How to display ASP.NET Core Pages error messages thrown by the framework

Question

In my ASP.NET application, when a file upload exceeds the default allowed size limit by ASP.NET, instead of displaying an error message, the application shows a blank page. How can I configure my application to properly handle and display error messages, such as those related to the default file size limits, instead of showing a blank page?

I have no default error handling page.

Answer 1

Hi @AT,

If you want to handle the 500 error inside the asp.net core, I suggest you could use the UseExceptionHandler method, this method is used to show the custom error page when the exception thrown.

More details, you could refer to below codes:

1.Create a Error handler view inside the home controller:

        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
        public IActionResult Error()
        {
            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
        }

View:

@model ErrorViewModel
@{
    ViewData["Title"] = "Error";
}
<h1 class="text-danger">Error.</h1>
<h2 class="text-danger">An error occurred while processing your request.</h2>
@if (Model.ShowRequestId)
{
    <p>
        <strong>Request ID:</strong> <code>@Model.RequestId</code>
    </p>
}
<h3>Development Mode</h3>
<p>
    Swapping to <strong>Development</strong> environment will display more detailed information about the error that occurred.
</p>
<p>
    <strong>The Development environment shouldn't be enabled for deployed applications.</strong>
    It can result in displaying sensitive information from exceptions to end users.
    For local debugging, enable the <strong>Development</strong> environment by setting the <strong>ASPNETCORE_ENVIRONMENT</strong> environment variable to <strong>Development</strong>
    and restarting the app.
</p>

Error model:

    public class ErrorViewModel
    {
        public string? RequestId { get; set; }
        public bool ShowRequestId => !string.IsNullOrEmpty(RequestId);
    }

Modify then program.cs:

...
app.UseExceptionHandler("/Home/Error");
...

Answer 2

Please consider the validation attribute to validate the file size at the client side using HTML5 File API and JavaScript. Below is sample which includes a custom validator of ASP.NET Core 3.1 MVC used for validating the size and type of uploading file:

Model and Custom Validator

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.ComponentModel.DataAnnotations;
using System.Text.RegularExpressions;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using System.Globalization;
using Microsoft.AspNetCore.Mvc.ModelBinding.Validation;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc.Rendering;
 
namespace MvcCoreApp.Models
{
    // Model
    // See attribute [FileValidation(50000, "image/jpeg")] below.
    // file size is limited to 50000 and file type to "image/jpeg"
    public class FileUpload
    {
        [Required(ErrorMessage = "{0} is required.")]
        public string Name { get; set; }
 
        [Display(Name = "Upload file")]
        [Required(ErrorMessage = "{0} is required.")]
        [FileValidation(50000, "image/jpeg")]
        public IFormFile PostedFile { get; set; }
    }
 
    // Custom validator attribute
    [AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
    public class FileValidationAttribute : ValidationAttribute, 
                                           IClientModelValidator
    {
        private readonly long size;
        private readonly string type;
 
        public FileValidationAttribute(long size, string type)
        {
            this.size = size;
            this.type = type;
            this.ErrorMessage = "{0} must be less than {1} byte, {2}";
        }
 
        public override string FormatErrorMessage(string displayName)
        {
            return String.Format(CultureInfo.CurrentCulture, 
                                 ErrorMessageString, displayName, 
                                 this.size, this.type);
        }
 
        public override bool IsValid(object value)
        {
            var postedFile = value as IFormFile;
 
            if (postedFile == null || postedFile.Length == 0)
            {
                return false;
            }
 
            if (postedFile.Length <= this.size && 
                postedFile.ContentType == this.type)
            {
                return true;
            }
 
            return false;
        }

        // method for IClientModelValidator
        public void AddValidation(ClientModelValidationContext context)
        {
            MergeAttribute(context.Attributes, "data-val", "true");
            var errorMessage = FormatErrorMessage(
                context.ModelMetadata.GetDisplayName());
            MergeAttribute(context.Attributes, 
                "data-val-filevalidation", errorMessage);
            MergeAttribute(context.Attributes, 
                "data-val-filevalidation-size", this.size.ToString());
            MergeAttribute(context.Attributes, 
                "data-val-filevalidation-type", this.type);
        }
 
        // helper used in above AddValidation method
        private bool MergeAttribute(IDictionary<string, string> attributes, 
                                    string key, string value)
        {
            if (attributes.ContainsKey(key))
            {
                return false;
            }
            attributes.Add(key, value);
            return true;
        }
    }
}

View

@model MvcCoreApp.Models.FileUpload
 
@{
    ViewData["Title"] = "Upload";
}
 
<h1>Upload</h1>
<hr />
<div class="row">
    <div class="col-md-4">
        <form asp-action="Upload" enctype="multipart/form-data">
            <div asp-validation-summary="ModelOnly" class="text-danger">
            </div>
            <div class="form-group">
                <label asp-for="Name" class="control-label"></label>
                <input asp-for="Name" class="form-control" />
                <span asp-validation-for="Name" class="text-danger"></span>
            </div>
 
            <div class="form-group">
                <label asp-for="PostedFile" class="control-label"></label>
                <input type="file" asp-for="PostedFile" />
                <br />
                <span asp-validation-for="PostedFile" class="text-danger">
                </span>
            </div>
 
            <div class="form-group">
                <input type="submit" value="Create" class="btn btn-primary" />
            </div>
        </form>
    </div>
</div>
 
@section Scripts {
    @{await Html.RenderPartialAsync("_ValidationScriptsPartial");}
 
 
<script type="text/javascript">
    //<![CDATA[
    $.validator.addMethod("filevalidation",
        function (value, element, parameters){
            // confirm if HTML5 File API is supported
            if (window.File && window.FileList) {
 
                if (element.files[0] == undefined ||
                    element.files[0] == null) {
                    return true;
                }
 
                if (element.files[0].type != parameters.type) {                    
                    return false;
                }
 
                if (element.files[0].size > Number(parameters.size)) {
                    return false;
                }
 
                return ture;
            } else {
                return ture;
            }
        });
 
    $.validator.unobtrusive.adapters.
        add("filevalidation", ["type", "size"], function (options) {
 
            var value = {
                size: options.params.size,
                type: options.params.type
            };
 
            setValidationValues(options, "filevalidation", value);
        });
 
    // helper method copied from jquery.validate.unobtrusive.js
    function setValidationValues(options, ruleName, value) {
        options.rules[ruleName] = value;
        if (options.message) {
            options.messages[ruleName] = options.message;
        }
    }
    //]]>
</script>
}

Comtroller / Action Method

using System;
using System.Collections.Generic;
using Microsoft.AspNetCore.Mvc;
using MvcCoreApp.Models;
using Microsoft.AspNetCore.Http;
using System.IO;
using Microsoft.AspNetCore.Mvc.Rendering;
 
namespace MvcCoreApp.Controllers
{
    public class ValidationController : Controller
    {
        public IActionResult Upload()
        {
            return View();
        }
 
        [HttpPost]
        [ValidateAntiForgeryToken]
        public IActionResult Upload(FileUpload model)
        {
            string result = "";
            IFormFile postedFile = model.PostedFile;
 
            if (ModelState.IsValid)            
            {
                string filename = Path.GetFileName(postedFile.FileName);
                 
                result = filename + " (" + postedFile.ContentType + ") - " +
                        postedFile.Length + " bytes uploaded";
            }
            else
            {
                result = "upload fail";
            }
 
            ViewBag.Result = result;
            return View();
        }
    }
}