How to concatenate a property expression and a lambda using Select?

Question

I would like to create the following expression dynamically:

e.Collection.Select(inner => inner.Property)

I created this code to do it, however I have an issue when I execute the expression call, someone knows what I'm doing wrong?

private static Expression InnerSelect<TInnerModel>(IQueryable source, ParameterExpression externalParameter, string complexProperty)
{
    // Creates the expression to the external property. // this generates: "e.Collection".
    var externalPropertyExpression = Expression.Property(externalParameter, complexProperty);

    // Creates the expression to the internal property. // this generates: "inner => inner.Property"
    var innerParameter = Expression.Parameter(typeof(TInnerModel), "inner");
    var innerPropertyExpression = Expression.Property(innerParameter, "Property");
    var innerLambda = Expression.Lambda(innerPropertyExpression, innerParameter);

    return Expression.Call(typeof(Queryable), "Select", new [] { typeof(TInnerModel) }, externalPropertyExpression, innerLambda);
}

Error:

No generic method 'Select' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

Answer 1

So, first off, the primary problem is very simple. As the error message says, you haven't passed enough type arguments to Select. But when you fix that, you'll still have a problem, and that problem will be much harder for you to see and understand.

Let's dig into that.

You wish to represent this as an expression tree:

e.Collection.Select(inner => inner.Property)

Let's begin by rewriting it in its non-extension-method form.

Queryable.Select<A, B>(e.Collection, inner => inner.Property)

Where A is the collection member type and B is the type of Property.

Now, suppose you had this expression in your program. What would it actually do at runtime? It would construct an expression tree for the lambda and pass it to Queryable.Select. That is, it would do something like:

var innerParameter = parameterFactory(whatever);
var lambdaBody = bodyFactory(whatever);
var lambda = makeALambda(lambdaBody, innerParameter);
Queryable.Select<TInnerModel>(e.Collection, lambda);

Right? You with me so far?

Now, suppose we wish to translate this program fragment to an expression tree that could itself be the body of a lambda. That would be:

var theMethodInfoForSelect = whatever;
var receiverE = valueFactory(whatever);
var thePropertyInfoForCollection = whatever;
var theFirstArgument = propertyFactory(receiverE, thePropertyInfoForCollection);
...

Again, with me so far? Now, the crucial question: what is the second argument? It is NOT the value of lambda, which is what you are passing. Remember, what we are doing here is constructing an expression tree which represents the code that the compiler is generating for that thing, and the expression tree that is in lambda is not that. You're mixing levels!

Let me put it this way: the compiler is expecting "add one and three". You are passing 4. Those are very different things! One of them is a description of how to obtain a number and the other one is a number. You are passing an expression tree. What the compiler is expecting is a description of how to obtain an expression tree.

So: do you have to now write code that generates expression trees for all of lambda's construction code? Thank goodness no. We provided you a handy way to turn an expression tree into a description of how to produce an expression tree, which is the Quote operation. You need to use it.

So, what is the right sequence of events that you need to do to build your expression tree? Let's walk through it:

First, you'll need a ParameterExpression of the type of e, which you already have in hand. Let's suppose that is:

ParameterExpression eParam = Expression.Parameter(typeof(E), "e");

Next, you will need a method info for the Select method. Let's suppose you can correctly get that.

    MethodInfo selectMethod = whatever;

That method takes two arguments, so let's make an array of argument expressions:

    Expression[] arguments = new Expression[2];

You'll need a property info for your Collection property. I assume you can get that:

    MethodInfo collectionGetter = whatever;

Now we can build the property expression:

    arguments[0] = Expression.Property(eParam, collectionGetter);

Super. Next we need to start building that lambda. We need a parameter info for inner:

    ParameterExpression innerParam = Expression.Parameter(typeof(Whatever), "inner");

We'll need a property info for Property, which I assume you can get:

    MethodInfo propertyGetter = whatever;

Now we can build the body of the lambda:

    MemberExpression body = Expression.Property(innerParam, propertyGetter);

The lambda takes an array of parameters:

    ParameterExpression[] innerParams = { innerParam };

Build the lambda from the body and the parameters:

    var lambda = Expression.Lambda<Func<X, int>>(body, innerParams);

Now the step you missed. The second argument is the quoted lambda, not the lambda:

    arguments[1] = Expression.Quote(lambda);

Now we can build the call to Select:

    MethodCallExpression callSelect = Expression.Call(null, selectMethod, arguments);

And we're done.

---

Give someone an expression tree and you give them an expression tree for a day; teach them how to find expression trees themselves and they can do it for a lifetime. How did I do that so fast?

Since I wrote the expression tree code generator, I had some immediate familiarity with the problem that you were likely to have. But that was ten years ago, and I did not do the above entirely from memory. What I did was I wrote this program:

using System;
using System.Linq.Expressions;
public interface IQ<T> {}
public class E
{
    public IQ<X> C { get; set; }
}
public class X
{
    public int P { get; set; }
}
public class Program
{
    public static IQ<R> S<T, R>(IQ<T> q, Expression<Func<T, R>> f) { return null; }
    public static void Main()
    {
        Expression<Func<E, IQ<int>>> f  = e => S<X, int>(e.C, c => c.P);
    }
}

Now I wished to know what code was generated by the compiler for the body of the outer lambda, so I went to https://sharplab.io/, pasted in the code, and then clicked on Results --> Decompile C#, which will compile the code to IL and then decompile it back to human-readable C#.

This is the best way I know of to quickly understand what the C# compiler is doing when it builds an expression tree, regardless of whether you know the compiler source code backwards and forwards. It's a very handy tool.