Question

So what I am trying to do is use expression trees to apply a predicate to each value in a collection (read map or list.All(predicate)). It appears that I am not getting the input parameter to the predicate bound to the value supplied by All, and I’m a little stuck. Here is the code (using linqpad) that I am working with::

public class SomeType
{
  public IEnumerable<bool> Collection { get; set; }
}

void Main()
{
  var list = new SomeType {
    Collection = new List<bool> { true, true, true }
  };
  var functor = Compiler((SomeType t) => t.Collection, (bool x) => x);
  functor(list).Dump();
}

MethodInfo FindMethod<TInput>(Type location, string name)
{
    var handle = location
        .GetMethods(BindingFlags.Static | BindingFlags.Public)
        .Where(method => method.Name == name).First();

    return handle.MakeGenericMethod(typeof(TInput));
}

Predicate<TObject> Compiler<TObject, TProperty>(
    Expression<Func<TObject, IEnumerable<TProperty>>> selector, 
    Expression<Predicate<TProperty>> predicate)
{
    var query = FindMethod<TProperty>(typeof(Enumerable), "All");
    var expression = Expression.Call(query,
        new Expression[] {
            Expression.Invoke(selector, selector.Parameters),
            Expression.Lambda<Func<TProperty, bool>>(predicate.Body,
                        Expression.Parameter(typeof(TProperty))),
        });         

    return Expression.Lambda<Predicate<TObject>>(expression,
        selector.Parameters).Compile();
}

Thanks and sorry if this was answered in another question (I looked for a while).

Answer 1

This does work, but I had to change the Predicate<TObject> to Func<TObject, bool>. If you want I can try to change it back.

static Predicate<TObject> Compiler<TObject, TProperty>(
    Expression<Func<TObject, IEnumerable<TProperty>>> selector,
    Expression<Func<TProperty, bool>> predicate)
{
    var query = FindMethod<TProperty>(typeof(Enumerable), "All");
    var expression = Expression.Call(
        query,
        Expression.Invoke(selector, selector.Parameters), 
        predicate);

    return Expression
        .Lambda<Predicate<TObject>>(expression, selector.Parameters)
        .Compile();
}

5 minutes later… And if you really want to use Predicate<TObject>…

static Predicate<TObject> Compiler<TObject, TProperty>(
    Expression<Func<TObject, IEnumerable<TProperty>>> selector,
    Expression<Predicate<TProperty>> predicate)
{
    var query = FindMethod<TProperty>(typeof(Enumerable), "All");

    var predicateAsFunc = Expression.Lambda<Func<TProperty, bool>>(
        predicate.Body, 
        predicate.Parameters);

    var expression = Expression.Call(
        query,
        Expression.Invoke(selector, selector.Parameters), 
        predicateAsFunc);

    return Expression
        .Lambda<Predicate<TObject>>(expression, selector.Parameters)
        .Compile();
}