c# - 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.