Scoping resolution - expression tree - compile inner lambda in outer lambda

c# dynamic-language-runtime expression-trees scope

Question

I am creating expression tree and there is a situation where I need to create one lambda in another lambda and store inner one in a class and add that class in expression tree. This is simple example of what I am trying to do (this code does not compile):

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;

namespace SimpleTest {
    public class LambdaWrapper {
        private Delegate compiledLambda;
        public LambdaWrapper(Delegate compiledLambda) {
            this.compiledLambda = compiledLambda;
        }
        public dynamic Execute() {
            return compiledLambda.DynamicInvoke();
        }
    }

    public class ForSO {

        public ParameterExpression Param;

        public LambdaExpression GetOuterLambda() {
            IList<Expression> lambdaBody = new List<Expression>();
            Param = Expression.Parameter(typeof(object), "Param");
            lambdaBody.Add(Expression.Assign(
                            Param, 
                            Expression.Constant("Value of 'param' valiable"))
                          );

            lambdaBody.Add(Expression.Call(
                            null, 
                            typeof(ForSO).GetMethod("Write"), 
                            Param)
                          );

            Delegate compiledInnerLambda = GetInnerLambda().Compile();
            LambdaWrapper wrapper = new LambdaWrapper(compiledInnerLambda);
            lambdaBody.Add(Expression.Constant(wrapper));
            //lambdaBody.Add(GetInnerLambda());
            return Expression.Lambda(
                        Expression.Block(
                                new ParameterExpression[] { Param }, 
                                lambdaBody));
        }

        public LambdaExpression GetInnerLambda() {
            return Expression.Lambda(
                    Expression.Block(
                        Expression.Call(null, 
                                typeof(ForSO).GetMethod("Write"), 
                                Expression.Constant("Inner lambda start")),
                        Expression.Call(null, 
                                typeof(ForSO).GetMethod("Write"), 
                                Param),
                        Expression.Call(null, 
                                typeof(ForSO).GetMethod("Write"), 
                                Expression.Constant("Inner lambda end"))
                    )
                );
        }

        public static void Write(object toWrite) {
            Console.WriteLine(toWrite);
        }

        public static void Main(string[] args) {
            ForSO so = new ForSO();
            LambdaWrapper wrapper = so.GetOuterLambda().Compile()
                                      .DynamicInvoke() as LambdaWrapper;
            wrapper.Execute();
            //(so.GetOuterLambda().Compile().DynamicInvoke() as Delegate).DynamicInvoke();
        }
    }
}

Problem is in GetInnerLambda().Compile() line in GetOuterLambda method. I am aware of one solution - it is in commented part of code. With that, everything works fine, but I need a wrapper as return value, not expression subtree (it might be ok to store inner lambda subtree in LambdaWrapper, and compile it later, but same problem occures).

Error I am getting is Unhandled Exception: System.InvalidOperationException: variable 'Param' of type 'System.Object' referenced from scope '', but it is not defined.

If I add Param to block variables in inner lambda, code compiles, but Param has not value assigned in outer lambda (and that makes sense).

How can this be solved?

1
6
5/19/2012 9:28:04 PM

Accepted Answer

With help from Balazs Tihanyi I found solution that works for me exactly as I need it. It is a bit more work because I had to create binders, but I my main project I already had them, so I created dummy binders for this example to work.

This is my final solution:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;
using System.Reflection;
using System.Dynamic;


namespace SimpleTest {
    public class MyCreateBinder : CreateInstanceBinder {
        public MyCreateBinder(CallInfo info) : base(info) { }

        public override DynamicMetaObject FallbackCreateInstance(
                                        DynamicMetaObject target,
                                        DynamicMetaObject[] args,
                                        DynamicMetaObject errorSuggestion) {
            var param = args[0].Value;

            Type toCreate = target.Value as Type;
            var ctors = toCreate.GetConstructors()
                        .Where(c => c.GetParameters().Length == args.Length)
                        .ToArray();

            if (ctors.Length == 0)
                throw 
                    new Exception(
                      String.Format(
                      "Can not find constructor for '{0}' with {1} parameters",
                      toCreate, args.Length));
            ConstructorInfo ctorToUse = ctors[0];
            return new DynamicMetaObject(
                            Expression.New(
                                ctorToUse,
                                args.Select(a => a.Expression).ToList()),
                       BindingRestrictions.Empty);
        }
    }

    public class MySetMemberBinder : SetMemberBinder {

        public MySetMemberBinder(string name) : base(name, false) { }

        public override DynamicMetaObject FallbackSetMember(
                                DynamicMetaObject target,
                                DynamicMetaObject value,
                                DynamicMetaObject errorSuggestion) {

            throw new NotImplementedException();
        }
    }

    public class MyGetMemberBinder : GetMemberBinder {
        public MyGetMemberBinder(string name) : base(name, false) { }

        public override DynamicMetaObject FallbackGetMember(
                                        DynamicMetaObject target,
                                        DynamicMetaObject errorSuggestion) {
            throw new NotImplementedException();
        }
    }

    public class MyInvokeMemberBinder : InvokeMemberBinder {
        public MyInvokeMemberBinder(string name, CallInfo callInfo) 
            : base(name, false, callInfo) { }

        public override DynamicMetaObject FallbackInvokeMember(
                                    DynamicMetaObject target,
                                    DynamicMetaObject[] args,
                                    DynamicMetaObject errorSuggestion) {
            var a = this;
            throw new NotImplementedException();
        }

        public override DynamicMetaObject FallbackInvoke(
                                    DynamicMetaObject target,
                                    DynamicMetaObject[] args,
                                    DynamicMetaObject errorSuggestion) {
            throw new NotImplementedException();
        }
    }

    public class LambdaWrapper : IDynamicMetaObjectProvider {
        private Delegate compiledLambda;
        private LambdaExpression exp;

        public LambdaWrapper(LambdaExpression exp) {
            this.exp = exp;
            this.compiledLambda = exp.Compile();
        }
        public dynamic Execute(dynamic param) {
            return compiledLambda.DynamicInvoke(param);
        }

        public DynamicMetaObject GetMetaObject(Expression parameter) {
            return new MetaLambdaWrapper(parameter, this);
        }
    }

    public class MetaLambdaWrapper : DynamicMetaObject {
        public MetaLambdaWrapper(Expression parameter, object value) : 
            base(parameter, BindingRestrictions.Empty, value) { }

        public override DynamicMetaObject BindInvokeMember(
                                    InvokeMemberBinder binder,
                                    DynamicMetaObject[] args) {
            MethodInfo method = this.Value.GetType().GetMethod(binder.Name);
            return new DynamicMetaObject(
                        Expression.Call(
                            Expression.Constant(this.Value),
                                method,
                                    args.Select(a => a.Expression)),
                        BindingRestrictions.GetTypeRestriction(
                            this.Expression, 
                            typeof(LambdaWrapper)));
        }
    }


    public class ForSO {
        public ParameterExpression Param;
        public LambdaExpression GetOuterLambda() {
            Expression wrapper;
            IList<Expression> lambdaBody = new List<Expression>();
            Param = Expression.Parameter(typeof(object), "Param");
            lambdaBody.Add(Expression.Assign(
                            Param,
                            Expression.Constant("Value of 'param' variable"))
                          );
            lambdaBody.Add(Expression.Call(
                            null,
                            typeof(ForSO).GetMethod("Write"),
                            Param)
                          );

            wrapper = Expression.Dynamic(
                                new MyCreateBinder(new CallInfo(1)),
                                typeof(object),
                                Expression.Constant(typeof(LambdaWrapper)),
                                Expression.Quote(GetInnerLambda()));


            lambdaBody.Add(
                Expression.Dynamic(
                    new MyInvokeMemberBinder("Execute", new CallInfo(1)),
                    typeof(object),
                    wrapper,
                Expression.Constant("calling inner lambda from outer")));

            lambdaBody.Add(wrapper);

            return Expression.Lambda(
                        Expression.Block(
                                new ParameterExpression[] { Param },
                                lambdaBody));
        }

        public LambdaExpression GetInnerLambda() {
            ParameterExpression innerParam = Expression.Parameter(
                                                typeof(object), 
                                                "innerParam");
            return Expression.Lambda(
                    Expression.Block(
                        Expression.Call(null,
                                typeof(ForSO).GetMethod("Write"),
                                Expression.Constant("Inner lambda start")),
                        Expression.Call(null,
                                typeof(ForSO).GetMethod("Write"),
                                innerParam),
                        Expression.Call(null,
                                typeof(ForSO).GetMethod("Write"),
                                Param),
                        Expression.Call(null,
                                typeof(ForSO).GetMethod("Write"),
                                Expression.Constant("Inner lambda end"))
                    ),
                    innerParam
                );
        }

        public static void Write(object toWrite) {
            Console.WriteLine(toWrite);
        }

        public static void Main(string[] args) {
            Console.WriteLine("-----------------------------------");
            ForSO so = new ForSO();

            LambdaWrapper wrapper = (LambdaWrapper) so.GetOuterLambda()
                                                    .Compile()
                                                    .DynamicInvoke();
            Console.WriteLine("-----------------------------------");
            wrapper.Execute("Calling from main");
        }
    }

}
0
5/21/2012 3:56:24 PM

Popular Answer

Well, since you can't use Param as a constant value in your inner lambda expression, I suggest you to add a lambda parameter to your expression:

public LambdaExpression GetInnerLambda()
{
    var param = Expression.Parameter(typeof(object));
    return Expression.Lambda(
        Expression.Block(
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                Expression.Constant("Inner lambda start")),
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                param),
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                Expression.Constant("Inner lambda end"))
        ),
        param
    );
}

Then store the parameter's value in your LambdaWrapper class, and use it later as an argument in the DynamicInvoke call:

public class LambdaWrapper
{
    private object param;
    private Delegate compiledLambda;

    public LambdaWrapper(Delegate compiledLambda, object param)
    {
        this.compiledLambda = compiledLambda;
        this.param = param;
    }

    public dynamic Execute()
    {
        return compiledLambda.DynamicInvoke(param);
    }
}

That works, but the only issue is that it will call WriteLine on Param, which is a ParameterExpression object. To solve this, you have to create the wrapper class dynamically in your expression tree:

//lambdaBody.Add(Expression.Constant(wrapper));
lambdaBody.Add(Expression.New(
    typeof(LambdaWrapper).GetConstructor(new[] { typeof(Delegate), typeof(object) }),
    Expression.Constant(compiledInnerLambda),
    Param)
);

Then it will use the assigned value of Param. And since you don't use Param outside of GetOuterLambda, you can now use it as a local variable.

EDIT:

Here is my second attempt to solve this issue:

public LambdaExpression GetOuterLambda()
{
    ...
    //Delegate compiledInnerLambda = GetInnerLambda().Compile();
    //LambdaWrapper wrapper = new LambdaWrapper(compiledInnerLambda);

    lambdaBody.Add(Expression.New(
        typeof(LambdaWrapper).GetConstructor(new[] { typeof(Delegate) }),
        Expression.Call(
            Expression.Call(
                typeof(ForSO).GetMethod("GetInnerLambda", BindingFlags.Public | BindingFlags.Static),
                Param
            ),
            typeof(LambdaExpression).GetMethod("Compile", Type.EmptyTypes)
        )
    ));
    ...
}

public static LambdaExpression GetInnerLambda(object param)
{
    return Expression.Lambda(
        Expression.Block(
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                Expression.Constant("Inner lambda start")),
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                Expression.Constant(param)),
            Expression.Call(null,
                typeof(ForSO).GetMethod("Write"),
                Expression.Constant("Inner lambda end"))
        )
    );
}

This approach compiles this inner lambda when you run the outer delegate. By doing this, Param will be assigned before the inner lambda gets compiled.



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