Question

I am asking this question, because I didn’t find yet any posts that are C# related and there might be some build in methods for that I couldn’t find. If there are, please tell me so and I can close this question.

Basically I have the common situation:

1. User types a function w.r.t. one or two variables into some TextBlock
2. I take this string analyse it
3. As a return I would like to have a delegate to a method that will take one or two inputs (the variables) and return the function value according to what the user typed in.

Now, I could probably think (and I would like to do this on my own, because I want to use my brain) of an algorithm of analysing the string step by step to actually find out, what has to be calculated first and in what way. E.g. First scan for parentheses, look for the expression within a group of parantheses and calculate that according to more general functions etc.

But in the end I would like to “create” a method of this analysis to be easily used as a normal delegate with a couple of arguments that will return the correct function value.
Are there any methods included in C# for that already, or would I have to go and program everything by myself?

As a remark: I don’t want to use anybody else’ library, only .NET libraries are acceptable for me.

Edit: After Matt pointed out expression trees, I found this thread which is a good example to my problem.

Edit2: The example pointed out does only include simple functions and will not be useful if I want to include more complex functions such as trigonometric ones or exponentials.

Answer 1

What you are describing is a parser. There are a number of different ways of implementing them, although generally speaking, for complex grammars, a “parser generator” is often used.

A parser generator will take a description of the grammar and convert it into code that will parse text that conforms to the grammar into some form of internal representation that can be manipulated by the program, e.g. a parse tree.

Since you indicate you want to avoid third-party libraries, I’ll assume that the use of a parser generator is similarly excluded, which leaves you with implementing your own parser (which fortunately is quite an interesting exercise).

The Wikipedia page on Recursive descent parsers will be particularly useful. I suggest reading through it and perhaps adapting the example code therein to your particular use case. I have done this myself a number of times for different grammars with this as a starting point, so can attest to its usefulness.

The output from such a parser will be a “parse tree”. And you then have a number of possibilities for how you convert this into an executable delegate. One option is to implement an Evaluate() method on your parse tree nodes, which will take a set of variables and return the result of evaluating the user’s expression. As others have mentioned, your parse tree could leverage .NET’s Expression trees, or you can go down the route of emitting IL directly (permitting you to produce a compiled .NET assembly from the user’s expression for later use as required).