How to differentiate between null and non existing data in JSON in Asp.Net Core model binding?

Question

I want to differentiate between these two json inputs in an action in Asp.Net Core:

{
  "field1": null,
  "field2": null
}

and

{
  "field1": null,
}

I have an ordinary class like this in C#:

public class MyData
{
   public string Field1 { get; set;}
   public string Field2 { get; set;}
}

I want to run a partial update of an object that can accept null as the value, but when the field will not be in the input it means I don't want to update this field at all (something else from setting it to null).

Answer 1

This is what I ended up doing, as all other options seem to be too complicated (e.g. jsonpatch, model binding) or would not give the flexibility I want.

This solution means there is a bit of a boilerplate to write for each property, but not too much:

public class UpdateRequest : PatchRequest
{
    [MaxLength(80)]
    [NotNullOrWhiteSpaceIfSet]
    public string Name
    {
       get => _name;
       set { _name = value; SetHasProperty(nameof(Name)); }
    }  
}

public abstract class PatchRequest
{
    private readonly HashSet<string> _properties = new HashSet<string>();

    public bool HasProperty(string propertyName) => _properties.Contains(propertyName);

    protected void SetHasProperty(string propertyName) => _properties.Add(propertyName);
}

The value can then be read like this:

if (request.HasProperty(nameof(request.Name)) { /* do something with request.Name */ }

and this is how it can be validated with a custom attribute:

var patchRequest = (PatchRequest) validationContext.ObjectInstance;
if (patchRequest.HasProperty(validationContext.MemberName) {/* do validation*/}