eval(string) to C# code

Question

Is it possible to evaluate the following in C# at runtime

I have a class that contains 3 properties (Field,Operator,Value)

Answer 1

You'd have to either use the CodeDOM libraries or create an Expression tree, compile it, and execute it. I think building up the expression tree is the best option.

Of course you could put in a switch statement on your operator, which is not bad because there is a limited number of operators you could use anyways.

Here's a way to do this with expression trees (written in LINQPad):

void Main()
{   
    var programmers = new List{ 
        new Programmer { Name = "Turing", Number = Math.E}, 
        new Programmer { Name = "Babbage", Number = Math.PI}, 
        new Programmer { Name = "Lovelace", Number = Math.E}};


    var rule0 = new Rule() { Field = "Name", Operator = BinaryExpression.Equal, Value = "Turing" };
    var rule1 = new Rule() { Field = "Number", Operator = BinaryExpression.GreaterThan,  Value = 2.719 };

    var matched0 = RunRule(programmers, rule0);
    matched0.Dump();

    var matched1 = RunRule(programmers, rule1);
    matched1.Dump();

    var matchedBoth = matched0.Intersect(matched1);
    matchedBoth.Dump();

    var matchedEither = matched0.Union(matched1);
    matchedEither.Dump();
}

public IEnumerable RunRule(IEnumerable foos, Rule rule) {

        var fieldParam = Expression.Parameter(typeof(T), "f");
        var fieldProp = Expression.Property (fieldParam, rule.Field);
        var valueParam = Expression.Parameter(typeof(V), "v");

        BinaryExpression binaryExpr = rule.Operator(fieldProp, valueParam);

        var lambda = Expression.Lambda>(binaryExpr, fieldParam, valueParam);
        var func = lambda.Compile();

        foreach(var foo in foos) {
            var result = func(foo, rule.Value);
            if(result)
                yield return foo;
        }

}

public class Rule {
    public string Field { get; set; }
    public Func Operator { get; set; }
    public T Value { get; set; }
}

public class Programmer {
    public string Name { get; set; }
    public double Number { get; set; }
}