Question

Is it possible to implement a ! (not) using expression trees. I’m interested in creating a C# eval class which will parse and evaluate logical expressions which contain true, false, ||, && and !. I know that && and || are currently supported by .NET 4 expression trees, but I was wondering if their is a way to implement summat like !(x && y) || z where z=false, y=true and z=false.

Currently I’m using a standard stack based tokenizer, parser, evaluator to evaluate these types of expression but would happly dump it, if a suitable expression tree could be created and executed on the fly.

Answer 1

I usually find that it’s worth coding a lambda expression which does what I want, and then seeing what the C# compiler does with it.

So this code:

Expression<Func<bool,bool>> func = x => !x;

Is compiled into:

ParameterExpression expression2;
Expression<Func<bool, bool>> expression = 
       Expression.Lambda<Func<bool, bool>>(Expression.Not(
            expression2 = Expression.Parameter(typeof(bool), "x")), 
                new ParameterExpression[] { expression2 });

(Apologies for the formatting – it’s hard to know what to do with that.)

That’s decompiled with Reflector, but with its “optimization” set at .NET 2.0 to avoid it using lambda syntax.

Once you’ve got past the junk, you’ll see it’s using Expression.Not. && uses Expression.AndAlso, and || uses Expression.OrElse.