Enity Framework regular error while command executing

I am developing a WPF Applicaton using Entity Framework for data access. As design pattern i use "MVVM" for tier organization, and "Repository" with "UnitOfWork" for data layer organization.

Generic Repository class:

    public class EFRepository : IRepository where T : class
    {
    public IUnitOfWork UnitOfWork { get; set; }
    private IObjectSet _objectset;
    private IObjectSet ObjectSet
    {
        get
        {
            if (_objectset == null)
                    {
                        _objectset = UnitOfWork.Context.CreateObjectSet();
            }
            return _objectset;
        }
    }
    public virtual IQueryable All()
    {
        return ObjectSet.AsQueryable();
    }
    ...

Unit of work interface:

    public interface IUnitOfWork: IDisposable
        {
            ObjectContext Context { get; set; }
            void Save();
            bool LazyLoadingEnabled { get; set; } 
            bool ProxyCreationEnabled { get; set; }
            string ConnectionString { get; set; }
        }

I build all L2E queries in model layer, like:

this.repository1.All().First(i => i.Field1 == some_value);

Sometimes here is thrown an EntityCommandExecutionException. It happens sometimes, but not regular. There is no rule to make this exception occur.

Exception Detail:

    EntityCommandExecutionException: 
    {"An error occurred while executing the command definition. See the inner exception for details."}
    InnerException:
     {"Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding."}
    StackTrace:
        at System.Data.EntityClient.EntityCommandDefinition.ExecuteStoreCommands(EntityCommand entityCommand, CommandBehavior behavior)
       at System.Data.Objects.Internal.ObjectQueryExecutionPlan.Execute[TResultType](ObjectContext context, ObjectParameterCollection parameterValues)
       at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 开发者_JS百科forMergeOption)
       at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable.GetEnumerator()
       at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)
       at System.Data.Objects.ELinq.ObjectQueryProvider.b__0[TResult](IEnumerable`1 sequence)
       at System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle[TResult](IEnumerable`1 query, Expression queryRoot)
       at System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute[S](Expression expression)
       at System.Linq.Queryable.First[TSource](IQueryable`1 source, Expression`1 predicate)
    

Please, help me to find out the problem, i really don`t know what to do now =(

p.s.:

I tried to provoke this error building and executing L2E queries in simple Console Application. I tried single L2E queries and through 1000-iterations cycles. But nothing caused this exception.

I can post any additional information if needed.

[23.03.2011]

Additional Info:

• Entity Framework 4.0
• MSSQL Server 2008
• this exception can be thrown any time the query take place. It can be l2e query to small table (<200 rows) or large (>500k rows). Also this exception can be caused by Function Import call
• when this exception is thrown,

    {"Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding."}

it is thrown immediately, but the connection timeout is set to 30 seconds! (i think that is "key feature")

• DB Server is situated in LAN, and there is no heavy traffic on DB.
• as i have found out, this exception can occur any time no matter what kind of queries or tables are used.
• I dont use transaction. This error occur even if i use only select queries.

I think the problem caused by using WPF with EF, because my "EF part" works fine in Console Application.

The ObjectContext has two timeout parameters:

int x = myObjectContext.Connection.ConnectionTimeout;

and

int? x = myObjectContext.CommandTimeout;

Looking at the exception and callstack I don't think that the ConnectionTimeout is the problem (that's the maximum time to try to establish a connection from the client to SQL Server). You could try to set the CommandTimeout to some high value (that's the timeout for queries and commits with SaveChanges; the value null means that the default of the underlying provider will be taken).