Using Microsoft Visual Basic Express 2010

This is the code i used:

Dim b As Integer = TextBox1.Text
Dim c As Integer = TextBox2.Text
Dim a As Integer = b * c

Why does this visual basic code not work and how do i fix it to make it work? It is supposed to multiply any two numbers I input into the text boxes but whenever I 'start debugging', it says:

"Conversion from string "" to type 'Integer' is not valid." and it highlights 'As Integer = TextBox1.Text' (part of first line of text)
Ive been trying to figure out why it wont work for a while now. Please help! And please be detailed in answer.

It looks like it's because TextBox1.Text is of type string. I'm not sure but it may attempt to convert the string into an int where applicable (with inputs such as "20" or "350") and error out otherwise. If the text box is empty, conversion may not be possible. You may need to check if the string can be converted and there's a built in function for this called IsNumeric. You can read more about it below:

Also, where have you placed the code to work out the value? If this is done as soon as the program loads, it may not work as expected. You will need to do this as part of a button click event handler.

You are trying to assign a string value to an integer variable. In order to do the assignment you should first (as stated by jwdunne) determine that the conversion is possible. If the conversion is possible you can use Convert to do the conversion as in

Dim b As Integer = Convert.ToInt32(textbox1.Text)

Replace "ToInt32" with the appropriate conversion.

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim b As Integer = Convert.ToInt32(TextBox1.Text)
    Dim c As Integer = Convert.ToInt32(TextBox2.Text)

    Dim a As Integer = b * c

    MsgBox(a, MsgBoxStyle.OkOnly, )

End Sub

Private Sub TextBox1_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles TextBox1.KeyDown

    Select Case e.KeyCode
        Case Keys.D0, Keys.D1, Keys.D2, Keys.D3, Keys.D4, Keys.D5, Keys.D6, Keys.D7, Keys.D8, Keys.D9
            e.SuppressKeyPress = False

        Case Keys.Subtract, Keys.OemMinus
            If Me.TextBox1.Text.StartsWith("-") Then e.SuppressKeyPress = True
        Case Keys.Delete, Keys.Back, Keys.Enter, Keys.Return
            e.SuppressKeyPress = False
        Case Else
            e.SuppressKeyPress = True
    End Select
End Sub

The keyDown sub will need to be for both text boxes, and you will not be allowed to type in text just +/- intergers

 Dim b As Integer = CInt(TextBox2.Text)
 Dim c As Integer = CInt(TextBox3.Text)
 Dim a As Integer = b * c

Or, you can shorten the whole thing to one line:

MsgBox(Val(TextBox1.Text) + Val(TextBox2.Text))
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