Injecting a variable into the Mono.CSharp.Evaluator (runtime compiling a LINQ query from string)

Question

I'm using the Mono.CSharp library to emit code. Following another question on SO (https://mcmap.net/q/394477/-mono-compiler-as-a-service-mcs) I managed to get Mono.CSharp evaluating correctly on the Microsoft CLR.

To add flexibility in my app I'd like to be able to customize a query at runtime - by allowing the user to provide a LINQ query as a string that gets parsed and hits the database when executed.

Given this basic snippet of code:

IQueryable<Contact> contacts = GetContacts();
string query = "from contact in contacts
                where contact.Name == \"name\"
                select contact";
var queryableResult = Mono.CSharp.Evaluator.Evaluate(query);

How can I 'inject' the contacts variable into the Mono.CSharp.Evaluator to be evaluated as part of the query? Am I going about this the right way? In the end I either need the resulting Expression or the IQueryable from the 'query' string.

Devoted

Check out my answer here: #3788596 – Kalinin

Answer 1

I think you have a few options:

1. Use static or ThreadStatic variables to exchange data between the caller and you string based code:

   namespace MyNs
   {
     public class MyClass
     {
    [ThreadStatic] // thread static so the data is specific to the calling thread
    public static string MyEnumerableVariable;
   
   
    public void DoSomething() 
    {
         Evaluator.ReferenceAssembly(Assembly.GetExecutingAssembly());
         Evaluator.Run("using MyNs;")
         // run the dynamic code
         var s = @"return (from contact in MyNs.MyClass.MyEnumerableVariable where contact.Name == ""John"" select contact).ToList();";
         Evaluator.Evaluate(s);
    }
   

   } }

2. Return a delegate from your string code:

   
    public void DoSomething() 
    {
   
   // run the dynamic code
     var s = @"return new Func<string, IQueryable<MyNs.Model.Contact>, IList>((s,q) => (from contact in q where contact.Name == s select contact).ToList());";
     var func = (Func<string, IQueryable<MyNs.Model.Contact>, IList>)Evaluator.Evaluate(s);
     var result = func("John", myQueryableOfContactsFromNHibernate);
   
   }
   

3. Go the full blown route.

string query = string.Format(
@"using (var dc = new DataContext()) 
{
  return (from contact in dc.Contacts where contact.Name == ""{0}"" select contact).ToList();
}", "John");

var result = Mono.CSharp.Evaluator.Evaluate(query);

Mellow

From a design point that's tightly coupling the code to the DataContext. I'm using NHibernate 3 with Ninject to get a new, clean ISession injected into my MVC controller's constructor for every request. If that's the only route then I can work around it with some static calls, just isn't ideal. – Devoted

Let me think some more, and I'm sure there are better ways, but my first thought is to make a ThreadStatic variable in your calling code and reference that from your string evaluated code. – Mellow

Attempting #2, <% var obj = Evaluator.Evaluate("return new Func<int> () => 3;"); %>. I'm receiving this error. If I remove the new, I get "Unexpected symbol '=>'". How do I return a Func from Evaluate into a var? – Contorted

The solution to my problem turned out to be to drop the return keyword and include the actual function as the parameter to the constructor, so... <% var obj = Evaluator.Evaluate("new Func<int>(() => 3);"); %>. Hopefully that helps someone else. – Contorted

I found a pretty good way to inject local variables and posted it under this question: #11569677 – Kalinin

Answer 2

I didn't try this, but I guess you could use Mono compiler to create a delegate vthat takes IQueryable<Contract> as argument and returns the filtered query. Something like:

IQueryable<Contact> contacts = GetContacts(); 
string query = "new Func<IQueryable<Contact>, IQueryable<Contact>>(contracts =>
                  from contact in contacts 
                  where contact.Name == \"name\" 
                  select contact)"; 
var res = Mono.CSharp.Evaluator.Evaluate(query); 

Then you just need to cast res to an appropriate Func<,> type and invoke it to get the result.

Verdie