Question

I came across a rather strange problem with linq-to-sql. In the following example,

var survey = (from s in dbContext.crmc_Surveys
                                   where (s.crmc_Retail_Trade_Id == tradeId) && (s.State_.Equals(state))
                                   select s).First();

If tradeId is null, it doesn’t behave as if I had specified null specifically like this instead,

var survey = (from s in dbContext.crmc_Surveys
                                   where (s.crmc_Retail_Trade_Id == null) && (s.State_.Equals(state))
                                   select s).First();

Which is my desired behavior. In fact it doesn’t return anything unless both values are non-null. I can’t figure out how to accomplish this short of several different linq queries. Any ideas?

Answer 1

Change where (s.crmc_Retail_Trade_Id == tradeId)
to

where (s.crmc_Retail_Trade_Id == tradeId || 
      (tradeId == null && s.crmc_Retail_Trade_Id == null))

Edit – based on this post by Brant Lamborn, it looks like the following would do what you want:

where (object.Equals(s.crmc_Retail_Trade_Id, tradeId))

The Null Semantics (LINQ to SQL) MSDN page links to some interesting info:

LINQ to SQL does not impose C# null or
Visual Basic nothing comparison
semantics on SQL. Comparison operators
are syntactically translated to their
SQL equivalents. The semantics reflect
SQL semantics as defined by server or
connection settings. Two null values
are considered unequal under default
SQL Server settings (although you can
change the settings to change the
semantics). Regardless, LINQ to SQL
does not consider server settings in
query translation.

A comparison with the literal null
(nothing) is translated to the
appropriate SQL version (is null or is
not null).

The value of null (nothing) in
collation is defined by SQL Server;
LINQ to SQL does not change the
collation.