Opening sharepoint site from a web service

See more:

C#3.5

webservice

SharePoint2010

Hello,

I have a sharepoint site that hold a very large list (more than 5000 items).
I was thinking of exposing a web service that eill return a list of guids.
I have a class that returns said list and I am trying to instantiate the class from the web service.
If I try to implment the class that returns the list of guid in an console application, everything works and I get the list of guids.
However, if i try to instantiate the class from a web service, when trying to open the site collection:

using (SPSite site = new SPSite(strSite))

I am getting an error

System.IO.FileNotFoundException: The Web application at http://npebwebstream could not be found. Verify that you have typed the URL correctly. If the URL should be serving existing content, the system administrator may need to add a new request URL mapping to the intended application.

Does anyone have an idea how to overcome this problem?

Here is the code

public void ProcessLargeList()
{
    ContentIterator iterator = new ContentIterator();

    //Here it works in a console application, and fails once being called from with in a
    //work service.

    using (var oSite = new SPSite(Site))
    {
        using (SPWeb oWeb = strWeb.Length > 0
            ? oSite.OpenWeb(Web)
            : oSite.RootWeb)
        {
            SPList spDoc = oWeb.Lists[DocumentLibrary] as SPDocumentLibrary;

            if (Query != null && Query.Length > 0)
            {
                SPQuery spquery = new SPQuery();
                spquery.Query = Query;
                spquery.QueryThrottleMode = SPQueryThrottleOption.Override;
                Status = IterationStatus.Started;

                //Runs through the content iterator and adds the items to the list of guids.

                iterator.ProcessListItems(spDoc, spquery, ProcessListItem,
                    ProcessListItemError);
            }
            else
            {
                iterator.ProcessListItems(spDoc, ProcessListItem,
                    ProcessListItemError);
            }
            Status = IterationStatus.Completed;
        }
    }
}

Thanks in advance,

Yossi