helping with the right code for a 15 shuffle game - wpf

Question

I am trying to build a 15 number shuffle game in VB.
Later on, I will have to submit the 15 pieces to a full image.
At the mintime, I tries to build it with numbers. it was all working fine until I got this messege:
Conversion from string "" to type 'Double' is not valid.
I got it in line:
If butt2.Content = "" Then
here is my code:
<pre lang="vb">Class MainWindow
Private Sub btn1_Click(sender As Object, e As RoutedEventArgs) Handles btn1.Click
'2,4
checkBtn(btn1, btn4)
checkBtn(btn1, btn2)
chechSolved()
End Sub
Private Sub btn2_Click(sender As Object, e As RoutedEventArgs) Handles btn2.Click
'1,3,5
checkBtn(btn2, btn1)
checkBtn(btn2, btn3)
checkBtn(btn2, btn5)
chechSolved()
End Sub
Private Sub btn3_Click(sender As Object, e As RoutedEventArgs) Handles btn3.Click
'2,6
checkBtn(btn3, btn2)
checkBtn(btn3, btn6)
chechSolved()
End Sub
Private Sub btn4_Click(sender As Object, e As RoutedEventArgs) Handles btn4.Click
'1,5,7
checkBtn(btn4, btn1)
checkBtn(btn4, btn5)
checkBtn(btn4, btn7)
chechSolved()
End Sub
Private Sub btn5_Click(sender As Object, e As RoutedEventArgs) Handles btn5.Click
'2,4,6,8
checkBtn(btn5, btn2)
checkBtn(btn5, btn4)
checkBtn(btn5, btn6)
checkBtn(btn5, btn8)
chechSolved()
End Sub
Private Sub btn6_Click(sender As Object, e As RoutedEventArgs) Handles btn6.Click
'3,5,9
checkBtn(btn6, btn3)
checkBtn(btn6, btn5)
checkBtn(btn6, btn9)
chechSolved()
End Sub
Private Sub btn7_Click(sender As Object, e As RoutedEventArgs) Handles btn7.Click
'4,8
checkBtn(btn7, btn4)
checkBtn(btn7, btn8)
chechSolved()
End Sub
Private Sub btn8_Click(sender As Object, e As RoutedEventArgs) Handles btn8.Click
'5,7,9
checkBtn(btn8, btn5)
checkBtn(btn8, btn7)
checkBtn(btn8, btn9)
chechSolved()
End Sub
Private Sub btn9_Click(sender As Object, e As RoutedEventArgs) Handles btn9.Click
'6,8
checkBtn(btn9, btn6)
checkBtn(btn9, btn8)
chechSolved()
End Sub
Sub checkBtn(ByVal butt1 As Button, ByVal butt2 As Button)
If butt2.Content = "" Then
butt2.Content = butt1.Content
butt1.Content = ""
End If
End Sub
Sub chechSolved()
If btn1.Content = "1" And btn2.Content = "2" And btn3.Content = "3" And btn4.Content = "4" And btn5.Content = "5" And btn6.Content = "6" And btn7.Content = "7" And btn8.Content = "8" And btn9.Content = "" Then
MsgBox("הצלחת")
End If
End Sub
Sub shuffle()
Dim a(8), i, j, RN As Integer
Dim flag As Boolean
flag = False
i = 1
a(j) = 1
Do While i <= 8
Randomize()
RN = CInt(Int((8 * Rnd()) + 1))
For j = 1 To i
If (a(j) = RN) Then
flag = True
Exit For
End If
Next
If flag = True Then
flag = False
Else
a(i) = RN
i = i + 1
End If
Loop
btn1.Content = a(1)
btn2.Content = a(2)
btn3.Content = a(3)
btn4.Content = a(4)
btn5.Content = a(5)
btn6.Content = a(6)
btn7.Content = a(7)
btn8.Content = a(8)
btn9.Content = ""
End Sub
Private Sub btnSH_Click(sender As Object, e As RoutedEventArgs) Handles btnSH.Click
shuffle()
End Sub
End Class</pre>
it would help a lot if you look at it.

Answer 1

The compiler might be having a problem comparing the string to the btn.Content because you set the Content to an Integer type (btn1.Content = a(1))