你可以写一个asp.net mvc 根据下拉框不同的值将不同类型的文件存在本地文件夹里，若下拉框未选择会弹出提示的代码例子吗

当然可以，下面是一个简单的ASP.NET MVC的代码例子： Controller代码： ``` public class FileController : Controller { private readonly string _baseFolder = @"C:\Files\"; public ActionResult Index() { return View(); } [HttpPost] public ActionResult Upload(UploadViewModel model) { if (model.SelectedFileType == FileType.None) { ModelState.AddModelError(nameof(model.SelectedFileType), "Please select a file type"); } if (model.File == null || model.File.ContentLength == 0) { ModelState.AddModelError(nameof(model.File), "Please select a file"); } if (!ModelState.IsValid) { return View("Index", model); } var fileName = $"{model.SelectedFileType}_{DateTime.Now:yyyyMMddHHmmss}{Path.GetExtension(model.File.FileName)}"; var fullPath = Path.Combine(_baseFolder, fileName); model.File.SaveAs(fullPath); ViewBag.Message = "File uploaded successfully"; return View("Index"); } } ``` View代码： ``` @model UploadViewModel @{ ViewBag.Title = "Upload"; } <h2>Upload a file</h2> @using (Html.BeginForm("Upload", "File", FormMethod.Post, new { enctype = "multipart/form-data" })) { @Html.ValidationSummary(true) <div class="form-group"> @Html.LabelFor(m => m.SelectedFileType) @Html.DropDownListFor(m => m.SelectedFileType, new SelectList(Enum.GetValues(typeof(FileType))), "- Select -", new { @class = "form-control" }) @Html.ValidationMessageFor(m => m.SelectedFileType) </div> <div class="form-group"> @Html.LabelFor(m => m.File) <input type="file" name="file" class="form-control" /> @Html.ValidationMessageFor(m => m.File) </div> <button type="submit" class="btn btn-primary">Upload</button> } @if (ViewBag.Message != null) { <div class="alert alert-success">@ViewBag.Message</div> } @section scripts { <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.19.2/jquery.validate.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery-validation-unobtrusive/3.2.11/jquery.validate.unobtrusive.min.js"></script> } ``` ViewModel代码： ``` public class UploadViewModel { [Required(ErrorMessage = "Please select a file type")] public FileType SelectedFileType { get; set; } [Required(ErrorMessage = "Please select a file")] [FileSize(1024 * 1024 * 10, ErrorMessage = "File size cannot exceed 10MB")] [FileTypes("txt,pdf,doc,docx,xls,xlsx,jpg,jpeg,png,gif", ErrorMessage = "File type is not supported")] public HttpPostedFileBase File { get; set; } } public enum FileType { None, Text, Image, Document, Spreadsheet } ``` 其中，FileType是一个枚举，表示不同类型的文件，例如Text表示文本文件，Image表示图像文件等。而FileSize和FileTypes是自定义的Validation Attribute，用于限制文件大小和文件类型。需要注意的是，由于涉及到文件上传，为了保证安全性，最好在IIS上将根目录设置为只读。