NotSupportedException thrown after call to .AsEnumerable() - linq

Question

I have some simple code that retrieves recorded ELMAH exceptions from a db:
HealthMonitoringEntities context = new HealthMonitoringEntities();
IQueryable<ELMAH_Error> exceptions = context.ELMAH_Error;
if (filter.ToDate != null)
exceptions = exceptions.Where(e => e.TimeUtc <= filter.ToDate.Value.AddHours(-4));
return exceptions.OrderByDescending(e => e.TimeUtc)
.Take(filter.Size)
.AsEnumerable()
.Select(e => new ElmahException()
{
ErrorId = e.ErrorId,
Application = e.Application,
Host = e.Host,
Type = e.Type,
Source = e.Source,
Error = e.Message,
User = e.User,
Code = e.StatusCode,
TimeStamp = e.TimeUtc.AddHours(-4).ToString()
}).ToList();
}
I get an exception on this line:
TimeStamp = e.TimeUtc.AddHours(-4).ToString()
The exception is:
LINQ to Entities does not recognize the method 'System.DateTime AddHours(Double)' method, and this method cannot be translated into a store expression.
When I call .AsEnumerable() before projecting with Select(), my sequence is enumerated and I project from a sequence that implements IEnumerable<ELMAH_Error>. Given that, why am I not working with the Linq-To-Objects API in my projection, which understands AddHours(), instead of still working with the Linq-To-Entities API?
UPDATE
There is a post on this topic by Jon Skeet here:
http://msmvps.com/blogs/jon_skeet/archive/2011/01/14/reimplementing-linq-to-objects-part-36-asenumerable.aspx
He has this query:
var query = db.Context
.Customers
.Where(c => some filter for SQL)
.OrderBy(c => some ordering for SQL)
.Select(c => some projection for SQL)
.AsEnumerable() // Switch to "in-process" for rest of query
.Where(c => some extra LINQ to Objects filtering)
.Select(c => some extra LINQ to Objects projection);
Note that after his call to AsEnumerable(), he indicated he is switching over to Linq-To-Objects. I am doing something similar in my function, but I am receiving a Linq-To-Entities exception where I had thought I would be executing against the Linq-To-Objects API.
Further Update
From Jim Wooley's blog: http://linqinaction.net/blogs/jwooley/archive/2009/01/21/linq-supported-data-types-and-functions.aspx
"As an example the following methods are shown as having translations for DateTime values: Add, Equals, CompareTo, Date, Day, Month, Year. In contrast methods like ToShortDateString, IsLeapYear, ToUniversalTime are not supported.
If you need to use one of the unsupported methods, you need to force the results to the client and evaulate them using LINQ to Objects at that point. You can do that using the .AsEnumerable extension method at any point in the query comprehension."
Is that not what I'm doing?

Answer 1

You have forgotten to protect your first call
if (filter.ToDate != null)
exceptions = exceptions.AsEnumerable()
.Where(e => e.TimeUtc <= filter.ToDate.Value.AddHours(-4));
Edit
Remember, IQueryables are not translated to sql until they are enumerated, so your debugger will execute that line but the error won't occur until you return. If filter.ToDate is null your code is equivalent to this:
if(filter.ToDate == null)
return exceptions
.Where(e => e.TimeUtc <= filter.ToDate
.Value
.AddHours(-4)) //uh-oh won't work in Linq-to-Entities
.OrderByDescending(e => e.TimeUtc)
.Take(filter.Size)
.AsEnumerable() //too late to protect you!
.Select(e => new ElmahException()
{
ErrorId = e.ErrorId,
Application = e.Application,
Host = e.Host,
Type = e.Type,
Source = e.Source,
Error = e.Message,
User = e.User,
Code = e.StatusCode,
TimeStamp = e.TimeUtc.AddHours(-4).ToString() //if we get this far we're OK
}).ToList();