string expression to c# function delegate

Question

I want to convert the following string into function delegate.

[Id]-[Description]

C# class:

public class Foo
{
    public string Id {get;set;}
    public string Description {get;set;}
}

Result function delegate:

Func<Foo, string> GetExpression = delegate()
{
    return x => string.Format("{0}-{1}", x.Id, x.Description);
};

I think compiled lambda or expression parser would be a way here, but not sure about the best way much. Any inputs?

Answer 1

It's possible as: to construct Linq Expression then compile it. Compiled expression is an ordinary delegate, with no performance drawbacks.

An example of implementation if type of argument(Foo) is known at compile time:

class ParserCompiler
{
    private static (string format, IReadOnlyCollection<string> propertyNames) Parse(string text)
    {
        var regex = new Regex(@"(.*?)\[(.+?)\](.*)");

        var formatTemplate = new StringBuilder();
        var propertyNames = new List<string>();
        var restOfText = text;
        Match match;
        while ((match = regex.Match(restOfText)).Success)
        {
            formatTemplate.Append(match.Groups[1].Value);
            formatTemplate.Append("{");
            formatTemplate.Append(propertyNames.Count);
            formatTemplate.Append("}");

            propertyNames.Add(match.Groups[2].Value);

            restOfText = match.Groups[3].Value;
        }

        formatTemplate.Append(restOfText);

        return (formatTemplate.ToString(), propertyNames);
    }

    public static Func<T, string> GetExpression<T>(string text) //"[Id]-[Description]"
    {
        var parsed = Parse(text); //"{0}-{1}  Id, Description"

        var argumentExpression = Expression.Parameter(typeof(T));

        var properties = typeof(T)
            .GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.GetField)
            .ToDictionary(keySelector: propInfo => propInfo.Name);

        var formatParamsArrayExpr = Expression.NewArrayInit(
            typeof(object), 
            parsed.propertyNames.Select(propName => Expression.Property(argumentExpression, properties[propName])));

        var formatStaticMethod = typeof(string).GetMethod("Format", BindingFlags.Static | BindingFlags.Public, null,new[] { typeof(string), typeof(object[]) }, null);
        var formatExpr = Expression.Call(
            formatStaticMethod,
            Expression.Constant(parsed.format, typeof(string)),
            formatParamsArrayExpr);

        var resultExpr = Expression.Lambda<Func<T, string>>(
            formatExpr,
            argumentExpression); // Expression<Func<Foo, string>> a = (Foo x) => string.Format("{0}-{1}", x.Id, x.Description);

        return resultExpr.Compile();
    }
}

And usage:

        var func = ParserCompiler.GetExpression<Foo>("[Id]-[Description]");
        var formattedString = func(new Foo {Id = "id1", Description = "desc1"});