C# - can I define a function on the fly from a user-supplied string formula?

Question

I can define a func with a lambda expression as in:

Func <int, int, int> Fxy = ( (a,b) => a + b ) );  // Fxy does addition

But what if I wanted to allow the user to supply the r.h.s. of the lambda expression at runtime ?

String formula_rhs = Console.Readline();
// user enters "a - b"

Can I somehow modify Fxy as if I had coded

Func <int, int, int> Fxy = ( (a,b) => a - b ) );  // Fxy does what the user wants 
                                                  // (subtraction)

I presently use a home built expression parser to accept user-defined formulas. Just starting up w .NET, and I get the feeling there may be a not-too-painful way to do this.

Have you seen Dynamic LINQ? weblogs.asp.net/scottgu/archive/2008/01/07/… — Gabe
– Gabe, Commented Mar 23, 2011 at 13:33

Zebi · Accepted Answer · 2011-03-23 13:38:35Z

1

Try the ncalc framework: http://ncalc.codeplex.com/ Its easy and lightweight.

answered Mar 23, 2011 at 13:38

Zebi

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Felice Pollano · Accepted Answer · 2011-03-23 13:37:06Z

0

Is not so easy. This library contains some Dynamic Linq features you can use to convert a lambda expression from text to Func<>, have a look at it.

answered Mar 23, 2011 at 13:37

Felice Pollano

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Tony Kh · Accepted Answer · 2011-03-28 06:35:48Z

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System.Reflection.Emit namespace is intended for code generation on-the-fly. It is quite complicated in general but may help for simple functions like subtraction.

answered Mar 28, 2011 at 6:35

Tony Kh

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Anton Semenov · Accepted Answer · 2011-03-28 07:08:20Z

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If you are planning to use not complex formulas, JScript engine would easy solution:

        Microsoft.JScript.Vsa.VsaEngine engine = Microsoft.JScript.Vsa.VsaEngine.CreateEngine();
        MessageBox.Show(Microsoft.JScript.Eval.JScriptEvaluate("((2+3)*5)/2", engine).ToString());

Dont forget to add reference to Microsoft.JScript assembly

answered Mar 28, 2011 at 7:08

Anton Semenov

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Patrick Huizinga · Accepted Answer · 2011-03-28 07:43:57Z

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Using Linq.Expressions could be an idea here.

Your example would tranlate to:

var a = Expression.Parameter(typeof(int), "a");
var b = Expression.Parameter(typeof(int), "b");
var sum = Expression.Subtract(a, b);
var lamb = Expression.Lambda<Func<int, int, int>>(sum, a, b);
Func<int, int, int> Fxy = lamb.Compile();

Ofcourse, you will have the fun of parsing a string into the correct expressions. :-)

answered Mar 28, 2011 at 7:43

Patrick Huizinga

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