Why does the ASP.Net MVC model binder bind an empty JSON array to null?

Question

Here is my model class:

public class MyModel {     public Employees[] MyEmpls{get;set;}     public int Id{get;set;}     public OrgName{get;set;} } 

Passing the below JSON structure object with MyEmpls as empty array to MVC controller.

["Id":12, "MyEmpls":[], "OrgName":"Kekran Mcran"] 

Controller

[HttpPost] public ActionResult SaveOrg(MyModel model) {   //model.MyEmpls is null here } 

I am expecting mode.MyEmpls to be an empty c# array, not a null. Is a custom model binder necessary to achieve an empty array?

Answer 1

I think that some of the other answers have missed the meaning of the question: why does the default MVC model binder bind an empty Json array to null instead of an empty C# array?

Well, I can't tell you why they did that, but I can show you where it happens. The source for MVC can be found on CodePlex here: http://aspnetwebstack.codeplex.com/SourceControl/latest. The file you're looking for is ValueProviderResult.cs where you can see:

    private static object UnwrapPossibleArrayType(CultureInfo culture, object value, Type destinationType)     {         if (value == null || destinationType.IsInstanceOfType(value))         {             return value;         }          // array conversion results in four cases, as below         Array valueAsArray = value as Array;         if (destinationType.IsArray)         {             Type destinationElementType = destinationType.GetElementType();             if (valueAsArray != null)             {                 // case 1: both destination + source type are arrays, so convert each element                 IList converted = Array.CreateInstance(destinationElementType, valueAsArray.Length);                 for (int i = 0; i < valueAsArray.Length; i++)                 {                     converted[i] = ConvertSimpleType(culture, valueAsArray.GetValue(i), destinationElementType);                 }                 return converted;             }             else             {                 // case 2: destination type is array but source is single element, so wrap element in array + convert                 object element = ConvertSimpleType(culture, value, destinationElementType);                 IList converted = Array.CreateInstance(destinationElementType, 1);                 converted[0] = element;                 return converted;             }         }         else if (valueAsArray != null)         {             // case 3: destination type is single element but source is array, so extract first element + convert             if (valueAsArray.Length > 0)             {                 value = valueAsArray.GetValue(0);                 return ConvertSimpleType(culture, value, destinationType);             }             else             {                 // case 3(a): source is empty array, so can't perform conversion                 return null;             }         }         // case 4: both destination + source type are single elements, so convert         return ConvertSimpleType(culture, value, destinationType);     } } 

The interesting part is "case 3":

else {     // case 3(a): source is empty array, so can't perform conversion     return null; } 

You can sidestep this issue by initialising your array on the model in its constructor. In my quick reading of the source I can't tell you why they can't return an empty array or why they decide not to, but it should make for interesting reading.

Answer 2

You're getting a null value as this is the default value for a reference type in C#. In order to get an empty array you will need to initialise the array in your model using a constructor. However as you will need to define the size of the array when it's initialized it might be better using another type of collection such as a List:

public class MyModel {     public List<Employees> MyEmpls{get;set;}     public int Id{get;set;}     public OrgName{get;set;}      public MyModel()      {          MyEmpls = new List<Employees>();     } } 

You will then get an empty list when an empty array is passed from the json.

If you really have to use an array just initialise it with a size:

public class MyModel {     public Employees[] MyEmpls{get;set;}     public int Id{get;set;}     public OrgName{get;set;}      public MyModel()      {          MyEmpls = new Employees[/*enter size of array in here*/];     } }