Asp.Net Core Model binding, how to get empty field to bind as a blank string?

Question

If a form like the one below is submitted and MyField is left blank on the form, then by default Asp.Net Core model binding will place null into the correspondi

Answer 1

Studying the Asp.Net Core code for the SimpleTypeModelBinder at https://github.com/aspnet/Mvc/blob/rel/1.1.3/src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/SimpleTypeModelBinder.cs I could see that there is a ModelMetadata.ConvertEmptyStringToNull setting which is set to true by default that is causing the blank string to be converted to null on data binding. But the property is read only so at first I couldn't figure out how to changes it's value.

@rsheptolut's post on this page https://github.com/aspnet/Mvc/issues/4988 led me to a solution.

Solution:

The value has to get set at startup. This can be done via this class:

       public class CustomMetadataProvider : IMetadataDetailsProvider, IDisplayMetadataProvider {
            public void CreateDisplayMetadata(DisplayMetadataProviderContext context) {

                if (context.Key.MetadataKind == ModelMetadataKind.Property) {

                    context.DisplayMetadata.ConvertEmptyStringToNull = false;
                }
            }
        }

when it's hooked int into MvcOptions in the ConfigureServices method of the startup.cs file like so

 services.AddMvc()
        .AddMvcOptions(options => options.ModelMetadataDetailsProviders.Add(new CustomMetadataProvider ())); 

Now site wide, the default for a blank field that is posted back will be for the data binder to set the corresponding model property to a blank string rather than to null. Yea!

Answer 2

Have you tried making the property have a default value of empty string?

public string MyField { get; set; } = string.Empty;

an uglier solution to try is:

private string myField  = string.Empty;
public string MyField 
{
    get { return myField ?? string.Empty; }
    set { myField = value; }
}

I think it should work