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。