实体框架，LINQ 查询

Question

How to loop through the Items below and update the Field which have empty values with the value (!) Error:

Cannot implicitly convert type System.threading.task to System.Collections.Generic.IEnumerable<Respositories.AssignmentMasterData>

using (SUPEntities db = new SUPEntities())
{
    IEnumerable<AssignementMasterData> masterDatas = null;

    masterDatas = db.AssignementMasterDatas
                    .Where(m => DbFunctions.TruncateTime(m.CreatedDateTime) >= DbFunctions.TruncateTime(criteria.FilterStartDate)
                        && DbFunctions.TruncateTime(m.CreatedDateTime) <= DbFunctions.TruncateTime(criteria.FilterEndDate)
                        && (m.AssignmentNoteNumber == criteria.AssigmentNumber || criteria.AssignmentNumber == null)
                        && (m.BaseCourseId == criteria.courseId || criteria.CourseId == 0)
                        && (m.AccountNumber == criteria.AccountNumber || criteria.AccountNumber == null)
                        && (m.ReferenceNumber == criteria.ReferenceNumber || criteria.ReferenceNumber == null)
                        && (m.FacultyCode == criteria.FAcultyCode || criteria.FacultyCode == null)
                        && (m.Processed == criteria.Processed)
                        && (m.ClassNumber == criteria.ClassNumber || criteria.ClassNumber == null))
                    .ForEachAsync(t => t.AssignmentNoteIdentifiedClasses.Select(e => String.IsNullOrEmpty(e.Category)? "(!)": e.Category));
                
}

Answer 1

Firstly, about the error message:

You are trying to assign the wrong type to your masterDatas variable. You declare it as a IEnumerable<Respositories.AssignmentMasterData> , but the ForEachAsync at the last line will return a Task , hence the error message.

See the ForEachAsync signature:

public static System.Threading.Tasks.Task ForEachAsync (this System.Linq.IQueryable source, Action action);

Secondly. You want to return an IEnumerable<Respositories.AssignmentMasterData>

For that you don't need the call to ForEachAsync.

You will need to some point transform your IQueryable into an IEnumerable . A call to AsEnumerable() does that. Then you need to replace some values. So you need to project your collection using a Select .

using (SUPEntities db = new SUPEntities())
{
    var masterDatas = db.AssignementMasterDatas
        .Where(m => DbFunctions.TruncateTime(m.CreatedDateTime) >= DbFunctions.TruncateTime(criteria.FilterStartDate)
            && DbFunctions.TruncateTime(m.CreatedDateTime) <= DbFunctions.TruncateTime(criteria.FilterEndDate)
            && (m.AssignmentNoteNumber == criteria.AssigmentNumber || criteria.AssignmentNumber == null)
            && (m.BaseCourseId == criteria.courseId || criteria.CourseId == 0)
            && (m.AccountNumber == criteria.AccountNumber || criteria.AccountNumber == null)
            && (m.ReferenceNumber == criteria.ReferenceNumber || criteria.ReferenceNumber == null)
            && (m.FacultyCode == criteria.FAcultyCode || criteria.FacultyCode == null)
            && (m.Processed == criteria.Processed)
            && (m.ClassNumber == criteria.ClassNumber || criteria.ClassNumber == null))
        .AsEnumerable()
        .Select(a =>
        {
            a.AssignmentNoteIdentifiedClasses = a.AssignmentNoteIdentifiedClasses
                .Select(e =>
                {
                    e.Category = string.IsNullOrWhiteSpace(e.Category) ? "(!)" : e.Category;
                    return e;
                })
                .ToList(); // Depending on the type of AssignmentNoteIdentifiedClasses, ToList() might be replaced.
            return a;
        });
    return masterDatas
}

Answer 2

• Use .Include( m => m.AssignmentNoteIdentifiedClasses ) to bring-in related data in a single query, this is much faster than loading each set of related AssignmentNoteIdentifiedClasses in your for-each-row loop.
  • This is also known as the "N+1 Problem" in ORMs .
• You don't need to use TruncateTime .
  • In fact, you shouldn't because that will mean your query isn't SARGable.
  • Avoid functions in SQL predicates.
  • Instead, just round criteria.FilterStartDate down to the start-of-day in application code and compare it normally with m => m.CreatedDateTime >= filterStart .
  • Similarly, FilterEndDate should be rounded-up and then compared like so: m => m.CreatedDateTime < filterEnd
    • Always use exclusive upper-bounds. It makes everything , especially date-range predicates, much easier to deal with.
• You don't need inline && in your Where . Use additional separate .Where() clauses instead. They'll be evaluated with AND
• I assume EF Core isn't sophisted enough to recognize the NULL -means-ignore anti-pattern for optional search predicates, in which case **DON'T USE NULL -means-ignore IN AN PREDICATE!"
  • This is bad for so many reasons: namely because query execution-plans are based on the structure ("shape") of the SQL query and not on parameter values, so the same cached execution-plan for non- NULL parameters will be used when some, or even all parameters are NULL - which is a problem.
  • Instead build your query by using IQueryable<T> 's Linq extensions and reassigning to itself.
    • eg IQueryable<T> query = db.Etc; query = query.Where( e => etc ); IQueryable<T> query = db.Etc; query = query.Where( e => etc );
    • Each .Where() is added as an AND condition. If you want to build-up an OR condition then use PredicateBuilder .

---

DateTime filterStart   = criteria.FilterStartDate.Date;
DateTime filterEndExcl = criteria.FilterEndDate  .Date.AddDays(1);

using (SUPEntities db = new SUPEntities())
{
    IQueryable<AssignementMasterData> query = db.AssignementMasterDatas
        .Include( m => m.AssignmentNoteIdentifiedClasses )
        .Where( m => m.CreatedDateTime >= filterStart   )
        .Where( m => m.CreatedDateTime <  filterEndExcl ) // Exclusive upper-bound.
        .Where( m => m.Processed       == criteria.Processed )
        .Where( m => m.ClassNumber     == criteria.ClassNumber )
    ;

    if( criteria.AssigmentNumber != null )
    {
        query = query.Where( m => m.AssignmentNoteNumber == criteria.AssigmentNumber );
    }

    if( criteria.AccountNumber != null )
    {
        query = query.Where( m => m.AccountNumber == criteria.AccountNumber );
    }

    if( criteria.CourseId != null && criteria.CourseId.Value > 0 )
    {
        query = query.Where( m => m.BaseCourseId == criteria.CourseId );
    }

    if( criteria.ReferenceNumber != null )
    {
        query = query.Where( m => m.ReferenceNumber == criteria.ReferenceNumber );
    }

    if( criteria.FacultyCode != null )
    {
        query = query.Where( m => m.FacultyCode == criteria.FacultyCode );
    }

    if( criteria.ClassNumber != null )
    {
        query = query.Where( m => m.ClassNumber == criteria.ClassNumber );
    }

    List<AssignementMasterData> rows = await query.ToListAsync().ConfigureAwait(false);
    
    List<String> categories = rows
        .SelectMany( r => r.AssignmentNoteIdentifiedClasses )
        .Select( String.IsNullOrEmpty(e.Category)? "(!)": e.Category) )
        .ToList();

    return categories;
}

---

The above can be simplified by adding a new extension-method (make sure you use Expression<Func<...>> and not just Func<> so that EF can still interpret the query:

public static class MyQueryableExtensions
{
    public static IQueryable<T> WhereIfNotNull<T,TValue>( this IQueryable<T> query, TValue? value, Expression<Func<T,Boolean>> predicate )
        where TValue : struct
    {
        if( value.HasValue && value.Value != default(TValue) )
        {
            return query.Where( predicate );
        }
        else
        {
            return query;
        }
    }
}

Used like so:

// `criteria` is now named `c` for brevity.

DateTime filterStart   = c.FilterStartDate.Date;
DateTime filterEndExcl = c.FilterEndDate  .Date.AddDays(1);

using (SUPEntities db = new SUPEntities())
{
    IQueryable<AssignementMasterData> query = db.AssignementMasterDatas
        .Include( m => m.AssignmentNoteIdentifiedClasses )
        .Where( m => m.CreatedDateTime >= filterStart    )
        .Where( m => m.CreatedDateTime <  filterEndExcl  ) // Exclusive upper-bound.
        .Where( m => m.Processed       == c.Processed    )
        .Where( m => m.ClassNumber     == c.ClassNumber  )
        .WhereIfNotNull( c.AssigmentNumber, m => m.AssignmentNoteNumber == c.AssigmentNumber )
        .WhereIfNotNull( c.AccountNumber  , m => m.AccountNumber        == c.AccountNumber   )
        .WhereIfNotNull( c.CourseId       , m => m.BaseCourseId       ​  == c.CourseId        )
        ​.WhereIfNotNull( c.ReferenceNumber, m => m.ReferenceNumberr     == c.ReferenceNumber )
        ​.WhereIfNotNull( c.FacultyCode    , m => m.FacultyCoder         == c.FacultyCode     )
        ​.WhereIfNotNull( c.ClassNumber    , m => m.ClassNumber          == c.ClassNumber     )
   ;

    List<AssignementMasterData> rows = await query.ToListAsync().ConfigureAwait(false);
    
    List<String> categories = rows
        .SelectMany( r => r.AssignmentNoteIdentifiedClasses )
        .Select( String.IsNullOrEmpty(e.Category)? "(!)": e.Category) )
        .ToList();

    return categories;
}