Dynamic logical expression parsing/evaluation in C# or VB?

Question

What is the best was to evaluate an expression like the following:
(A And B) Or (A And C) Or (Not B And C)
or
(A && B) || (A && C) || (!B && C)

At runtime, I was planning on converting the above expressions to the following:
(True And False) Or (True And False) Or (Not False And True)
or
(True && False) || (True && False) || (! False && True)

Conditions: 1) The logical expression is not known until runtime. 2) The number variable and their values are not known until runtime. 3) Variable values are never null.

I know I could create a simple assemble with a class and a method that I generate at runtime based on the inputs, but is there a better way. I have done this before. Use a string builder to write the code, then call the compiler. After that, you load the assembly and call the method.

Suggestions?

Thanks.

Answer 1

Be warned: the two final conditions you're talking about are not necessarily equivalent. The && operators in C# will use short-circuit evalution, while the logical And operator in VB does not. If you want to be sure the statements are equivalent, translate a user And to AndAlso and a user Or to OrElse.

For simple expresssions you probably won't notice a difference. But if the conditions can have side effects or if the performance difference between the two is a concern, this can be important.

Answer 2

If you're using .NET3.5 then you can parse the text and create an abstract sytax tree using the Expression classes. Then create a suitable LambdaExpression instance and compile it into a delegate, which you can then execute.

Constructing a parser and syntax tree builder for this kind of fairly simple grammer is quite an interesting exercise, and will execute somewhat faster than invoking the compiler (and it's neater in my view as well).

If you're not using .NET3.5, then it's also not complicated to implement an interpreted abstract syntax tree yourself.