Foreach loop async-await

Question

I have a question about good practice with async-await in foreach loop.

My method looks like this

 public async Task<List<Category>> GetCategoriesAsync(string partOfName, CancellationToken ct = default(CancellationToken)) { string lowerCasePartOfName = partOfName.ToLower(); var categories = await _context.ECATEGORIES .Where(a => a.NAME.ToLower().Contains(lowerCasePartOfName)) .ProjectTo<Category>() .ToListAsync(ct); //version1 #Beginning var i = 0; foreach (var parentId in categories) { var categoryParent = await _context.ECATEGORIES .Where(a => a.ID == parentId.ParentId) .Select(s => s.NAME) .FirstOrDefaultAsync(ct); categories[i].CategoryParent = categoryParent; i++; } //version1 #End //version2 #Beginning categories.ForEach(async x => x.CategoryParent = await _context.ECATEGORIES .Where(a => a.ID == x.ParentId) .Select(s => s.NAME).FirstOrDefaultAsync(ct)); //version2 #End return categories; }

Version1 and version2 gives same result but I would like to ask which is better practice for async tasks or maybe none of them.

Thanks in advance.

Both are bad in that the code could be rewritten to make use of a proper join. That would make 1 db call instead of 1 call per category + 1 (for the initial call). But strictly answering your question, it does not matter: pick the one you feel most comfortable with. — Igor
– Igor, Commented Nov 21, 2018 at 10:13
Problem I had is that, parentId is often duplicated and I couldn't use .Contains — KJanek
– KJanek, Commented Nov 21, 2018 at 10:16

Igor · Accepted Answer · 2018-11-21 11:31:47Z

Both are bad in that the code could be rewritten to make use of a proper join. That would make 1 db call instead of 1 call per category + 1 (for the initial call). But strictly answering your question, it does not matter: pick the one you feel most comfortable with

Problem I had is that, parentId is often duplicated and I couldn't use .Contains

You can use a left join to do the same thing but all in 1 DB call which is cheaper than 1 call per result.

string lowerCasePartOfName = partOfName.ToLower(); var categories = await (from category in _context.ECATEGORIES.Where(a => a.NAME.ToLower().Contains(lowerCasePartOfName)) from parent in _context.ECATEGORIES.Where(parent => parent.ID == category.ParentId).DefaultIfEmpty() select new Category { Id = category.ID, CategoryParent = parent.NAME, }).ToListAsync();

If your schema is setup to be case insensitive then you can omit the ToLower calls as well. You can check this by looking at the COLLATION.

.Where(a => a.NAME.Contains(lowerCasePartOfName))

Sorry but I don't know how to use it in my method:/ In your code parent is IEnumarable<ECATEGORY> so I can't reference like parent.NAME .
var categories = await _context.ECATEGORIES .Where(a => a.NAME.ToLower().Contains(lowerCasePartOfName)) .GroupJoin(_context.ECATEGORIES, child => child.PARENTID, parent => parent.ID, (category, parent) => new { Category = category, ParentName = parent.FirstOrDefault()}) .Select(s => new Category { Id = s.Category.ID, CategoryParent = s.ParentName.NAME, ParentId = s.Category.PARENTID, }) .ToListAsync(ct);
I think now is ok? If yes I post as my answer to my question. I changed parent.Name to parent.FirstOrDefault() and then retrieve Name

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