Model state validation filter ASP.NET Core

ModelStateValidationFilter.cs

using System;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;

namespace Example
{
    public class ModelStateValidationFilter : Attribute, IActionFilter
    {
        public void OnActionExecuting(ActionExecutingContext context)
        {
            if (!context.ModelState.IsValid) {
                context.Result = new BadRequestObjectResult(context.ModelState);
            }
        }

        public void OnActionExecuted(ActionExecutedContext context) {}
    }
}

I hereby place this to public domain.

You can inherit from ActionFilterAttribute, override OnActionExecuting, and then you can delete the empty method of OnActionExecuted

Just for understanding, why it was not included in the Asp.Net core? It's so obvious implementation.

public class ModelStateValidationFilter : ActionFilterAttribute
{
     public override void OnActionExecuting(ActionExecutingContext context)
     {
         if (!context.ModelState.IsValid)
         {
             List<string> list = (from modelState in context.ModelState.Values from error in modelState.Errors select error.ErrorMessage).ToList();
             context.Result = new BadRequestObjectResult(list);
         }

         base.OnActionExecuting(context);
     }
}

public class ModelStateValidationFilter : ActionFilterAttribute
  {
    public override void OnActionExecuting(ActionExecutingContext context)
    {
      if (!context.ModelState.IsValid)
      {
        List<string> list = (from modelState in context.ModelState.Values from error in modelState.Errors select error.ErrorMessage).ToList();

        //Also add exceptions.
        list.AddRange(from modelState in context.ModelState.Values from error in modelState.Errors select error.Exception.ToString());

        context.Result = new BadRequestObjectResult(list);
      }

      base.OnActionExecuting(context);
    }
  }

What if I want to return the same view for which the request has been made and pass the modelstate errors along. ??

@waqaskhan540 I have same question as you, did you solved?

@lousaibiao try this:

public override void OnActionExecuting(ActionExecutingContext context)
{
   if (!context.ModelState.IsValid)
   {
      var controller = context.Controller as Controller;
      var model = context.ActionArguments?.Count > 0
         ? context.ActionArguments.First().Value
         : null;
      context.Result = (IActionResult)controller?.View(model)
         ?? new BadRequestResult();
   }
   
   base.OnActionExecuting(context);
}

Alternatively, you can invoke the action directly:

      public override void OnActionExecuting(ActionExecutingContext context)
      {
         if (!context.ModelState.IsValid)
         {
            var action = context.ActionDescriptor as ControllerActionDescriptor;
            var controller = context.Controller as Controller;
            context.Result = (IActionResult)action?.ControllerTypeInfo.InvokeMember(
               action.ActionName,
               BindingFlags.InvokeMethod | BindingFlags.Public | BindingFlags.Instance,
               null,
               controller,
               null)
               ?? new BadRequestResult();
         }

         base.OnActionExecuting(context);
      }