Lambda on List<T> and using Reflection to get the property names

Question

Let's say you have a Generic Class which have a List<T> Items;

Now think of this basic lambda expression:

var result = Items.FindAll(x => x.Name = "Filip");

This will only work as long as we Know the Properties of T, which you don't when it's a generic type.

Therefore I would like to fetch the properties using Reflection like this:

PropertyInfo[] properties = typeof(T).GetProperties(BindingFlags.Public);

and somehow combind that with the above Lambda-expression so that it searches All the public properties of the Type and see if it contains "Filip", at this time I do not care if the property-name is Name or not.

Is this possible?

Answer 1

var result = Items.FindAll(x => 
    properties.Any(p => p.PropertyType == typeof(string) && 
                        p.GetValue(x, null) == "Filip"));

Obviously this is a simplistic, optimistic string comparison (you might want to use string.Compare, for example), but this should make the idea clear.

Edit

dtb makes a good suggestion in using expression trees. You could accomplish what you're after in a faster fashion like this:

public static class PropertyScanner
{
    static Func<TType, bool> CreatePredicate<TType, TValue>(TValue value, IEqualityComparer<TValue> comparer)
    {
        var arg = Expression.Parameter(typeof(TType), "arg");

        Expression body = null;

        Expression<Func<TValue, TValue, bool>> compare = (val1, val2) => comparer.Equals(val1, val2);

        foreach (PropertyInfo property in typeof(TType).GetProperties(BindingFlags.Public))
        {
            if (property.PropertyType == typeof(TValue) || typeof(TValue).IsAssignableFrom(property.PropertyType))
            {
                Expression prop = Expression.Equal(Expression.Invoke(compare, new Expression[]
                                       {
                                           Expression.Constant(value),
                                           Expression.Property(arg, property.Name)
                                       }),
                                               Expression.Constant(0));

                if (body == null)
                {
                    body = prop;
                }
                else
                {
                    body = Expression.OrElse(body, prop);
                }
            }
        }

        return Expression.Lambda<Func<TType, bool>>(body, arg).Compile();
    }

    public static IEnumerable<TType> ScanProperties<TType, TValue>(this IEnumerable<TType> source, TValue value)
    {
        return ScanProperties<TType, TValue>(source, value, EqualityComparer<TValue>.Default);
    }

    public static IEnumerable<TType> ScanProperties<TType, TValue>(this IEnumerable<TType> source, TValue value, IEqualityComparer<TValue> comparer)
    {
        return source.Where(CreatePredicate<TType, TValue>(value, comparer));
    }
}

This will allow you to do something like this:

var result = Items.ScanProperties("Filip").ToList();

Answer 2

You can use expression trees to construct a lambda on-the-fly:

Func<T, bool> CreatePredicate<T>()
{
    var arg = Expression.Parameter(typeof(T), "arg");
    var body = Expression.Equal(Expression.Property(arg, "Name"),
                                Expression.Constant("Filip"));
    return Expression.Lambda<Func<T, bool>>(body, arg).Compile();
}

IEnumerable<T> GetTWhereNameIsFilip<T>(IEnumerable<T> source)
{
    Func<T, bool> predicate = CreatePredicate<T>();
    // cache predicate for max performance

    return source.Where(predicate);
}