Question

I’m using linq to filter a selection of MessageItems. The method I’ve written accepts a bunch of parameters that might be null. If they are null, the criteria for the file should be ignored. If it is not null, use it to filter the results.

It’s my understanding that when doing an || operation is C#, if the first expression is true, the second expression should not be evaluated.

e.g.

if(ExpressionOne() || ExpressionTwo())
{
     // only ExpressionOne was evaluated because it was true
}

now, in linq, I’m trying this:

var messages = (from msg in dc.MessageItems
where  String.IsNullOrEmpty(fromname) || (!String.IsNullOrEmpty(fromname) && msg.FromName.ToLower().Contains(fromname.ToLower()))
select msg);

I would have thought this would be sound, because String.IsNullOrEmpty(fromname) would equal true and the second part of the || wouldn’t get run.

However it does get run, and the second part

msg.FromName.ToLower().Contains(fromname.ToLower()))

throws a null reference exception (because fromname is null)!! – I get a classic “Object reference not set to an instance of an object” exception.

Any help?

Answer 1

Have a read of this documentation which explains how linq and c# can experience a disconnect.

Since Linq expressions are expected to be reduced to something other than plain methods you may find that this code breaks if later it is used in some non Linq to Objects context.

That said

String.IsNullOrEmpty(fromname) || 
(   !String.IsNullOrEmpty(fromname) && 
    msg.FromName.ToLower().Contains(fromname.ToLower())
)

Is badly formed since it should really be

String.IsNullOrEmpty(fromname) || 
msg.FromName.ToLower().Contains(fromname.ToLower())

which makes it nice and clear that you are relying on msg and msg.FromName to both be non null as well.

To make your life easier in c# you could add the following string extension method

public static class ExtensionMethods
{
    public static bool Contains(
        this string self, string value, StringComparison comparison)
    {
        return self.IndexOf(value, comparison) >= 0;
    }

    public static bool ContainsOrNull(
        this string self, string value, StringComparison comparison)
    {
        if (value == null)
            return false;
        return self.IndexOf(value, comparison) >= 0;
    }
}

Then use:

var messages = (from msg in dc.MessageItems
where  msg.FromName.ContainsOrNull(
    fromname, StringComparison.InvariantCultureIgnoreCase)
select msg);

However this is not the problem. The problem is that the Linq to SQL aspects of the system are trying to use the fromname value to construct the query which is sent to the server.

Since fromname is a variable the translation mechanism goes off and does what is asked of it (producing a lower case representation of fromname even if it is null, which triggers the exception).

in this case you can either do what you have already discovered: keep the query as is but make sure you can always create a non null fromname value with the desired behaviour even if it is null.

Perhaps better would be:

IEnumerable<MessageItem> results;
if (string.IsNullOrEmpty(fromname))
{ 
    results = from msg in dc.MessageItems 
    select msg;    
}
else
{
    results = from msg in dc.MessageItems 
    where msg.FromName.ToLower().Contains(fromname) 
    select msg;    
}

This is not so great it the query contained other constraints and thus invovled more duplication but for the simple query actually should result in more readable/maintainable code. This is a pain if you are relying on anonymous types though but hopefully this is not an issue for you.