What is wrong with my prime function - vb.net

Question

I'm a beginner in vb and I'm wondering why this code doesn't work.
I wrote the function which I can't seem to find why it doesn't. When I run the program, it seems to print out nothing.
I am supposed to find all the prime numbers between 1 and the input
Option Strict On
Public Class Lab4
Dim endCounter As Integer
Dim sum As Integer
Dim msg As String
Dim input As Integer
Public Function isPrime(input As Integer) As Boolean
endCounter = input - 1
For primeCounter As Integer = 1 To endCounter
If input Mod primeCounter <> 0 Then
Return True
Else
Return False
End If
Next
Return False
End Function
Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
Me.Close()
End Sub
Private Sub btnPrime_Click(sender As Object, e As EventArgs) Handles btnPrime.Click
input = Convert.ToInt32(txtNumber.Text)
msg = "The prime numbers are: "
If input < 0 Then
msg = "Number cannot be negative!"
For inputCounter As Integer = 1 To input
If isPrime(inputCounter) = True Then
msg += inputCounter & " "
End If
Next
MsgBox(msg, , "Prime Number")
End Sub
End If
End Class

Answer 1

This test will always immediately return something on the very first test:
Public Function isPrime(input As Integer) As Boolean
endCounter = input - 1
For primeCounter As Integer = 1 To endCounter
If input Mod primeCounter <> 0 Then
Return True
Else
Return False
End If
Next
Return False
End Function
because you use Return immediately after the 'has modulus' line. Remove the Return True and else lines, so it only returns False if a modulus value is found. The very last line should be Return True – no modulus values are found, so it is a prime.
For this to work, you need to change your test to
If input Mod primeCounter == 0 Then
and change the start test from 1 to 2.
You don't need to test against 1, and you don't need to test all the way up to input or (you probably misunderstood something there) input - 1. The most logical endpoint is ceil(sqrt(input)), the next higher number of the square root of your starting value.