Validate file upload control using Custom Validator

1. Code in the ASPX page. Add a file upload control and a custom validator

<form id="form1" runat="server">
    <asp:FileUpload ID="fileupload" runat="server" />
    <asp:CustomValidator ID="cv" runat="server" ControlToValidate="fileupload"
        ErrorMessage="Message" OnServerValidate="cv_ServerValidate" />
    <asp:Button ID="btnUpload" runat="server" Text="Upload" OnClick="btnUpload_Click" />
    </form>

2. Add a server side validator function for validation of upload control

protected void cv_ServerValidate(object source, ServerValidateEventArgs args)
        {
            if (System.IO.Path.GetExtension(fileupload.FileName) == ".pdf")
            {
                if (fileupload.FileBytes.Length > 20971520)
                {
                    cv.ErrorMessage = "File size exceeds maximum limit 20 MB.";
                    args.IsValid = false;
                }
                else
                {
                    args.IsValid = true;
                }
            }
            else
            {
                cv.ErrorMessage = "File type should be pdf.";
                args.IsValid = false;
            }
        }

3. You can implement your logic to upload a file in btnUpload_Click event

protected void btnUpload_Click(object sender, EventArgs e)
        {
            if (Page.IsValid)
            {
               // your logic here 
            }
       }