how to use a delegate to get by name a CompiledQuery

Question

I'm trying to find and run a CompiledQuery given the name. How do I access the compiled query by name and how do I then invoke the delegate?

Here's as far as I can get - I get the error 'Error binding to target method'

public class ActivityRepository
{
    private readonly ActivityDataContext _db;

    public ActivityRepository()
    {
        _db = new ActivityDataContext();
    }

    public static Func<ActivityDataContext, int, IQueryable<ProjectObject>>
        GetCompiledLatestProjects = CompiledQuery.Compile
            ((ActivityDataContext db, int projectId) =>
             from c in db.projectObjects
             where c.projectId == projectId
             select c);

    public static Func<ActivityDataContext, Guid, IQueryable<Report>>
        GetCompiledReports = CompiledQuery.Compile
            ((ActivityDataContext db, Guid itemId) =>
             from c in db.Reports
             where c.reportObjectId == itemId
             select c);

// Other compiled queries ommitted, but the results are IQueryable objects that implement a common interface IProjectObject

    delegate IQueryable<IProjectObject> MyDelegate();

    static MyDelegate GetByName(object target, string methodName)
    {
            return (MyDelegate)Delegate.CreateDelegate
                (typeof(MyDelegate), target, methodName);
    }

    public IList<Results> GetResults(string reportName)
    {
            IQueryable<ProjectObject> projectItems = GetLatestProjectObjects(projectId, quantity);
        foreach (projectObject o in projectItems)
        {
            MyDelegate del = GetByName(this, reportName);
             var dbReport = (IProjectObject) GetCompiledReports(_db, o.itemId).FirstOrDefault();
// add results to List and return
         }
     }
}

Answer 1

You can probably do some long winded way of trying to identify any fields of type Func<...> with a DataContext as the first argument that return IQueryable, but you might find it a lot easier to just add a custom attribute:

[AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
public class DynamicQueryAttribute : Attribute { }

...which you can then add to your field:

[DynamicQuery]
public static Func<ActivityDataContext, int, IQueryable<ProjectObject>>
    GetCompiledLatestProjects = CompiledQuery.Compile
        ((ActivityDataContext db, int projectId) =>
         from c in db.projectObjects
         where c.projectId == projectId
         select c);

You can then select the query based on that, and its name:

public static IQueryable<IProjectObject> ExecuteQuery(Type ownerType, string name, params object[] args)
{
    var query = typeof(ownerType)
        .GetFields(BindingFlags.Public | BindingFlags.Static)
        .Where(f =>
                (f.GetCustomAttributes(typeof(DynamicQueryAttribute), false).Length > 0)
                && (f.Name.ToLowerInvariant() == name.ToLowerInvariant()))
        .Select(f => (Delegate) f.GetValue(null))
        .SingleOrDefault();

    if (query == null)
        return null;

    return (IQueryable<IReportObject>)query.DynamicInvoke(args);
}

Usage:

var results = ExecuteQuery(
    typeof(ActivityRepository),
    "GetCompiledLatestProjects", 
    dataContext, 
    projectId);

Hope that helps :)