How can I check for a null inside of a linq query?

Question

I have the following code:

protected IEnumerable<string> GetErrorsFromModelState() {

    var exceptions = ModelState.SelectMany(x => x.Value.Errors
        .Select(error => error.Exception.Message));

    var errors = ModelState.SelectMany(x => x.Value.Errors
        .Select(error => error.ErrorMessage));

    return exceptions.Union(errors);
}

Is there a way that I could stop this giving a nullReference Exception if:

error.Exception is null  or if error.Exception.Message is null

Both cases are giving me problems and I'm not sure how I can code this with a IsNullOrEmpty check for both cases.

Nikhil Agrawal · Accepted Answer · 2012-09-01 09:40:56Z

2

How about

protected IEnumerable<string> GetErrorsFromModelState()
{        
    var exceptions = ModelState.SelectMany(x => x.Value.Errors
                            .Where(error => (error != null) && 
                                            (error.Exception != null) &&
                                            (error.Exception.Message != null))
                            .Select(error => error.Exception.Message));

    var errors = ModelState.SelectMany(x => x.Value.Errors
                            .Where(error => (error != null) && 
                                            (error.ErrorMessage != null))
                            .Select(error => error.ErrorMessage));

    return exceptions.Union(errors);
}

edited Sep 1, 2012 at 9:40

answered Sep 1, 2012 at 9:16

Nikhil Agrawal

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2 Comments

Angela Over a year ago

Thanks. this works good. Couple of spelling errors with Exeception :-)

Jon Hanna Over a year ago

I would leave out the Exception.Message check. If an exception somewhere is having its message set to null, I want to know about that bug so I can track it down at source.

George Duckett · Accepted Answer · 2012-09-01 09:12:05Z

2

Add in a .Where(error.Exception != null && error.Exception.Message != null) before your select to only include ones where the values you want are not null.

answered Sep 1, 2012 at 9:12

George Duckett

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1 Comment

George Duckett Over a year ago

Then add that to I, before what I answered with

Saeed Amiri · Accepted Answer · 2012-09-01 09:41:32Z

You could use May be Monad, First write an static class and put With extension method in that class, then simply use With method. In the link there are some other similar types which are also helpful.

public static TResult With<TInput, TResult>(this TInput o, 
       Func<TInput, TResult> evaluator)
       where TResult : class where TInput : class
{
  if (o == null) return null;
  return evaluator(o);
}

And use it simply:

var exceptions = ModelState.SelectMany(x => x.With(y=>y.Value).With(z=>z.Errors))
        .Select(error => error.With(e=>e.Exception).With(m=>m.Message));

Update: for making more clear (similar sample also exists in link), assume you have Person class hierarchy:

public class Person
{
   public Address Adress{get;set;}
}

public class Address
{
   public string PostCode{get;set;}
}

Now you want to get person's related post code and you don't know input person is null or not:

var postCode = 
// this gives address or null, if either person is null or its address
person.With(x=>x.Address) 
// this gives post code or returns null, 
// if previous value in chain is null or post code is null 
      .With(x=>x.PostCode);

Jon Hanna · Accepted Answer · 2012-09-01 09:17:42Z

1

To filter out the nulls, then .Where(error => error.Exception != null) (the Exception object should always have a non-null message, so I'd count it as a bug to detect and fix there if there's an exception).

To behaved differently in such a case, then e.g. .Select(error => error.Exception == null ? "No Error" : error.Exception.Message).

answered Sep 1, 2012 at 9:17

Jon Hanna

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