避免使用linq多次浏览列表，具有动态条件（过滤器）

Question

I have a list with Article objects. Parameters for this object are

Category, Description, Status, Class

The user can filter the list with the combinaison he wants. Only Description, or Category + Class, an so on.

So if the user choose a criterion i'll get an int >= 0, otherwise i'll get -1.

For example if he choose to filter with Status and Category , i'll get

FilterCategory = x, FilterDescription = -1, FilterStatus = y, FilterClass = -1

My way to filter the list is the following:

if (FilterCategory != -1)
    list = list.Where(a => a.Category == FilterCategory);
if (FilterDescription != -1)
    list = list.Where(a => a.Description == FilterDescription);
if (FilterStatus != -1)
    list = list.Where(a => a.Status == FilterStatus);
if (FilterClass != -1)
    list = list.Where(a => a.Class == FilterClass);

In this way, i have to iterate the list 4 time, it's not efficient with a lot of items. I would browse the list once, and check the 4 condition in an unique where. But i don't know how to do that with the specific condition != -1.

Thank you

Answer 1

As Juharr mentions in the comments, Linq will not evaluate your .Where right after calling it. Only when calling ToList/ToArray/First/Single/foreach will it evaluate the clause and it will automatically combine your filters.

Answer 2

You can do a HUGE Where query moving all your conditions it a single Where by noticing that:

if (FilterCategory != -1)
    list = list.Where(a => a.Category == FilterCategory);

is equivalent to:

list = list.Where(a => FilterCategory == -1 || a.Category == FilterCategory);

Then the query is:

list = list.Where(a => (FilterCategory == -1 || a.Category == FilterCategory)
                    && (FilterDescription == -1 || a.Description == FilterDescription)
                    && (FilterStatus == -1 || a.Status == FilterStatus)
                    && (FilterClass == -1 || a.Class == FilterClass));

But I really doubt it will improve your enumeration performance.

Answer 3

list = list.Where(a => (FilterCategory != -1 || a.Category == FilterCategory) && 
                                       (FilterDescription != -1 || a.Description == FilterDescription) &&
                                       (FilterStatus != -1 || a.Status == FilterStatus) &&
                                       (FilterClass != -1 || a.Class == FilterClass));