check if expression has free parameters

Question

As part of a funcletizer, i wish to replace c# expressions, that do not contain parameters, with their evaluated constants:

double d = 100.0;

Expression<Func<double, double>> ex1 = x => -x;
Expression<Func<double>> ex2 = () => -d;

Expression result;
result = Funcletize(ex1); // should return ex1 unmodified
result = Funcletize(ex2); // should return Expression.Constant(-100.0)

I know I can evaluate an expression by wrapping it in a lambda expression and invoking that:

object result = Expression.Lambda(ex2).Compile().DynamicInvoke(); 
// result == -100

When the expression contains unbound parameters, as ex1 above, this will of course fail, throwing InvalidOperationException, since I supplied no parameters. How can I check whether an expression contains such parameters?

My current solution involves a try{} catch(InvoalidOperationException), but that seems a very inelegant and error-prone way:

// this works; by catching InvalidOperationException
public static Expression Funcletize(Expression ex)
{
    try
    {
        // Compile() will throw InvalidOperationException, 
        // if the expression contains unbound parameters
        var lambda = Expression.Lambda(ex).Compile(); 

        Object value = lambda.DynamicInvoke();
        return Expression.Constant(value, ex.Type);
    }
    catch  (InvalidOperationException)
    {
        return ex;
    }
}

Answer 1

Sure, most things are possible. There are two separate things in play here:

• removing unused parameters, by tracking which are seen in the expression
• evaluating and inlining captured variables (emphasis: this is a semantic change) - which we do by trying to recognising the field->[field->]...field-> pattern (although the code shown might actually perform false-positives here in some cases)

Code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reflection;
static class Program
{
    static void Main()
    {
        double d = 100;

        Expression<Func<double, double>> ex1 = x => -x;
        Expression<Func<double>> ex2 = () => -d;

        var result1 = Demungify(ex1); // (x) => -x
        var result2 = Demungify(ex2); // () => -100
    }
    public static LambdaExpression Demungify(LambdaExpression ex)
    {
        var visitor = new Demungifier();
        var newBody = visitor.Visit(ex.Body);
        var args = ex.Parameters.Where(visitor.WasSeen).ToArray();
        var lambda = Expression.Lambda(newBody, args);
        if (!args.Any() && !(lambda.Body is ConstantExpression))
        {
            // evaluate that!
            object result = lambda.Compile().DynamicInvoke();
            lambda = Expression.Lambda(Expression.Constant(result, newBody.Type));    
        }
        return lambda;
    }
    class Demungifier : ExpressionVisitor
    {
        readonly HashSet<ParameterExpression> parameters = new HashSet<ParameterExpression>();

        public bool WasSeen(ParameterExpression param)
        {
            return parameters.Contains(param);
        }
        protected override Expression VisitParameter(ParameterExpression node)
        {
            parameters.Add(node);
            return base.VisitParameter(node);
        }
        protected override Expression VisitMember(MemberExpression node)
        {
            object value;
            if(TryEvaluate(node, out value))
            {
                return Expression.Constant(value, ((FieldInfo)node.Member).FieldType);
            }
            return base.VisitMember(node);
        }
        bool TryEvaluate(Expression expression, out object value)
        {
            if(expression == null)
            {
                value = null;
                return true;
            }
            if(expression.NodeType == ExpressionType.Constant)
            {
                value = ((ConstantExpression)expression).Value;
                return true;
            }
            // captured variables are always fields, potentially of fields of fields
            // eventually terminating in a ConstantExpression that is the capture-context
            MemberExpression member;
            if(expression.NodeType == ExpressionType.MemberAccess
                && (member= (MemberExpression)expression).Member.MemberType == System.Reflection.MemberTypes.Field)
            {
                object target;
                if(TryEvaluate(member.Expression, out target))
                {
                    value = ((FieldInfo)member.Member).GetValue(target);
                    return true;
                }
            }
            value = null;
            return false;
        }

    }
}

Answer 2

In this case you can cast the Expression to LambdaExpression and see if it has parameters.

public static Expression Funcletize(Expression ex)
{
   var l = ex as LambdaExpression;

    if (l != null && l.Parameters.Count == 0)
    {
        var lambda = Expression.Lambda(ex).Compile();
        Object value = lambda.DynamicInvoke();
        return Expression.Constant(value, ex.Type);
    }

    return ex;
}