Question

I’m having problems with the following Linq query (demandNb is a filter option, as a string):

Return (From auto In entity.DEMDT_AUTMT _
    Where (String.IsNullOrEmpty(demandNb) OrElse auto.DEMND_NB = demandNb) _
    Order By auto.NO_DEMND Descending _
    Select auto).ToList()

My goal is to get all demands for the database if the filter fields is empty. If there is a filter, the function should return all demands that match the filter.

Currently, when I try to convert the IQueryable collection to a list using .ToList(), the following error is thrown:

Conversion from string "" to type 'Double' is not valid.

The problem is, when demandNb is equal to “”, this instruction auto.DEMND_NB = demandNb is still executed, throwing a conversion error.

When I remove the part after OrElse, everything works fine (although this is not the behavior I want, I want the filter to work afterall!). I have also tried to put the result in a variable (which works), and then convert it to a list, and the error is only thrown when .ToList() is called. Wierd…

Is there something I am not seeing, and why is not OrElse working as intended?

Edit:

I will be implementing several more filter fields like this (like demand owner, demand date, etc), so I cannot verify the if the filter is empty before executing the query

Answer 1

If I had to guess, I’d say auto.DEMND_NB is of type Double, and demandNb is obviously a string. This has nothing to do with the OrElse: you’re just trying to compare two values of non-compatible types.

Since the value of demandNb is not tied to each row in the data, I’d suggest doing something like this (pardon the C#):

var autos = entity.DEMDT_AUTMT;
if(!string.IsNullOrEmpty(demandNb))
{
    var demandNbDouble = double.Parse(demandNb);
    autos = autos.Where(auto => auto.DEMND_NB == demandNbDouble);
}
autos = autos.OrderBy(...);