Save Uploaded FilePath in variable in ASP.Net using C#

Article: Upload Image file and display in Image control in ASP.Net using C# and VB.Net

I am trying to upload the file same like this and trying to store path in one variable but as soon as image gets upload the variables get empty.

string imageId = "", fileExtension = "", filePath = "";
protected void btnAttachImage_Click(object sender, EventArgs e)
{
    if (fileUpload.HasFile)
    {
        // Generate a unique ID for the image
        imageId = GenerateImageId();

        // Get the file extension
        fileExtension = Path.GetExtension(fileUpload.FileName);

        // Save the file with the ID and extension
        filePath = Server.MapPath("~/Images/" + imageId + fileExtension);
        fileUpload.SaveAs(filePath);

        // Set the image URL to display the uploaded image
        Image1.ImageUrl = "~/Images/" + imageId + fileExtension;
        //lblFileName.Text = fileUpload.FileName;
    }
    else
    {
        // Handle case when no file is selected
    }
}

private string GenerateImageId()
{
    string code = txtCmpId.Text;
    return code;
}

<asp:Image ID="Image1" runat="server" Width="150" Height="150" CssClass="custom-image" />
<asp:FileUpload ID="fileUpload" runat="server" />
<asp:Button ID="btnAttachImage" runat="server" Text="Attach Image" OnClick="btnAttachImage_Click" Style="display: none" />