Why do I get null instead of empty string when receiving POST request in from Razor View?

Question

I used to receive empty string when there was no value:

[HttpPost]
public ActionResult Add(string text)
{
    // text is "" when there's no value provided by user
}

But now I'm passing a model

[HttpPost]
public ActionResult Add(SomeModel Model)
{
    // model.Text is null when there's no value provided by user
}

So I have to use the ?? "" operator.

Why is this happening?

Merta

An alternative solution to using attributes on each model property, as described below, is using a custom model binder, see #12734583 – Pioneer

Answer 1

You can use the DisplayFormat attribute on the property of your model class:

[DisplayFormat(ConvertEmptyStringToNull = false)]

Artful

This helped me with implicit model binding before my action was executed. Used it along with MetadataTypeAttribute to mark auto-generated property of Entity Framework Model-First class – Conscientious

This is definitively a Should Know. – Coward

I am using: public ActionResult Quotations(string projectName, string brandName, string modelName, string clientName) { var model = _dataAccess.Quotations_Select(projectName, brandName, modelName, clientName); return View(model); } calling a stored procedure and supplying values, so, the attribute will not work for me. What should I do? – Ethylethylate

It's a long road, but also, in MVC6, if you (like me) for some reason don't want to set this via an attribute and want to store your metadata elsewhere, you can write your own metadata provider and register it in Startup.cs like: AddMvc(o => ModelMetadataDetailsProviders.Add(new YourProvider())). There in GetDisplayMetadata you can do everying that attributes do. For this question it'll be context.DisplayMetadata.ConvertEmptyStringToNull = false; – Xenophanes

This conflicted with my other validation I had on the property. I ended up creating a property with a backing field where the get returns empty string if the backing field is null. – Refugia

Answer 2

The default model binding will create a new SomeModel for you. The default value for the string type is null since it's a reference type, so it's being set to null.

Is this a use case for the string.IsNullOrEmpty() method?

Night

Yeah, that's what I thought. I didn't realize that default value for the string type is null, not String.Empty though. No, it's not the case for the IsNullOrEmpty method, I have NOT NULL columns in my SQL table, so I get an exception when there'are null strings. So I guess I'll have to continue to use the ?? "" operator. Thanks! – Merta

You might also use a property with a backing store and return string.empty instead of null from the ViewModel, eg: – Night

woops! here's some psuedocode: private string _text; public string Text { get { return _text; } set { _text = value ?? string.empty; }} -- something like that – Night

Answer 3

I am trying this in Create and Edit (my object is called 'entity'):-

        if (ModelState.IsValid)
        {
            RemoveStringNull(entity);
            db.Entity.Add(entity);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        return View(entity);
    }

Which calls this:-

    private void RemoveStringNull(object entity)
    {
        Type type = entity.GetType();
        FieldInfo[] fieldInfos = type.GetFields(BindingFlags.Instance | BindingFlags.GetField | BindingFlags.Public | BindingFlags.NonPublic);
        for (int j = 0; j < fieldInfos.Length; j++)
        {
            FieldInfo propertyInfo = fieldInfos[j];
            if (propertyInfo.FieldType.Name == "String" )
            {
                object obj = propertyInfo.GetValue(entity);
                if(obj==null)
                    propertyInfo.SetValue(entity, "");
            }
        }
    }

It will be useful if you use Database First and your Model attributes get wiped out each time, or other solutions fail.

Yuu