Ajax passing empty value but Controller get null in ASP.NET MVC

I decided summary from @Rahul Sharma's and @rhytonix's answers along with giving you examples and more detailed explanations.

1. Why is it that when in Ajax I am sending empty, I get null value in my controller?

This is simply because MVC 2.0 defaults to initializing strings to null. To be more precise, if an empty string means has no value, So .NET sets the default value of its. And the default string (belonging to reference type) is null.

More details in Model String Property Binding Breaking Change

2. Is there a better way and more elegant to resolve my problem like below?

There are some ways to bind String property as string.Empty instead of null

1. From C# 6, You can use DefaultValueAttribute to have auto-property an initial value like below

public string LastName => string.Empty; 

Basically, This way is the same as the OP's solution mentioned in the post, Just more elegant.

2. Custom default implementation of IModelBinder by inheriting from DefaultModelBinder and changing the ConvertEmptyStringToNull value to false on the internal ModelMetaData object.

public sealed class EmptyStringModelBinder : DefaultModelBinder 
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        bindingContext.ModelMetadata.ConvertEmptyStringToNull = false;
        return base.BindModel(controllerContext, bindingContext);
    }
}

Then in Application_Start() method of Global.asax.cs you need to do like below to complete

protected void Application_Start()
{
    ModelBinders.Binders.DefaultBinder = new EmptyStringModelBinder();
    RegisterRoutes( RouteTable.Routes );
}

3. Use DisplayFormatAttribute.ConvertEmptyStringToNull Property like below

[DisplayFormat(ConvertEmptyStringToNull = false)]
public string LastName { get; set; }

Simply because in ModelMetadata

true if empty string values are automatically converted to null; otherwise, false. The default is true

Why is it that when in Ajax I am sending empty, I get null value in my controller?

string is a reference type, and its default value is null. The ModelBinder sets the properties to their default value if no value is provided in the request.

Is there a better way and more elegant to resolve my problem like below?

1. You can annotate the property with [DisplayFormat(ConvertEmptyStringToNull = false)], so the empty string value is preserved.

2. You can write a custom ModelBinder that sets ConvertEmptyStringToNull to false, and apply it globally.

public class NullStringModelBinder : DefaultModelBinder {
    public override object BindModel(ControllerContext controllerContext,
                                     ModelBindingContext bindingContext) {
        bindingContext.ModelMetadata.ConvertEmptyStringToNull = false;
        return base.BindModel(controllerContext, bindingContext);
    }
}

//register it in Application_Start()
ModelBinders.Binders.Add(typeof(string), new NullStringModelBinder());

This particular change has been documented here and it is one of the breaking changes from MVC 1.0. This logic of binding empty string to nulls is controlled with the ModelMetadata.ConvertEmptyStringToNull property which is used by the DefaultModelBinder.

Now if you do not want to annotate all your properties, you can create a custom model binder:

public class EmptyStringModelBinder : DefaultModelBinder 
{
    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        bindingContext.ModelMetadata.ConvertEmptyStringToNull = false;
        Binders = new ModelBinderDictionary() { DefaultBinder = this };
        return base.BindModel(controllerContext, bindingContext);
    }
}

And set it in your Global.asax:

ModelBinders.Binders.DefaultBinder = new EmptyStringModelBinder();

Or in your specific action:

[HttpPost]
public JsonResult ActionSubmit([ModelBinder(typeof(EmptyStringModelBinder))SampleViewModel model)

Why was this done?

This was done because the default value of a string is null and because string is a reference type and the default value for all reference types is null. Hence, this change of the framework might be reasonable. But on the other hand, we should try to avoid null values at all, therefore we need to write a custom model binder to avoid such cases.

There is a question on why the default value of the string type null instead of an empty string?. You can go over this to understand more about why this change was done.

As per @Anton: In c# 8.0 you can turn on null checks to avoid NullReferenceException and set to reference types default values instead of null