FileExtensions Validator in ASP.NET MVC tutorial with examples

FileExtensions Validator in ASP.NET MVC: A Tutorial with Code Examples. File Extensions play an important role in the organization and management of digital files. They help to identify the format and type of file, which can be crucial for determining the best method for opening, processing, or sharing the file. In .NET MVC, FileExtensions can be used to annotate data, providing additional information about the data being processed. In this blog post, we'll explore the concept of FileExtensions data annotation in .NET MVC and provide a code example to illustrate how it works.

How FileExtensions Data Annotation Works

FileExtensions data annotation allows you to specify the FileExtensions that are allowed when uploading a file. This annotation can be applied to a property in your data model, and it will enforce the specified file extension restriction when the file is uploaded. If a user attempts to upload a file with an extension that is not allowed, an error message will be displayed, indicating that the file extension is not valid.

Example of FileExtensions Data Annotation in .NET MVC

Here's an example of how FileExtensions data annotation can be used in .NET MVC. Suppose you have a data model for storing information about images, and you want to allow only JPG and PNG FileExtensions. You would define the data model like this:
public class ImageModel
    [Display(Name = "Image")]
    [FileExtensions(Extensions = "jpg,png", ErrorMessage = "Only JPG and PNG files are allowed.")]
    public HttpPostedFileBase ImageFile { get; set; }
In this example, the FileExtensions attribute has been applied to the ImageFile property. The Extensions parameter specifies the allowed FileExtensions, and the ErrorMessage parameter provides a custom error message that will be displayed if a user attempts to upload a file with an invalid extension.

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To enforce the file extension restriction, you would need to add some code to your controller action method to check the file extension before uploading the file. Here's an example of what that code might look like:
public ActionResult Upload(ImageModel model)
    if (ModelState.IsValid)
        // Check if the file has a valid extension
        if (!model.ImageFile.FileName.EndsWith(".jpg") && !model.ImageFile.FileName.EndsWith(".png"))
            ModelState.AddModelError("ImageFile", "Only JPG and PNG files are allowed.");
            return View(model);

        // Continue with file upload logic

    return View(model);
In this example, the code checks the file extension of the uploaded file by examining the FileName property of the ImageFile property. If the file extension is not ".jpg" or ".png", a custom error message is added to the ModelState object, and the user is returned to the view with the error message displayed.
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