Question

I know this is a crazy thing to ask.. but Ill give it a shot 🙂
Is there anyway to build up your linq/query and then later on be able to build even more on it and then execute it.. for instance like this:

        public void Test()
    {
        var k = TestRepo();
        k = k.Where(e => e.SomeColumn == someValue);

        rp.DataSource = k.Select(t => t.Id);
        rp.DataBind();
    }


    public IEnumerable<ApplicationEntity> TestRepo()
    {
        using (var x = new MyEntityContext())
        {
            return from q in x.MyColumn
                   select q;
        }
    }

If im not misstaken this whould never work :)… but whouldnt it be just great if you first could “build” the basics of the query and then extend it somemore and then once its finaly ready actually execute it and get the result..
So is there anyway to do this as in the example above with EF or NHibernate.. or both of em?

Thanks in advance!

Answer 1

The trick is not to dispose the MyEntityContext. For instance, you can cache it for the lifetime of a request and dispose it at the end of an request. This might look like this:

public static class ContextFactory
{
private static bool disposeRegistered = false;

public static MyEntityContext GetContext()
{
    RegisterDispose();

    var instance = (MyEntityContext)HttpContext.Current
        .Items["MyEntityContext"];

    if (instance == null)
    {
        instance = new MyEntityContext();

        HttpContext.Current.Items["MyEntityContext"] =
            instance;
    }

    return instance;    
}

private static void RegisterDispose()
{
    if (disposeRegistered)
        return;

    HttpContext.Current.ApplicationInstance
       .EndRequest += (s,e) =>
    {
        var instance = HttpContext.Current
            .Items["MyEntityContext"] as IDisposable;

        if (instance != null)
        {
            instance.Dispose();
        }
    };

    this.disposeRegistered = true;
}
}

The TestRepo method than looks like this:

public IQueryable<Employee> TestRepo()
{
    var x = ContextFactory.GetContext()

    return 
        from employee in x.Employees
        where employee.Salary > 1000
        select employee;
}

Note how the TestRepo now returns IQueryable. This allows other methods to change the query and only when you start iterating it, the query will get sent to the database.

NOTE: Instead of taking a dependency on a static ContextFactory class, you could also inject an MyEntityContext into types that wish to use it. You can use this by using a technique called Dependency Injection.