GetAllErrorMessages

using System.Text; using System.Web.Http.ModelBinding; public static class MyExtensionMethods { public static string GetAllErrorMessages(this ModelStateDictionary modelState, string separator = " ") { if (separator == null) { separator = " "; } StringBuilder sbErrors = new StringBuilder(); foreach (ModelState modelStateValues in modelState.Values) { foreach (ModelError error in modelStateValues.Errors) { if (string.IsNullOrWhiteSpace(error.ErrorMessage)) { if (error.Exception != null) { sbErrors.Append(error.Exception.Message); sbErrors.Append(separator); } } else { sbErrors.Append(error.ErrorMessage); sbErrors.Append(separator); } } } if (sbErrors.Length > separator.Length) { sbErrors.Remove(sbErrors.Length - separator.Length, separator.Length); } string errorMessage = sbErrors.ToString(); if (string.IsNullOrWhiteSpace(errorMessage)) { errorMessage = string.Empty; } return errorMessage; } } using System.Web.Http; public class ExampleController : ApiController { [HttpPost] public IHttpActionResult Usage([FromBody]YourClass body) { if (!this.ModelState.IsValid) { return this.BadRequest(this.ModelState.GetAllErrorMessages()); } } }