Question

I’m trying to pass a GUID as a string over web services, however I’ve hit a bump in the road. The GUID is stored in SQL Server as a UNIQUEIDENTIFIER and EF pulls it as a GUID. What I would want to do is in my query do this:

var query = myEntities.Foo.Where(q => q.Id== Id)
                .Select(bar =>
                        new Bar()
                            { BString = bar.GUID }
                        );

Now, C# throws a build error that you cannot convert System.Guid to string, so I’d think that you could do this:

{ BString = bar.GUID.ToString() }

or even

{ BString = Convert.ToString(bar.GUID) }

But both end with runtime exception:

There was an exception, it was System.NotSupportedException: LINQ to
Entities does not recognize the method ‘System.String
ToString(System.Object)’ method, and this method cannot be translated
into a store expression.

So I gather that in both cases, it’s not able to figure out a T-SQL equivalent of going from UniqueIdentifier to varchar.

Is there a way around this besides retrieving the object, then iterating over my return set copying each object to a new object and converting guid->string at that point?

Thanks.

Answer 1

Convert it to a string outside the query part of the LINQ statement.

var query = myEntities.Foo.Where(q => q.Id== Id).Select(r => r.GUID)
  .AsEnumerable()
  .Select(guid =>
    new Bar()
    { BString = guid.ToString() }
  );

The .AsEnumerable() is the important part: it casts the result from IQueryable to IEnumerable, having the effect of running any further operations on the client after the database query has been made. (You could also use .ToArray() or .ToList() in its place.)