Visual basic error message for blank field, password and data retrieval system - vb.net

Question

I am writing a program in visual basic that, when a user enters their email and password (from a list of email and passwords in a CSV file) it will give them their name, address, password etc. I have successfully achieved this however when I input nothing into the field the program simply enters 5 blank lines, how do I fix this? I need an error message to say that nothing has been input and for it to reject it. How do I do this? My code is below:
Imports System.Text.RegularExpressions
Public Class Form1
Private Sub Label2_Click(sender As Object, e As EventArgs) Handles Password.Click
End Sub
Private Sub Label1_Click(sender As Object, e As EventArgs) Handles EmailAddress.Click
End Sub
Private Sub Submit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Submit.Click
Dim currentRow As String()
Dim foundmatch As Boolean
Using parserDetails As New Microsoft.VisualBasic.FileIO.TextFieldParser("CSV_File.csv")
parserDetails.SetDelimiters(",")
While Not parserDetails.EndOfData
currentRow = parserDetails.ReadFields()
If EmailAddress.Text = currentRow(0) And Password.Text = currentRow(1) Then
Me.DataGridView1.Rows.Add(currentRow)
foundmatch = True
End If
End While
End Using
If Not foundmatch Then
MsgBox("The email and/or password entered cannot be found or is incorrect.", MsgBoxStyle.Critical, "Invalid Email")
End If
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
End Sub
Private Sub Input(p1 As String)
Throw New NotImplementedException
End Sub
End Class

Answer 1

You'd use the If construct to check for conditions. For example...
Private Sub Submit_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Submit.Click
If String.IsNullOrWhiteSpace(EmailAddress.Text) Then
MsgBox('Email Address is required!')
Return
End If
' The rest of your logic
End Sub
You can check multiple conditions at once, sequentially, etc.
A couple of notes:
(I realize this is just a learning exercise but...) If you can load the file into a database-like structure (DataTable?) then you may be able to perform more effective searches than just scanning line-by-line. (Or if you could use an actual database, even better.) For small amounts of data it may not be a big deal, but scaling to larger amounts will make a difference.
FYI - Storing user passwords in plain text is a famously bad idea. While you're learning and practicing, it would be good to familiarize yourself with password hashing. Store the hash of the password (not encrypted, hashed) and when a user enters their password you hash their input and compare the hashes. It should be impossible for you or anyone else to be able to recover and see the original password.

Answer 2

You would want to make use of something like this:
If(Not String.IsNullOrWhitespace(EmailAddress.Text)) Then
'put
Else 'null/empty or white space
Msgbox("You cannot leave the email address empty...")
End If
I.e. you could change your while block to this:
While Not parserDetails.EndOfData
currentRow = parserDetails.ReadFields()
If (Not String.IsNullOrWhiteSpace(EmailAddress.Text)) Then
If EmailAddress.Text = currentRow(0) And Password.Text = currentRow(1) Then
Me.DataGridView1.Rows.Add(currentRow)
foundmatch = True
Exit While
End If
Else 'null/empty or white space
MsgBox("You cannot leave the email address empty...")
foundmatch = False
Exit While
End If
End While
Note the addition of the Exit Whiles, which makes it more efficient because it no longer needs to loop after the correct values are found.