Hello, Given I don't know if the parameter will be null or not and I want to use it in the following way: if Param != null then compare with its id if Param == null then compare with null.

Something like that:

var c = from cat in context.Categories
where ParamCat != null && cat.ParentId == ParamCat.Id 
||
ParamCat == null && cat.ParentId == null
select c;

If ParamCat is null as soon as I try to get something from c (for example c.Count()) it throws the exception.

Usually when we use some kind of condition it stop comparison as soon as condition fail especially if we use AND. For example this code will not cause any exception:

if (ParamCat != null &&  cat.ParentId == RaramCat.Id)
{
}

If so, why the linq code above throw exception? (Null reference)

Thanks