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asp.net - How to validate a FileUpload control

FileUpload control validation
FileUpload is an ASP.NET web server control that allows us to upload a file to the web server from the client browser. FileUpload server control renders a TextBox and a browse Button in the web browser that enable users to select a file from the client computer and upload it to the web server machine.

Users can specify the file to upload by entering the full path of the local computer file in the TextBox of FileUpload control or using the browse button to select a file from the local computer file system.

RequiredFieldValidator control allows us to check whether the FileUpload control has a file selected. So users must input a file path in FileUpload control before submitting the form. We can make the FileUpload control a required field in web form by attaching a RequiredFieldValidator control with it.

RequiredFiledValidator is an asp.net validation web server control that makes an input control to a required field. RequiredFieldValidator control's ControlToValidate property gets or sets the input control to validate. So we can set the ControlToValidate property value by inputting the FileUpload control's ID.

RequiredFieldValidator control's ErrorMessage property allows us to set an error message to display in the web browser when the user tries to submit the form without inputting any file path in FileUpload control. Finally, we can ensure that users select a file in FileUpload control before they submit the form to the web server.

FileUpload control's HasFile property allows us to verify on the server side that FileUpload control contains a file. Using FileUpload control's SaveAs method we can save the client's uploaded file to the web server machine.

The following ASP.NET C# example code demonstrates to us how can we validate a FileUpload web server control.
FileUploadValidation.aspx

<%@ Page Language="C#" %>
<%@ Import Namespace="System.IO" %>

<!DOCTYPE html>

<script runat="server">
    protected void Button1_Click(object sender, System.EventArgs e) { 
        string uploadFolder = Request.PhysicalApplicationPath+"Upload\\" ;
        FileUpload1.SaveAs(uploadFolder + FileUpload1.FileName);
        Label1.Text = "File uploaded successfully: " + FileUpload1.PostedFile.FileName;
    }
</script>

<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
    <title>asp.net FileUpload validation example: how to validate FileUpload control (file upload)</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <h2 style="color:Green">asp.net FileUpload example: Validate FileUpload</h2>
        <asp:Label 
             ID="Label1" 
             runat="server" 
             Font-Size="Large"
             ForeColor="Crimson"
             >
        </asp:Label>
        <br /><br />
        <asp:Label 
             ID="Label2" 
             runat="server" 
             ForeColor="DarkBlue"
             Text="Choose a file for upload it."
             Font-Bold="true"
             >
        </asp:Label>
        <br />
        <asp:FileUpload 
             ID="FileUpload1" 
             runat="server" 
             BackColor="DarkBlue" 
             ForeColor="AliceBlue" 
             />
        <asp:RequiredFieldValidator 
             ID="RequiredFieldValidator1"
             runat="server"
             ControlToValidate="FileUpload1"
             ErrorMessage="Choose a file!"
             >
        </asp:RequiredFieldValidator>    
        <br />         
        <asp:Button 
             ID="Button1" 
             runat="server" 
             Font-Bold="true" 
             ForeColor="DodgerBlue" 
             OnClick="Button1_Click"
             Text="Upload Now"
             />   

        <br /><br />
    </div>
    </form>
</body>
</html>
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