Лучший способ создать Динамический запрос LINQ

Question

Лучший способ создать Динамический запрос LINQ

Решение:

private async void ListView_ItemTapped(object sender, ItemTappedEventArgs e)
{
   string result= await  DisplayActionSheet("Details", "Close", null, "Cash", "Delete", "");

   if(result=="Delete")
    {
        int position = DebtsList.TemplatedItems.GetGlobalIndexOfItem(e.Item);

        mySource.RemoveAt(position);

        DebtsList.ItemsSource = mySource;

    }
}

mySource - это ItemsSource в DebtsList, например,

public ObservableCollection<DebtEntries> mySource { get; set; }

. . .

mySource = new ObservableCollection<DebtEntries>();
mySource.Add(new DebtEntries { Name = "xxx", Usage="xxx",Value="xxx",CreationDate="xxx"});
//. . .
DebtsList.ItemsSource = mySource;

7

linq asp.net

задан Vikas 10 April 2009 в 07:23

1 ответ

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score 21 · Accepted Answer

Okay, it's not entirely clear what you want, but if you're trying to only add where clauses for the parameters which are non-null, you could do:

public IQueryable<Student> FindByAllStudents
    (int? id, string name, int? courseID, bool? isActive)
{    
    IQueryable<Student> query = db.Student;
    if (id != null)
    {
        query = query.Where(student => student.ID == id.Value);
    }
    if (name != null)
    {
        query = query.Where(student => student.Name.Contains(name));
    }
    if (courseID != null)
    {
        query = query.Where(student => student.CourseID == courseID.Value);
    }
    if (isActive != null)
    {
        query = query.Where(student => student.IsActive == isActive.Value);
    }
    return query;
}

I haven't tried that, and it's possible that LINQ to SQL would get confused by the code to find the value of the nullable value types. You may need to write code like this:

    if (courseID != null)
    {
        int queryCourseID = courseID.Value;
        query = query.Where(student => student.CourseID == queryCourseID);
    }

It's worth trying the simpler form first though :)

Of course, all this gets a bit irritating. A helpful extension method could make life more concise:

public static IQueryable<TSource> OptionalWhere<TSource, TParameter>
    (IQueryable<TSource> source,
     TParameter? parameter, 
     Func<TParameter, Expression<Func<TSource,bool>>> whereClause)
    where TParameter : struct
{
    IQueryable<TSource> ret = source;
    if (parameter != null)
    {
        ret = ret.Where(whereClause(parameter.Value));
    }
    return ret;
}

You'd then use it like this:

public IQueryable<Student> FindByAllStudents
    (int? id, string name, int? courseID, bool? isActive)
{    
    IQueryable<Student> query = db.Student
        .OptionalWhere(id, x => (student => student.ID == x))
        .OptionalWhere(courseID, x => (student => student.CourseID == x))
        .OptionalWhere(isActive, x => (student => student.IsActive == x));
    if (name != null)
    {
        query = query.Where(student => student.Name.Contains(name));
    }
    return query;
}

Using a higher order function like this could get confusing if you're not really comfortable with it though, so if you're not doing very many queries like this you might want to stick with the longer but simpler code.