Custom Expression Parser in C#

Question

I am working on a calculator program for a finance application. I need to parse and evaluate complex financial expressions like mentioned below.

The expression is a mix of custom functions and arithmetic expressions. I am using NCalc to resolve the Arithmetic expressions. However, I am having trouble in resolving the Custom Functions.

IF((COALESCE(X1,X2)-X3+IF(X4<=0,0,X5))>0, CUSTOM_FUNCTION(X6), X7)

Any suggestion on best approach?

I am currently working on a complex logic Involving Recursive calls and Stack Push/Pop. But it is not working.

Mishax · Accepted Answer · 2015-08-23 05:05:11Z

6

This is an old problem that was solved a long time ago. The solution is to use a parser generator, not to write your own parser from scratch. There are many options available, one of the more popular ones being ANTLR.

Using a parser generator like ANTLR you can describe your problem using easy-to-understand EBNF-like production rules. The parser generator will generate the complicated logic it seems you are now trying to write by hand, in the language of your choosing, in your case C#. Probably the grammar of your language is already available in this format or you have used this format to describe the language to your users and in your development team.

edited Aug 23, 2015 at 5:05

answered Aug 23, 2015 at 4:57

Mishax

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1 Comment

user5256439 Over a year ago

Thanks a lot. I resolved it by incorporating ideas from Mathos expression parser (link below). Using ANTLR make the expressions little complex. So I ended with a mix of my own and Mathos parser. link

greenoldman · Accepted Answer · 2015-08-23 07:02:09Z

2

Mishax is right, the way you write the rules is more clear than coding them directly in C#. If you have problems with ANTLR and C# (I had) there are other generators dedicated for C# -- Irony, Coco/R, GOLD, ANTLR, LLLPG, Sprache, or my NLT.

And not to leave you just hanging on faith (if you want just the result):

expr -> e1:expr "+" e2:expr { e1+e2 }
      | e1:expr "*" e2:expr { e1*e2 }
      ... and so on ...
      ;

The equivalent piece in raw C# would be much longer.

answered Aug 23, 2015 at 7:02

greenoldman

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Comments

Adrian Stanculescu · Accepted Answer · 2015-08-23 06:11:19Z

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I think you have to use a finite state machine. Compilators are based on it. You have to parse the string letter by letter, and define your operations: Something like that:

Do { //your parser i++; } While(i

answered Aug 23, 2015 at 6:11

Adrian Stanculescu

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Akash Kava · Accepted Answer · 2015-08-23 08:11:42Z

I would recommend little alternate approach by using JavaScript Interpretor.

https://github.com/sebastienros/jint

JInt has a powerful parser, however there are few things you will need to change.

IF and COALESCE does not exist in JavaScript, however they can be converted for example.

IF expression

x = IF(CONDITION,THEN,ELSE)

JavaScript equivalent is

x = CONDITION ? THEN : ELSE

COALESCE expression

x = COALESCE(x1,x2,x3)

JavaScript equivalent is

x = x1 != null ? x1 : 
        (x2 != null ? x2 : 
             (x3 != null ? x3 : null ))

JInt will give you a parsed AST, you can manipulate AST and convert corresponding IF and COALESCE operators.

Using well organized JInt parser and simply post process AST will be lot easier then reinventing the wheel.

By using JInt, you can also use wide variety of already existing .NET methods and you can easily inject your own classes to represent context values.

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