C#•13mo ago

Is it possible to remove predicate by condition?

I have following:

var notificationModels = await _repository
    .Query()
    .Where(model => DateTimeOffset.Now <= model.ExpiresAt)
    .Where(model => model.To
        .Contains(operatorName))
    .Where(model => model.Id <= arguments.LastNotificationId)
    .OrderByDescending(model => model.Id)
    .Limit(arguments.TotalCount)
    .ToList();

var notificationModels = await _repository
    .Query()
    .Where(model => DateTimeOffset.Now <= model.ExpiresAt)
    .Where(model => model.To
        .Contains(operatorName))
    .Where(model => model.Id <= arguments.LastNotificationId)
    .OrderByDescending(model => model.Id)
    .Limit(arguments.TotalCount)
    .ToList();

LastNotificationId can be null or 0 and this case I wanna remove .Where(model => model.Id <= arguments.LastNotificationId) and return just limited number of items

13 Replies

cap5lut•13mo ago

u can split the method chaining up:

var query = _repository
    .Query()
    .Where(model => DateTimeOffset.Now <= model.ExpiresAt)
    .Where(model => model.To
        .Contains(operatorName));

if (check last modification id here)
    query = query.Where(model => model.Id <= arguments.LastNotificationId);

var notificationModels = await query
    .OrderByDescending(model => model.Id)
    .Limit(arguments.TotalCount)
    .ToList();

var query = _repository
    .Query()
    .Where(model => DateTimeOffset.Now <= model.ExpiresAt)
    .Where(model => model.To
        .Contains(operatorName));

if (check last modification id here)
    query = query.Where(model => model.Id <= arguments.LastNotificationId);

var notificationModels = await query
    .OrderByDescending(model => model.Id)
    .Limit(arguments.TotalCount)
    .ToList();

SWEETPONYOP•13mo ago

Angius•13mo ago

You can also make a nice extension method like

public IQueryable<T> WhereIf<T>(this IQueryable<T> query, Func<T, bool> predicate, bool condition)
{
    return condition
        ? query.Where(predicate)
        : query;
}

public IQueryable<T> WhereIf<T>(this IQueryable<T> query, Func<T, bool> predicate, bool condition)
{
    return condition
        ? query.Where(predicate)
        : query;
}

SWEETPONYOP•13mo ago

it is good idea but can we just remove predicate from linq query?

Angius•13mo ago

No, you cannot

cap5lut•13mo ago

the predicate needs to be Expression<Func<T, bool>>, i think

SWEETPONYOP•13mo ago

ur right

Angius•13mo ago

Yep yep, right

SWEETPONYOP•13mo ago

thanks for helping me! I'm sorry but how to use predicate in my case? harold

cap5lut•13mo ago

the predicate is the same as in the normal Where call, just afterwards u pass a boolean value that if true actually adds the check

Angius•13mo ago

someQuery.WhereIf(x => x.Foo == bar, 2 == 2)

someQuery.WhereIf(x => x.Foo == bar, 2 == 2)

cap5lut•13mo ago

never tried it with IQueryable and Expression but i guess theoretically u could do something like

.Where(condition ? model => model.Id <= arguments.LastNotificationId : model => true)

.Where(condition ? model => model.Id <= arguments.LastNotificationId : model => true)

so it would simply add your perdicate or an "always true" predicate. but in normal linq its a horrible idea because this will check the "always true" predicate on each element as well (even if not needed) and i guess the generated sql would look quite weird as well (and maybe the dbms would do the check for every row as well)

SWEETPONYOP•13mo ago

ah I see thanks!

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Is it possible to remove predicate by condition?