lambda - Adding a method call to a Linq Expression whilst remaining an full expression

Question

How I do extend an linq expression whilst keeping it an expression? I've simplified this quite a bit (to avoid pasting pages) - .e.g I working with Queryable rather than Enumerable, but the solution for this will suffice, ultimately I need to keep it as an expression whilst adding a method call to it.

For exampleL

        var p1 = new Person() {Name = "RDA1", Age = 27};
        var p2 = new Person() {Name = "RDA2", Age = 28};
        var p3 = new Person() {Name = "RDA3", Age = 29};

        var people = new[] {p1, p2, p3};


        Expression<Func<IEnumerable<Person>, IEnumerable<Person>>> filterExp
            = list => list.Take(2);


        Expression<Func<Person, int>> sortExp = l => l.Age;


        MethodCallExpression orderByCallExpression = Expression.Call(
            typeof (Enumerable),
            "OrderByDescending",
            new Type[] {typeof (Person), typeof (int)},
            filterExp.Parameters[0],
            sortExp); 

var combinedExpression = Expression.Lambda<Func<IEnumerable<Person>, IEnumerable<Person>>>
(filterExp.AddMethodCall(orderByCallExpression)); // made up AddMethodCall but you get the idea

I've searched dozens of SO posts for the past few hours and I can't seem to figure this out, I can do it if I compile filterExp but not without keeping both expressions and end result an expression.

Answer 1

First, you don't need to switch everything to IEnumerable: AsQueryable() will let you use your test collection with expression trees:

var p1 = new Person() { Name = "RDA1", Age = 27 };
var p2 = new Person() { Name = "RDA2", Age = 28 };
var p3 = new Person() { Name = "RDA3", Age = 29 };

var people = new[] { p1, p2, p3 }.AsQueryable();

You were off to a good start, you just need to shift around what you're using where:

Expression<Func<IQueryable<Person>, IQueryable<Person>>> filterExp
    = list => list.Take(2);

Expression<Func<Person, int>> sortExp = l => l.Age;

MethodCallExpression orderByCallExpression = Expression.Call(
    typeof(Queryable),
    "OrderByDescending",
    new Type[] { typeof(Person), typeof(int) },
    filterExp.Body,
    sortExp);

var combinedExpression =
    Expression.Lambda<Func<IQueryable<Person>, IQueryable<Person>>>(
        orderByCallExpression, filterExp.Parameters[0]);

We use filterExp.Body to extract list.Take(2) as the first parameter to OrderByDescending, then move the filterExp parameter to the lambda expression.

I assume this is your intended usage?

var compiled = combinedExpression.Compile();
var res = compiled(people);

foreach (var r in res)
{
    // Do something
}