetCustomAttribute() is not an available method because this isn't a concrete class.

What does typeof(this) in the abstract class need to change to in order to have access to the inheriting class? Or is this not good practice and should I implement the method in the inheriting class altogether?

Answer 1

You should use this.GetType(). That will provide you with the actual concrete type of the instance.

So in this case:

public virtual object GetAttributeValue<T>(string propertyName) where T : Attribute {
    var attribute = this.GetType().GetCustomAttribute(typeof(T));
    return attribute != null ? attribute.GetType().GetProperty(propertyName).GetValue(attribute, null) : null;
}

Note, that it will return the top-most class. That is, if you had:

public class AdministrativeUser : User
{

}

public class User : MongoEntityBase
{

}

Then this.GetType() will return AdministrativeUser.

---

Also, this means that you could implement the GetAttributeValue method outside of the abstract base class. You wouldn't require implementers to inherit from MongoEntityBase.

public static class MongoEntityHelper
{
    public static object GetAttributeValue<T>(IMongoEntity entity, string propertyName) where T : Attribute 
    {
        var attribute = (T)entity.GetType().GetCustomAttribute(typeof(T));
        return attribute != null ? attribute.GetType().GetProperty(propertyName).GetValue(attribute, null) : null;
    }
}

(could also implement it as an extension method if you wish to)

Answer 2

typeof(this) won't compile.

What you're searching for is this.GetType().