ASP.NET Core: Prevent Automatic HTTP 400 responses for individual action

Update: you can just use the following code in ConfigureServices in Startup.cs:

services.Configure<ApiBehaviorOptions>(apiBehaviorOptions => {
    apiBehaviorOptions.SuppressModelStateInvalidFilter = true;
});

---

Based on Simon Vane's answer, I had to modify the attribute for ASP.Net Core 2.2 as follows:

/// <summary>
/// Suppresses the default ApiController behaviour of automatically creating error 400 responses
/// </summary>
[AttributeUsage(AttributeTargets.Method)]
public class SuppressModelStateInvalidFilterAttribute : Attribute, IActionModelConvention {
    private static readonly Type ModelStateInvalidFilterFactory = typeof(ModelStateInvalidFilter).Assembly.GetType("Microsoft.AspNetCore.Mvc.Infrastructure.ModelStateInvalidFilterFactory");

    public void Apply(ActionModel action) {
        for (var i = 0; i < action.Filters.Count; i++) {
            if (action.Filters[i] is ModelStateInvalidFilter || action.Filters[i].GetType() == ModelStateInvalidFilterFactory) {
                action.Filters.RemoveAt(i);
                break;
            }
        }
    }
}

You could play with ApiBehaviorOptions.InvalidModelStateResponseFactory property to handle specific cases based on actionContext details:

services.Configure<ApiBehaviorOptions>(options =>
{
    options.InvalidModelStateResponseFactory = actionContext => 
    {
        // Do what you need here for specific cases with `actionContext` 
        // I believe you can cehck the action attributes 
        // if you'd like to make mark / handle specific cases by action attributes. 

        return new BadRequestObjectResult(context.ModelState);
    }
});

I had a response from Microsoft - https://github.com/aspnet/Mvc/issues/8575

The following worked a charm.

[AttributeUsage(AttributeTargets.Method)]
public class SuppressModelStateInvalidFilterAttribute : Attribute, IActionModelConvention
{
    public void Apply(ActionModel action)
    {
        for (var i = 0; i < action.Filters.Count; i++)
        {
            if (action.Filters[i] is ModelStateInvalidFilter)
            {
                action.Filters.RemoveAt(i);
                break;
            }
        }
    }
}

In my controller I could then make changes to the model before re-validating it (note the ModelState.Clear(), TryValidateModel add to existing model state):

if (model == null)
{
    return BadRequest(ModelState);
}

model.Property = valueFromPath;

ModelState.Clear();
if (TryValidateModel(model) == false)
{
    return BadRequest(ModelState);
}