Linq Query Help - Detecting Null Values

Question

I have the following Linq query:

(from container in Container
join containerType in ContainerType on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container).Max (row => Convert.ToInt64(row.SerialNumber))

This query works great as long as at least one Container row meets the criteria. If no rows meet the criteria, I get the following error:

The null value cannot be assigned to a member with type System.Int64 which is a non-nullable value type

Is there a way I can rewrite this so that if no rows satisfy the query an arbitrary value gets returned, say a -1?

frabiacca · Accepted Answer · 2010-06-30 15:57:44Z

2

you can store first part of your query in a list and then do the Max on the list. Something similar to:

var query = (from container in Container
join containerType in ContainerType on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container);

if(query != null)
int64 maxvalue = query.Max (row => Convert.ToInt64(row.SerialNumber))

(i coded it on the fly, pls check it)

answered Jun 30, 2010 at 15:57

frabiacca

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2 Comments

Tahbaza Over a year ago

you might have to add a .SingleOrDefault() on the end of the first query, like so: select container).SingleOrDefault();

Toby Over a year ago

query wouldn't be null, it would be empty.

Toby · Accepted Answer · 2010-06-30 16:12:15Z

1

This will give you a -1 if there are no results:

(
from container in Container
join containerType in ContainerType
    on container.ContainerType equals containerType
where containerType.ContainerTypeID == someIDValue
select container.SerialNumber as long?
).DefaultIfEmpty().Max(sn => sn ?? -1)

edited Jun 30, 2010 at 16:12

answered Jun 30, 2010 at 16:05

Toby

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