<form method="post"
      asp-action="Index"
      asp-controller="TestFileUpload"
      enctype="multipart/form-data">
    <input type="file" name="files" multiple />
    <input type="submit" value="Upload" />
</form>

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Index(IList<IFormFile> files)

public class TestFileUploadController : Controller
{
    private readonly IHostingEnvironment _environment;
    public TestFileUploadController(IHostingEnvironment environment)
    {
        _environment = environment;
    }

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Index(IList<IFormFile> files)
{
    var uploadsRootFolder = Path.Combine(_environment.WebRootPath, "uploads");
    if (!Directory.Exists(uploadsRootFolder))
    {
        Directory.CreateDirectory(uploadsRootFolder);
    }
 
    foreach (var file in files)
    {
        if (file == null || file.Length == 0)
        {
            continue;
        }
 
        var filePath = Path.Combine(uploadsRootFolder, file.FileName);
        using (var fileStream = new FileStream(filePath, FileMode.Create))
        {
            await file.CopyToAsync(fileStream).ConfigureAwait(false);
        }
    }
    return View();
}

public class UserViewModel
{
    [Required(ErrorMessage = "Please select a file.")]
    [DataType(DataType.Upload)]
    public IFormFile Photo { get; set; }
}

@model UserViewModel
 
<form method="post"
      asp-action="UploadPhoto"
      asp-controller="TestFileUpload"
      enctype="multipart/form-data">
    <div asp-validation-summary="ModelOnly" class="text-danger"></div>
 
    <input asp-for="Photo" />
    <span asp-validation-for="Photo" class="text-danger"></span>
    <input type="submit" value="Upload"/>
</form>

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> UploadPhoto(UserViewModel userViewModel)
{
    if (ModelState.IsValid)
    {
        var formFile = userViewModel.Photo;
        if (formFile == null || formFile.Length == 0)
        {
            ModelState.AddModelError("", "Uploaded file is empty or null.");
            return View(viewName: "Index");
        }
 
        var uploadsRootFolder = Path.Combine(_environment.WebRootPath, "uploads");
        if (!Directory.Exists(uploadsRootFolder))
        {
            Directory.CreateDirectory(uploadsRootFolder);
        }
 
        var filePath = Path.Combine(uploadsRootFolder, formFile.FileName);
        using (var fileStream = new FileStream(filePath, FileMode.Create))
        {
            await formFile.CopyToAsync(fileStream).ConfigureAwait(false);
        }
 
        RedirectToAction("Index");
    }
    return View(viewName: "Index");
}

 [FileExtensions(Extensions = ".png,.jpg,.jpeg,.gif", ErrorMessage = "Please upload an image file.")]

[AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
public class UploadFileExtensionsAttribute : ValidationAttribute
{
    private readonly IList<string> _allowedExtensions;
    public UploadFileExtensionsAttribute(string fileExtensions)
    {
        _allowedExtensions = fileExtensions.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries).ToList();
    }
 
    public override bool IsValid(object value)
    {
        var file = value as IFormFile;
        if (file != null)
        {
            return isValidFile(file);
        }
 
        var files = value as IList<IFormFile>;
        if (files == null)
        {
            return false;
        }
 
        foreach (var postedFile in files)
        {
            if (!isValidFile(postedFile)) return false;
        }
 
        return true;
    }
 
    private bool isValidFile(IFormFile file)
    {
        if (file == null || file.Length == 0)
        {
            return false;
        }
 
        var fileExtension = Path.GetExtension(file.FileName);
        return !string.IsNullOrWhiteSpace(fileExtension) &&
               _allowedExtensions.Any(ext => fileExtension.Equals(ext, StringComparison.OrdinalIgnoreCase));
    }
}

public class UserViewModel
{
    [Required(ErrorMessage = "Please select a file.")]
    //`FileExtensions` needs to be applied to a string property. It doesn't work on IFormFile properties, and definitely not on IEnumerable<IFormFile> properties.
    //[FileExtensions(Extensions = ".png,.jpg,.jpeg,.gif", ErrorMessage = "Please upload an image file.")]
    [UploadFileExtensions(".png,.jpg,.jpeg,.gif", ErrorMessage = "Please upload an image file.")]
    [DataType(DataType.Upload)]
    public IFormFile Photo { get; set; }
}

To improve the initial page request load time, we normally apply two performance techniques called bundling and minification. In this tutorial, I will give you a step by step overview of different tools and techniques we can use to apply bundling and minification in ASP.NET Core 5 applications. 

Best practices for private config data and connection strings in configuration in ASP.NET and Azure

• انجام پی در پی اعمال Async به کمک Iterators - قسمت اول

• انجام پی در پی اعمال Async به کمک Iterators - قسمت دوم

• What is Visual Studio Async?

• Visual Studio Async CTP for the rest of us…

• Making Asynchronous Programming Easy

• C# 5 Async Exception Handling

• C# 5.0 Asynchrony – A Simple Intro and A Quick Look at async/await Concepts in the Async CTP

• C# 5 Async, Part 1: Simplifying Asynchrony – That for which we await

• Asynchrony in C# 5, Part One

• Async, Await and C# vNext

• C# 5.0 Asynchronous Made Easy

• What’s Next in C#? Get Ready for Async!

• مصاحبه با تیم طراح

• مصاحبه با Anders Hejlsberg

• We improved solution load performance by optimizing design time build.
• We've added installation progress details on Visual Studio Installer.
• You can pause your installation and resume at a later time.
• We streamlined the update process so the notification takes you directly to the Installer.
• Non-administrators can create a VS layout.
• We added a new shortcut for Edit.Duplicate in the keyboard mapping.
• We made significant improvements to the F# language and tools, particularly for .NET Core SDK projects.
• The C++ compiler optimizes your code to run faster through improved optimizations.
• C++ Mapfile generation overhead is reduced in full linking scenarios.
• Debug options are available for Embedded ARM GCC support.
• We added strong name signing on CoreCLR for the C# compiler.
• Visual Studio Tools for Xamarin has lots of new productivity updates for iOS and Android developers.
• Python no longer requires a completion DB, and Anaconda users have support for conda.
• The Performance Profiler's CPU Usage Tool can display logical call stacks for asynchronous code.
• The CPU Usage tool displays source line highlighting and async/await code with logical 'Call Stack Stitching'.
• The debugger supports thread names set via SetThreadDescription APIs in dump debugging.
• Snapshot Debugging can be started from the Debug Target dropdown for ASP.NET applications.
• We've launched the initial implementation of Navigate to decompiled sources for .NET code navigation.
• New enhancements for Configure Continuous Delivery include support for TFVC, Git authentication over SSH, and containerized projects.
• You can now click on the Continuous Delivery tile in Team Explorer to configure automated build and deployments for your application.
• Team Explorer supports Git tags and checking out pull request branches.
• Service Fabric Tooling for the 6.1 Service Fabric release is now available.
• The Windows 10 Insider Preview SDK can be installed as an optional component.
• File versions for a number of Visual Studio executables now reflect the minor release.
• Test Explorer has a hierarchy view and real time test discovery is now on by default.
• We have added support for testing Win10 IoT Core applications.
• Visual Studio Build Tools supports TypeScript and Node.js.
• ClickOnce Tools support signing application and deployment manifests with CNG certificate.
• You can access Azure resources such as Key Vault using your Visual Studio accounts. 

ASP.NET 5 is an entirely new web stack, rebuilt from the ground up to run fast, cross-platform, and completely configurable. Join David Fowler, one of the architects of ASP.NET 5, as he goes deep into the core of ASP.NET 5. What's the hosting model? How does the HTTP stack work? How do middleware and NuGet packages get pulled in to compose your application? How do the different servers (IIS, Kestrel, WebListener, etc.) actually work? How can you take advantage of these things to build lean, fast, optimized web apps on the new ASP.NET 5 platform? Come find out!

This article explains how to update an existing ASP.NET Core 2.2 project to ASP.NET Core 3.0.