LINQ Boolean expressions

Question

I don't know if this is the right question for here. Let's say I have a String with Values:

A = "ab1, eb4, gr3, ..."

Then there are two more Strings B containing must-have values and C not-allowed values. Unfortunately B and C can contain logical AND / OR within the strings.

e.g.

B = "(ut2 | eg1) & ab1"

C = "ke2 & pa5"

Now I am looking for a way to check wether A is valid for B and C. True or False.

For A it's pretty easy to convert the String to List of String containing the values seperately and enabling LINQ expressions on that list.

But how can I best "parse" the information in B and C to evaluate it with an LINQ expression on List A ?

Think ideal solution would be to have B and C each as just one logical expression to directly have List A checked on them. But how to start with that?

Thanks for your support and best regards.

Answer 1

What works is:

1) Checking all values in B and C if they are present in A, and replace them with "True" oder "False". Leads from:

B = "(ut2 | eg1) & ab1"

to

B = "(False| False) & True"

2) Evaluating this String.

For Evaluation I found this Method: https://stackoverflow.com/a/8476664/6552728

    Dim dt As New System.Data.DataTable()
    dt.Columns.Add("", GetType(Boolean))
    dt.Columns(0).Expression = "(False| False) & True"

    Dim r As System.Data.DataRow = dt.NewRow()
    dt.Rows.Add(r)
    Dim result As Boolean = DirectCast(r(0), Boolean)
    MsgBox(result.toString)

Thats maybe not very nice but it works. Is there a way evaluating logical expressions stored in String using LINQ Expression or Binary Expression? I tried for the last 2 hours but can't get it working...

    Dim exp As Expression(Of Func(Of String, Boolean)) = Function(p) p = "True Or False"
    Dim o As Object
    o = Expression.Lambda(exp).Compile().DynamicInvoke()

Answer 2

StackOverflow isn't a code writing service, but some code is fun to write.

Here is a class that parses the test phrase into an Expression tree and then compiles the tree to return a Func to test a HashSet<string>:

public static class PhraseCompiler {
    public static Regex tokenRE = new Regex(@"\(|\)|&|\||\w+", RegexOptions.Compiled);
    static IEnumerable<string> Tokens(string phrase) {
        foreach (Match m in tokenRE.Matches(phrase))
            yield return m.Value;
    }

    static MethodInfo HSContainsMI = typeof(HashSet<string>).GetMethod("Contains");
    static Expression subCompile(IEnumerator<string> te, ParameterExpression g) {
        var stack = new Stack<Expression>();

        while (te.MoveNext()) {
            switch (te.Current) {
                case "(":
                    break;
                case ")":
                    goto exit_while;
                case "|":
                case "&":
                    var op = te.Current;
                    var lhs = stack.Pop();
                    var rhs = subCompile(te, g);
                    var opExpr = Expression.MakeBinary(op == "|" ? ExpressionType.OrElse : ExpressionType.AndAlso, lhs, rhs);
                    stack.Push(opExpr);
                    break;
                default:
                    var v = Expression.Constant(te.Current, typeof(string));
                    var test = Expression.Call(g, HSContainsMI, v);
                    stack.Push(test);
                    break;
            }
        }
    exit_while:

        return stack.Pop();
    }

    public static Func<HashSet<string>, bool> Compile(string phrase) {
        var g = Expression.Parameter(typeof(HashSet<string>), "g");

        var te = Tokens(phrase).GetEnumerator();
        var e = subCompile(te, g);

        var ef = Expression.Lambda<Func<HashSet<string>, bool>>(e, g);

        return ef.Compile();
    }
}

Given your initial values

var A = "ab1, eb4, gr3";

var B = "((ut2 | eg1) | gr3) & ab1";
var C = "ke2 & pa5";

Then you can convert the test value to a HashSet<string>:

var As = A.Split(", ").ToHashSet();

And then you can compile the tests:

var fB = PhraseCompiler.Compile(B);
var fC = PhraseCompiler.Compile(C);

and run the tests:

var testBresult = fB(As);
var testCresult = fC(As);

NOTE: The compiler does no error checking and is likely to throw an exception for bad test phrases.