How to select multiple file in ASP.NET using C#

You can add the allowmultiple property

<asp:FileUpload runat="server" ID="UploadImages" AllowMultiple="true" />

Take a look at this example

.NET 4.5 & above :

<asp:FileUpload ID="fileImages" AllowMultiple="true" runat="server" />

.NET 4 and below :

 <asp:FileUpload ID="fileImages" Multiple="Multiple" runat="server" />

html :

<div>
    <asp:FileUpload runat="server" ID="UploadImages" AllowMultiple="true" />
    <asp:Button runat="server" ID="uploadedFile" Text="Upload" OnClick="uploadFile_Click" />
    <asp:Label ID="listofuploadedfiles" runat="server" />
</div>

Button click code :

protected void uploadFile_Click(object sender, EventArgs e)
{
   if (UploadImages.HasFiles)
   {
       foreach (HttpPostedFile uploadedFile in UploadImages.PostedFiles)
       {
           uploadedFile.SaveAs(System.IO.Path.Combine(Server.MapPath("~/Images/"),
           uploadedFile.FileName)); listofuploadedfiles.Text += String.Format("{0}<br />", uploadedFile.FileName);
       }
   }
} 

more reference

You just could write a Submit Controller Method like:

[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Submit(IEnumerable<HttpPostedFileBase> files)
{
    //your file code here 
}

and then just offering a corresponding view with any widget for the upload you like and post back the files. e.g. Jquery file upload widget: https://blueimp.github.io/jQuery-File-Upload/

Kendo Ui example in ASP.NET MVC:

 <form method="post" action='@Url.RouteUrl("Default", new {Action =     "Submit"})' style="width:45%">
    @Html.AntiForgeryToken()
    @Html.ValidationSummary(true)
    <div class="float-left">
    //widget here 
        <p>
            <input type="submit" value="Submit" class="k-button" />
        </p>
    </div>
  </form>