asparallel plinq vs linq

Question

I have a code block

var result = db.ProductReceives.Where(x => x.CustomerName.ToLower().Contains(searchTxt)).ToList();

works fine but when i write same query

 var result = db.ProductReceives.AsParallel().Where(x => x.CustomerName.ToLower().Contains(searchTxt)).ToList();

It Shows a error message Object reference not set to an instance of an object.

I want to execute the query AsParallel query.can some one help?

Answer 1

If you hit the database as db suggests, then:

• The non-parallel code will be translated to a SQL query and executed by the DBMS directly.
  This is fast . Especially if you have an index on CustomerName as it'll be able to only scan through that index (you'll end up with a full table scan otherwise but it will probably still be fast enough).

• The second one will:

  • Download the whole ProductReceives table. All of it.
  • It will then create an object for each row.
  • Then it will feed these objects to your parallel check.

  This will be much slower than the first solution.

You're getting a NullReferenceException because one of these rows has a NULL CustomerName . So you end up calling ((string)null).ToLower() .

The error doesn't occur in the first case because the DMBS will take care of filtering that out by itself.

Answer 2

也许你应该在调用ToLower之前检查客户名是否为null。

db.Tv_ProductReceive.AsParallel().Where(i =>i.CustomerName != null && i.CustomerName.ToLower().Contains(searchTxt)).ToList();