Global / Class variable value is getting skipped inside button click event in same class

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VB

ASP.NET

Hello Everyone,
I am getting stuck inside this problem,

Here is my simple application using Master Page Concept and created one simple Page. which contains AsyncFileUpload Control and Submit Button

Code Page :

Partial Class Pages_ArtWork_UploadImages
    Inherits BasePage

    Public imgFilePath As String = Nothing

.....

Protected Sub imgFileUpload_UploadedComplete(ByVal sender As Object, ByVal e As AjaxControlToolkit.AsyncFileUploadEventArgs)
        System.Threading.Thread.Sleep(1500)
        If imgFileUpload.HasFile Then
            If checkFileType(System.IO.Path.GetExtension(e.filename)) Then

                ' Getting the stream of the selected image and assign it to the Image Object.
                Dim UploadedImage As System.Drawing.Image = System.Drawing.Image.FromStream(imgFileUpload.PostedFile.InputStream)
                ' Checking if the Width or height is greater than 250px
                If UploadedImage.PhysicalDimension.Width > 250 OrElse UploadedImage.PhysicalDimension.Height > 250 Then
                    Dim alert As String = ("alert('Image size should be exactly 250*250 dimension. Your Current Image width is " + UploadedImage.PhysicalDimension.Width.ToString + " and height is " + UploadedImage.PhysicalDimension.Height.ToString + ".');")
                    ScriptManager.RegisterStartupScript(Me, [GetType](), "JScript", alert, True)
                Else
                    'Save File in Application.
                    imgFilePath = Server.MapPath("../../Uploads/Images/") + (ddl_EditHw.SelectedValue & System.IO.Path.GetExtension(e.filename))
                    imgFileUpload.SaveAs(imgFilePath)
                    lblResult.Visible = False
                    ScriptManager.RegisterClientScriptBlock(Me, Me.GetType(), "Script", "alert('File Uploaded Successfully');", True)
                End If
            Else
                ScriptManager.RegisterClientScriptBlock(Me, Me.GetType(), "Script", "alert('Uploading Failed - Check File Type (.jpg, .png, .gif, .jpeg)');", True)
            End If
        End If
        ClearContents(sender)
        
    End Sub

Public Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click

        If Not imgFilePath = Nothing Then

....
...
     Else
      lblResult.Text = "Please Upload Image first."
            lblResult.Visible = True

        End If

In above submit click event , when I try to get Image File path from imgFilePath variable. It returns "Nothing". If I use same variable inside other functions It works perfectly. but dont know what is problem with Click event.
Waiting for your kind reply.

Thanking You


Regards :
Rohit More