将空参数传递给LINQ where子句

Question

I have a table where some items may be null. I can query then easily by using a query like so:

db.SomeTable.Where(s => s.SomeCol == null)

Simple enough, but this does not work (No results. I suspect it is searching for an empty string instead; "") when using a variable that happens to be null, like so:

string variable = null;
db.SomeTable.Where(s => s.SomeCol == variable)

Do I have to do something special to get this to work?

Answer 1

Using LinqPad you can see the difference.

The former creates a query like:

select ...
from SomeTable as t0
where t0.SomeCol IS NULL

whereas the latter is

select ...
from SomeTable as t0
where t0.SomeCol = @p0

Instead you can use object.Equals in your test. Eg,

string test = null;
var q = from c in SomeTable where object.Equals(c.SomeCol, test) select c;

This will generate the appropriate where clause based on the value of the variable used in the condition.