In asp.net core, are empty string converted to NULL?

Question

Assuming I have a form, and the value of "" is selected when the form is sent from the view to the controller to the action method, will asp.net core convert the empty string to a NULL value?

If I do not make the Boolean property nullable for the [required] attribute, it gives me this error: The value '' is invalid.

Does this mean that: "" is evaluated as NULL, the Boolean property does not allow NULL, asp.net core returns a error saying that you can not pass a empty string to the Model property because it is not nullable because asp.net core converts the empty string to a NULL?

user_4139809 · Answer

MVC model binding does support binding an empty string as either a null or empty string, depending on metadata.

You can control that behaviour per field with attributes;

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

Or override that behaviour globally by implementing a custom IDisplayMetadataProvider.

public class DisplayProvider : IDisplayMetadataProvider
{
    public void CreateDisplayMetadata(DisplayMetadataProviderContext context)
    {
        if (context.Key.ModelType == typeof(string))
            context.DisplayMetadata.ConvertEmptyStringToNull = false;
    }
}
// in .AddMvc(o => ...) / AddControllers(o => ...) / or an IConfigure service
[MvcOptions].ModelMetadataDetailsProviders.Add(new DisplayProvider());

Or convert values in any way you like by providing your own IModelBinder / IModelBinderProvider.

public class StringBindProvider : IModelBinderProvider
{
    private StringBinder stringBinder = new StringBinder();
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context.Metadata.ModelType == typeof(string))
            return stringBinder;
        return null;
    }
}
public class StringBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var value = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (value != ValueProviderResult.None)
        {
            bindingContext.ModelState.SetModelValue(bindingContext.ModelName, value);
            var str = value.FirstValue?.Trim();
            if (bindingContext.ModelMetadata.ConvertEmptyStringToNull && string.IsNullOrWhiteSpace(str))
                str = null;
            bindingContext.Result = ModelBindingResult.Success(str);
        }
        return Task.CompletedTask;
    }
}
// see above
[MvcOptions].ModelBinderProviders.Insert(0, new StringBindProvider());

user_12884742 · Answer

The first thing you need to understand is that the Options field here is the type of bool(not string).

The only content it can receive is true or false or null, whether you enter an empty string or a string other than true or false, it will recognized as null.

The attribute of Reuqired indicates that the Options field cannot be null,because Options is a bool type, so after you enter an empty string, the empty string is converted to a null value, and due to the restriction of the reuqired attribute, it cannot be null, so it will remind you invalid.

If you want to allow Options to receive null values, you only need to remove the reuqired attribute.

In the premise of the Required attribute limitation, I did a code test, you can refer to:

user_2294065 · Answer

No they are not - the message says "The value '' is invalid." which is how an empty string looks like.

user_9549856 · Answer

you need to set value for boolean if you don't want to set required field. You need to send false if it is empty.

In asp.net core, are empty string converted to NULL?

4 Answers