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;
}
}
}