How to return string empty if value is null [duplicate]

Question

I want to return a empty string when the SomethingName is null. plants.Something.Name != null ? plants.Something.Name : string.Empty still gets me a null pointer. I have debugged my application to know this is causing a NullPointerException.

When I set data in the database for SomethingName, I dont get a null pointer exception.

Is there a better way to handle this?

    public FlowerMetaData GetLeafByFlowerId(int flowerId, string flowerName)
    {
        _flowerContext = _contextUtility.GetFlowerContext(flowerName);

        var flowerData = (from flowers in _flowerContext.Flowers
                            where flowers.FlowerId == flowerId
                            join plants in _flowerContext.Plants on flowers.PlantId equals plants.PlantId
                            join leafs in _flowerContext.Leafs on flowers.LeafAk equals leafs.LeafAK
                            select new FlowerMetaData
                            {
                                PlantId = plants.PlantId,
                                PlantName = plants.PlantName,
                                FlowerName = FlowerName.ToUpper(),
                                FlowerNumber = leafs.FlowerNumber,
                                SomethingName = plants.Something.Name != null ? plants.Something.Name : string.Empty,
                                CreatedId = plants.CreatedId,
                            }).FirstOrDefault();

        return flowerData;
    }

StackTrace

    "Message": "An error has occurred.",
    "ExceptionMessage": "No flower found with id = 37.",
    "ExceptionType": "System.NullReferenceException",
    "StackTrace": "   at Flower.Services.FlowerService.GetLeafByFlowerId(Int32 flowerId, String flowerName)
at Flower.Controllers.Controllers.FlowerController.GetFlower(Int32 id, String client)\r\n   at lambda_method(Closure , Object , Object[] )
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c__DisplayClass10.<GetExecutor>b__9(Object instance, Object[] methodParameters) at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.ActionFilterResult.<ExecuteAsync>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
  at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Filters.AuthorizationFilterAttribute.<ExecuteAuthorizationFilterAsyncCore>d__2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
  at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
  at Sentinel.Services.AuthFilters.AddChallengeOnUnauthorizedResult.<ExecuteAsync>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
  at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Controllers.AuthenticationFilterResult.<ExecuteAsync>d__0.MoveNext()\r\n--- End of stack trace from previous location where exception was thrown ---\r\n   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\n   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()"
}

@PeterSmith But not with the way you identity the source of NRE. — Ivan Stoev
– Ivan Stoev, Commented Apr 4, 2018 at 18:29
If you could provide the stack trace you're seeing, that would help a lot in determining whether this is a "normal" NRE or not. — Jon Skeet
– Jon Skeet, Commented Apr 4, 2018 at 18:30
@DaisyShipton, I added the stack trace, not sure it it will help, but thank you. — Maddy
– Maddy, Commented Apr 4, 2018 at 18:58

Jon Skeet · Accepted Answer · 2018-04-04 18:28:30Z

4

Usually you can use the null-conditional operator from C# 6, combined with the null-coalescing operator from C# 2 to make this really easy:

SomethingName = plants.Something?.Name ?? ""

The expression plants.Something?.Name will evaluate to null (rather than throwing an exception) if plants.Something evaluates to null. The ?? "" part then just says "If the result of the expression is null, use the empty string instead."

If plants itself could be null, you could use:

SomethingName = plants?.Something?.Name ?? ""

... but in this case you shouldn't need to.

However, there's a wrinkle: if you're doing this in an LINQ provider that uses IQueryable<T> (rather than IEnumerable<T> as LINQ to Objects usually does), each part of your query expression will be converted into an expression tree, and the null-conditional operator isn't supported in expression trees. As such, you may need to perform a projection in the query, then do the last part in-process. For example:

var entry = (from flowers in _flowerContext.Flowers
             where flowers.FlowerId == flowerId
             join plants in _flowerContext.Plants on flowers.PlantId equals plants.PlantId
             join leafs in _flowerContext.Leafs on flowers.LeafAk equals leafs.LeafAK
             select new { plants, leafs }).FirstOrDefault();
return entry == null 
    ? null // No entry in the database
    : new FlowerMetaData
      {
          PlantId = entry.plants.PlantId,
          PlantName = entry.plants.PlantName,
          FlowerName = FlowerName.ToUpper(),
          FlowerNumber = entry.leafs.FlowerNumber,
          SomethingName = entry.plants.Something?.Name ?? "",
          CreatedId = entry.plants.CreatedId,
       };

edited Apr 4, 2018 at 18:28

answered Apr 4, 2018 at 17:41

Jon Skeet

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17 Comments

Maddy Over a year ago

what can I do if plant or something is not nullable.

Jon Skeet Over a year ago

@Maddy: Then the compiler won't allow you to use ?., but you wouldn't get a NullReferenceException anyway for that part, because you couldn't be dereferencing a null value.

Ivan Stoev Over a year ago

?. operator cannot be used in expression trees though.

Jon Skeet Over a year ago

@IvanStoev: You're absolutely right. Will edit.

Jon Skeet Over a year ago

@IvanStoev: Have a look now.

|

Peter Smith · Accepted Answer · 2018-04-04 17:40:09Z

1

Not tested but

SomethingName = plants.Something?.Name ?? string.empty

should do what's required. What Daisy said!

answered Apr 4, 2018 at 17:40

Peter Smith

5,5529 gold badges54 silver badges80 bronze badges

3 Comments

Maddy Over a year ago

What can I do if Something is not nullable?

Jon Skeet Over a year ago

@Maddy: Then Something.Name shouldn't throw a NullReferenceException.

Peter Smith Over a year ago

Daisy has it all covered!

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