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?

Answer 1

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<MvcOptions> 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());