I cannot figure out where my error is here is my entire code. I am using Visual Basic and using the program Microsoft visual. I am not sure what I did I'm sure it's one error causing them all maybe a few because they all seem to be the same error. This is due tom and is a final and I had it working until the program crashed and I had to start over again. Any assistance is helpful
Thanks
Public Class IT450Final
Public Science, Mathematics, Writing, Reading As Integer

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim Sum As Integer
'If user did not enter score
If TextBoxa.Text = "" Then
MsgBox("You Forgot to enter a score in Science")
Return
End If
If TextBoxb.Text = "" Then
MsgBox("You forgot to enter a score in Mathematics")
Return
End If
If TextBoxc.Text = "" Then
MsgBox(" Your forgot to enter a score in Writing")
Return
End If
If TextBoxd.Text = "" Then
MsgBox(" You forgot to enter a score in Reading")
Return
End If
'converting to integer
TextBoxa = CInt(TextBoxa.Text)
'converting to inteteger
TextBoxb = CInt(TextBoxb.Text)
'converting to integer
TextBoxc = CInt(TextBoxc.Text)
'converting to integer
TextBoxd = CInt(TextBoxd.Text)
If TextBoxa < 0 Or TextBoxa > 5 Then
MsgBox("Please enter score between 0-50 in Mathematics score Box Please")
Return
End If
If TextBoxb < 0 Or TextBoxb > 5 Then

End If
MsgBox("Please enter score between 0-50 in Science score Box Please")
Return
If TextBoxc < 0 Or TextBoxc > 5 Then

End If
Return
MsgBox("Please enter score between 0-50 in Writing score Box Please")
Return
End
If TextBoxd < 0 Or TextBoxd > 5 Then
MsgBox("Please enter score between 0-50 in Reading score Box Please")
End If
'Calculate Exam Total
Sum = TextBoxa + TextBoxb + TextBoxc + TextBoxd

'Determine letter grade
If Sum >= 90 Then
lblgrade.Text = "5"
ElseIf Sum >= 80 Then
lblgrade.Text = "4"
ElseIf Sum >= 70 Then
lblgrade.Text = "3"
ElseIf Sum >= 60 Then
lblgrade.Text = "2"
Else
lblgrade.Text = "1"
End If


End Sub
Private Sub Buttonclear_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Buttonclear.Click
TextBoxa.Text = ""
TextBoxa.Text = 0
TextBoxb.Text = 0
TextBoxc.Text = 0
TextBoxd.Text = 0
TextBoxa.Focus()



End Sub
End Class


Errors that I am receiving are
Error 13 Operator '+' is not defined for types 'System.Windows.Forms.TextBox' and
'System.Windows.Forms.TextBox'. Line 51 column 15

Error 5 Operator '<' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 31 column12

Error 7 Operator '<' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 35 column 12

Error 9 Operator '<' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. Line 40 column 12

Error 11 Operator '<' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 47 column 12

Error 6 Operator '>' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 31 column 28

Error 8 Operator '>' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 35 column 28

Error 10 Operator '>' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 40 column 28

Error 12 Operator '>' is not defined for types 'System.Windows.Forms.TextBox' and 'Integer'. line 47 column 28

Error 1 Value of type 'Integer' cannot be converted to 'System.Windows.Forms.TextBox'. line 24 column 20

Error 2 Value of type 'Integer' cannot be converted to 'System.Windows.Forms.TextBox'. line 26 column 20

Error 3 Value of type 'Integer' cannot be converted to 'System.Windows.Forms.TextBox'. line 28 column 20

Error 4 Value of type 'Integer' cannot be converted to 'System.Windows.Forms.TextBox'. line 30 column 20