Using CSharpCodeProvider to allow user created functions

Question

I have an existing asp.net (c#) application. I need to provide users with a way to create flexibles rules to calculate an effective date given a hiredate and an enrollmentdate.<

Answer 1

See my recent answer here: Parsing "DateTime.Now"?

Essentially, you can easily leverage an existing library like FLEE to parse expressions and emit IL for these rules. If you take a look at the examples, you can see how to set up variables for the user expressions to leverage. For example, you may define a "rule" that consists of some input variables (like HireDate or EnrollmentDate), and a user expression/predicate that returns the date. If you expose the DateTime members like I have in the linked answer, then users can leverage those as well.

Just as a quick example, not tested but should give you an idea.

You can setup some custom functions to help, like getting the first day of a month:

public static class CustomFunctions
{
    public static DateTime GetFirstDayOfMonth(DateTime date)
    {
        return new DateTime(date.Year, date.Month, 1);
    }
}

A basic FLEE setup (you'll have to customize/tweak as necessary)

ExpressionContext context = new ExpressionContext();

//Tell FLEE to expect a DateTime result; if the expression evaluates otherwise, 
//throws an ExpressionCompileException when compiling the expression
context.Options.ResultType = typeof(DateTime);

//Instruct FLEE to expose the `DateTime` static members and have 
//them accessible via "DateTime".
//This mimics the same exact C# syntax to access `DateTime.Now`
context.Imports.AddType(typeof(DateTime), "DateTime");
context.Imports.AddType(typeof(CustomFunctions));

//Expose your key variables like HireDate and EnrollmentDate
context.Variables["HireDate"] = GetHireDate(); //DateTime I suppose
context.Variables["EnrollmentDate"] = GetEnrollmentDate(); //DateTime I suppose

//Parse the expression, naturally the string would come from your data source
IGenericExpression expression = context.CompileGeneric(GetYourRule(), context);

DateTime date = expression.Evaluate();

Then your rules might look like:

string rule1 = "if(HireDate > EnrollmentDate, HireDate, EnrollmentDate)";
string rule2 = "HireDate.AddDays(90)";
string rule3 = "GetFirstDayOfMonth(EnrollmentDate.AddMonths(1))";
string rule4 = "GetFirstDayOfMonth(EnrollmentDate.AddMonths(if(EnrollmentDate.Day < 15, 1, 2)))";