web api action parameter convert empty string parameter to null

Question

I have a some web Api actions with a lot of string parameter. for some of these parameters , client sends empty string instead of null but I need to save null in database in case of empty string. I tried with model binder and JSONconvertor but failed.

FYI; I need a generic solution as I don't want check parameter inside the method body and replace them with null.

I have answered something similar to this for the question in stackoverflow.com/questions/28430031/…. Instead of creating a new instance, you can find only the empty string parameters (if the parameter type is string) and replace them with null in the action arguments dictionary. — Parthasarathy
– Parthasarathy, Commented Feb 11, 2015 at 12:14
Thanks mate. your solution also works but I developed a custom model binder. see below. Thanks again — behtash
– behtash, Commented Feb 12, 2015 at 1:37

Justin · Accepted Answer · 2015-02-10 23:54:40Z

4

You can use the DisplayFormat attribute on your string properties to automatically convert empty strings to null.

[DisplayFormat(ConvertEmptyStringToNull = true)]
public string MyString { get; set; }

answered Feb 10, 2015 at 23:54

Justin

694 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

julealgon Over a year ago

I tested this right before giving you a downvote. If you can point to a working example I'll gladly revert it to a upvote. I needed this exact functionality and wasn't able to make it work.

JARRRRG Over a year ago

I've just gotten this to work, you must put this on your entity framework poco (in my case Code First database entity). My view model parsed it in as an empty string "".. With this annotation when it came to try persist it to the database it changed it to null. Thank you Justin.

behtash · Accepted Answer · 2015-02-12 01:36:21Z

Thanks Sarathy , your solution may also work but I ended with following solution: 1)Creating custom model binder like following

 public class EmptyStringModelBinder : System.Web.Mvc.IModelBinder
    {
        public object BindModel(System.Web.Mvc.ControllerContext controllerContext, System.Web.Mvc.ModelBindingContext bindingContext)
        {
            string key = bindingContext.ModelName;
            ValueProviderResult val = bindingContext.ValueProvider.GetValue(key);
            if (val != null)
            {
                var s = val.AttemptedValue as string;
                if (s != null && (s.IsEmpty() || s.Trim().IsEmpty()))
                {
                    return null;
                }

                return val.AttemptedValue;
            }

            return null;
        }
    }

2)Mark action method parameter with ModelBinder attribute

public ActionResult UpdateAttribute(int id, 
                                     int AttributeTypeId,
                                     int? Number_Value, 
                                     decimal? Money_Value,
                                            [ModelBinder(typeof(EmptyStringModelBinder))]string Text_Value)

or you could add this model binder at configuration. it will inspect all string parameters and replace empty string with null(maybe not desired)

Sean · Accepted Answer · 2016-09-23 05:35:10Z

3

If you use the [Required] attribute on the string property, but make it nullable in the database, WebApi will convert the empty string it receives in the Json to a null value.

My requirement was for the opposite. I have a non-nullable field in the database, and wanted to save the empty string to the database. For this requirement, I had to remove the [Required] attribute and add [DisplayFormat(ConvertEmptyStringToNull = false)]

Then the empty strings in the JSON were persisted to the database.

answered Sep 23, 2016 at 5:35

Sean

15.2k17 gold badges83 silver badges139 bronze badges

1 Comment

crucifery Over a year ago

+1. It is handy for validation tasks with WebAPI where it is usually required to have everything in place before a check ModelState.IsValid. I had the identical case like Sean's requirement. But! there were some thousand models with up to 150 properties. Instead of doing work with DisplayFormat for each property, I came to such solution: link

Collectives™ on Stack Overflow

web api action parameter convert empty string parameter to null

3 Answers 3

2 Comments

Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related