Linq2SQL查询中无法进行强制转换

Question

I've had trouble with a Linq2SQL query. Dissection of the offending query yields this minimal example of its baffling behavior.

NorthwindDataContext db = 
  new NorthwindDataContext();
IEnumerable<int> q =
  from x in db.Categories
  select (int)(object)new System.Collections.ArrayList(x.CategoryID);
int[] ouch = q.ToArray();

( CategoryID is an int .) At the end, ouch will be filled with zeroes (one zero for every category in the database). I have used int and ArrayList in this example; the exact types are irrelevant. The main points needed to repeat this phenomenon are:

1. Linq2SQL. Using a local data source will produce the expected cast exception
2. Use of a property of the queried database table in the select expression. If no column of the queried tables is used, a cast exception will be raised.

My question is, why doesn't the above code produce an exception trying to cast the ArrayList into an int ?

The generated SQL code according to LINQPad:

SELECT NULL AS [EMPTY]
FROM [Categories] AS [t0]

As a background to my question: My original code read something like this:

IEnumerable<ParentClass> q =
  (from x in db.SomeTable
  select (ParentClass) new ChildClass { SomeProperty = x.SomeColumn })
  .ToArray();

ChildClass inherits from ParentClass . This code though correctly typed and semantically sound, raises an exception. This happens only if db is a Linq2SQL connection, not if it's a local data source. Trying to understand the cause of this behavior led me to the code I posted above.

Answer 1

No cast exception occurs because the casts are sent into the database where they are performed by rules outside of .net.

It's the same reason this method doesn't throw.

It's the same reason that string comparison in a query is (by default) case-insensitive.

Answer 2

NorthwindDataContext db = new NorthwindDataContext();
IEnumerable<int> q =
  from x in db.Categories
  select x.CategoryID;

int[] ouch = q.ToArray();