C#: An item with the same key has already been added, when compiling expression

Question

Ok, here\'s a tricky one. Hopefully there is an expression guru here who can spot what I am doing wrong here, cause I am just not getting it.

I am building up expression

Answer 1

This is not an answer, but I hope it will help someone find the answer. I've simplified the code further so that it is just one single file and still fails in the same way. I have renamed the variables so that "x" is not used twice. I have removed the Range class and replaced it with hardcoded constants 0 and 1.

using System;
using System.Linq;
using System.Linq.Expressions;

class Program
{
    static Expression> And(Expression> first,
                                           Expression> second)
    {
        var x = Expression.Parameter(typeof(int), "x");
        var body = Expression.AndAlso(Expression.Invoke(first, x), Expression.Invoke(second, x));
        return Expression.Lambda>(body, x);
    }

    static Expression> GetPredicateFor(Expression> selector)
    {
        var param = Expression.Parameter(typeof(int), "y");
        var member = Expression.Invoke(selector, param);

        Expression body =
            Expression.AndAlso(
                Expression.GreaterThanOrEqual(member, Expression.Constant(0, typeof(int))),
                Expression.LessThanOrEqual(member, Expression.Constant(1, typeof(int))));

        return Expression.Lambda>(body, param);
    }

    static void Main()
    {
        Expression> predicate = a => true;
        predicate = And(predicate, GetPredicateFor(b => b)); // Comment out this line and it will run without error
        var z = predicate.Compile();
    }
}

The expression looks like this in the debugger:

x => (Invoke(a => True,x) && Invoke(y => ((Invoke(b => b,y) >= 0) && (Invoke(b => b,y) <= 1)),x))

Update: I've simplified it down to about the most simple it can be while still throwing the same exception:

using System;
using System.Linq;
using System.Linq.Expressions;

class Program
{
    static void Main()
    {
        Expression> selector = b => true;
        ParameterExpression param = Expression.Parameter(typeof(int), "y");
        InvocationExpression member = Expression.Invoke(selector, param);
        Expression body = Expression.AndAlso(member, member);
        Expression> predicate = Expression.Lambda>(body, param);
        var z = predicate.Compile();
    }
}