asp.net - Get result using condition not works properly (C#)

Question

I trying to get data from datatabase and assign it to list

Here is 2 conditions. 1 - I have only categoryId and 2 - I have category and subcategoryId

I wrote code like this

public async Task<List<TimelineItemDto>> GetFilteredClasses(int? categoryId, int? subcategoryId)
    {
        List<TimelineItemDto> filteredClasses;
        if (categoryId != null)
        {
            filteredClasses = await _context.Events
                .Where(x => x.CategoryId == categoryId && x.EventType == EntityType.Class)
                .ProjectTo<TimelineItemDto>()
                .ToListAsync();
        }

        if (categoryId != null && subcategoryId != null)
        {
            filteredClasses = await _context.Events
                .Where(x => x.CategoryId == categoryId && x.SubcategoryId == subcategoryId &&
                            x.EventType == EntityType.Class)
                .ProjectTo<TimelineItemDto>()
                .ToListAsync();
        }

        else
        {
            filteredClasses = await _context.Events.Where(x =>
                    x.EventType == EntityType.Class).ProjectTo<TimelineItemDto>()
                .ToListAsync();
        }

        return filteredClasses;
    }

but at first if I got this

A second if and at else all okay and it's executed and filling list

How I can solve this?

question from:https://stackoverflow.com/questions/65859125/get-result-using-condition-not-works-properly-c

Answer 1

Well I would do something like below :

public async Task<List<TimelineItemDto>> GetFilteredClasses(int? categoryId, int? subcategoryId)
{
       var  filteredClasses = _context.Events.Where(x => x.EventType == EntityType.Class);
        if (categoryId != null)
        {
            filteredClasses = filteredClasses.
                .Where(x => x.CategoryId == categoryId);                
        }

        if (categoryId != null && subcategoryId != null)
        {
            filteredClasses = filteredClasses.Where(x =>  x.SubcategoryId == subcategoryId );              
        }      

        return await filteredClasses.ProjectTo<TimelineItemDto>()
                .ToListAsync();;
}

This way you will avoid materializing multiple queries.