Linq where column == (null reference) not the same as column == null

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.

Another option to solve this, as I ran across this problem as well.

where (tradeId == null ? s.crmc_Retail_Trade_Id == null : s.crmc_Retail_Trade_Id == tradeId)