How to collect a single property in a list of objects?

Question

Is it possible to create an extension method to return a single property or field in a list of objects?

Currently I have a lot of functions like the following.

public static List<int> GetSpeeds(this List<ObjectMotion> motions) {
    List<int> speeds = new List<int>();
    foreach (ObjectMotion motion in motions) {
        speeds.Add(motion.Speed);
    }
    return speeds;
}

This is "hard coded" and only serves a single property in a single object type. Its tedious and I'm sure there's a way using LINQ / Reflection to create an extension method that can do this in a generic and reusable way. Something like this:

public static List<TProp> GetProperties<T, TProp>(this List<T> objects, Property prop){
    List<TProp> props = new List<TProp>();
    foreach (ObjectMotion obj in objects) {
        props.Add(obj.prop??);
    }
    return props;
}

Apart from the easiest method using LINQ, I'm also looking for the fastest method. Is it possible to use code generation (and Lambda expression trees) to create such a method at runtime? I'm sure that would be faster than using Reflection.

You function could be written as "motions.Select( motion => motion.Speed)". That's not exactly generic, but for a start it's a lot shorter than writing and calling a method each time. — nvoigt
– nvoigt, Commented Mar 12, 2013 at 13:58
Only calling my method may be faster. (LINQ is usually slower than non-generic code) — Robin Rodricks
– Robin Rodricks, Commented Mar 12, 2013 at 13:58

Lee · Accepted Answer · 2013-03-12 14:06:30Z

7

You could do:

public static List<TProp> GetProperties<T, TProp>(this IEnumerable<T> seq, Func<T, TProp> selector)
{
    return seq.Select(selector).ToList();
}

and use it like:

List<int> speeds = motions.GetProperties(m => m.Speed);

it's questionable whether this method is better than just using Select and ToList directly though.

edited Mar 12, 2013 at 14:06

answered Mar 12, 2013 at 13:59

Lee

145k20 gold badges243 silver badges292 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Sten Petrov · Accepted Answer · 2013-03-12 13:56:09Z

7

It is, no reflection needed:

List<int> values = motions.Select(m=>m.Speed).ToList();

answered Mar 12, 2013 at 13:56

Sten Petrov

11.1k1 gold badge45 silver badges63 bronze badges

3 Comments

Robin Rodricks Over a year ago

How do I create an extension method to do this? I don't want to write this snippet everytime. And how do I use code generation to create the fastest method?

Sten Petrov Over a year ago

public static List<O> GetValues<T,O>(this List<T> target, Func<T,O> selector){ return target.Select(selectoror);}

Sten Petrov Over a year ago

but no need to make a function, Select is a function already

p.s.w.g · Accepted Answer · 2013-03-12 14:00:15Z

1

A for loop would be the fastest I think, followed closely by linq (minimal overhead if you don't do use closures). I can't image any other mechanism would be any better than that.

You could replace the List<int> with a int[] or initialize the list with a certain capacity. That would probably do more to speed up your code than anything else (though still not much).

answered Mar 12, 2013 at 14:00

p.s.w.g

150k31 gold badges307 silver badges339 bronze badges

Collectives™ on Stack Overflow

How to collect a single property in a list of objects?

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related