对List的每个项执行自定义操作<T>

Question

I would like to do a custom operation on a list's members and be able to specify on which property I would perform it but I'm having a hard time finding the correct syntax for assigning the result back to the property.

Example :

I have a list of terms such as the one below and would like to normalize their 'Frequency'.

public class Term
{
    public string Name { get; set; }
    public double Frequency { get; set; }
    public double Weight { get; set; }
}

Using some syntax like this I should be able to specify the property I am doing the operation on :

List<Term> list = Normalize(artist.Terms, s => s.Frequency);

(here it's 'Frequency' on 'Term' but I should be able to do that on any property of any type, property type will always be a double)

So that's what I crafted but I can't find out how to perform the operation nor assigning the result back to the property :

private static List<T> Normalize<T>(List<T> elements, Func<T, double> func)
{
    List<T> list = new List<T>();
    double fMin = elements.Min(func);
    double fMax = elements.Max(func);
    double fDelta = fMax - fMin;
    double fInv = 1.0d / fDelta;
    for (int i = 0; i < elements.Count; i++)
    {
        T t = elements[i];

        // What should I do from here ?
        //double invoke = func.Invoke(term);
        //term.Frequency = (term.Frequency - fMin) * fInv;
     }
    return list;
}

How would you achieve this ?

Answer 1

If you want to avoid using expressions and reflection, you can simply provide both a getter function and a setter function. I also slightly updated the method since it would be altering the objects in original list, so I didn't think it'd be wise to create and return a new list; the API would be deceptive. (you can easily switch it back if desired)

private static void Normalize<T>(List<T> elements, Func<T, double> getter, Action<T, double> setter)
{
    double fMin = elements.Min(getter);
    double fMax = elements.Max(getter);
    double fDelta = fMax - fMin;
    double fInv = 1.0d / fDelta;
    for (int i = 0; i < elements.Count; i++)
    {
        T t = elements[i];

        double initialValue = getter(t);
        double newValue = (initialValue - fMin) * fInv;
        setter(t, newValue);
    }
}

With usage like:

Normalize(terms, t => t.Frequency, (t, normalizedFrequency) => t.Frequency = normalizedFrequency);

And of course it's easy to update to be an extension method so it could be used like:

terms.Normalize(t => t.Frequency, (t, normalizedFrequency) => t.Frequency = normalizedFrequency);

With this change in the Normalize signature, it's clear you are altering the existing list and not producing a new one. With the old signature, it would appear you are leaving the old list and objects untouched which is not the case.

Answer 2

You can use an Expression to get the handle to the property:

private static List<T> Normalize<T>(List<T> elements, Expression<Func<T, double>> func)
{
    //...
    var expr = (MemberExpression)func.Body;
    var property = expr.Member as PropertyInfo;
    if (property != null)
        property.SetValue(/* element */, /* new value */);
    //...
}

Furthermore, I would recommend using IEnumerable . This is more flexible and can be converted to lists at any time:

 private static IEnumerable<T> Normalize(...)
 {
     foreach(...)
     {
         yield return ...;
     }
 }