Object reference not set to an instance of an object

[hrutu]

I am getting an error "object reference not set to an instance of an object"

Partial Class dynamic_array
    Inherits System.Web.UI.Page

    Dim s() As String
    Dim i As Integer = 0

    Protected Sub btn_set_Click(sender As Object, e As System.EventArgs) Handles btn_set.Click

        ReDim s(CInt(txt_no.Text))
    End Sub

    Protected Sub btn_save_Click(sender As Object, e As System.EventArgs) Handles btn_save.Click
        Dim i As Integer = 0

        Try
            For i = 0 To s.Length - 1
                s(i) = InputBox("enter name " + (i + 1).ToString)
            Next
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
       

    End Sub

    Protected Sub btn_get_Click(sender As Object, e As System.EventArgs) Handles btn_get.Click

        Dim str As String = ""

        Dim i As Integer = 0

        Try
            For i = 0 To s.Length - 1
                str &= s(i) + vbCrLf
            Next

            MsgBox(str)

        Catch ex As Exception
            MsgBox(ex.Message)

        End Try
    End Sub
End Class

[Frinavale]

Hi there!

You are experiencing a NullRefereceExc eption. This exception is thrown when you attempt to use a variable that you have declared but have not instantiated it beforehand.

For example, the following will throw a NullRefrenceExc eption:

Dim x As SomeObject
x.PropertyOfSomeObject = "a value"

The reason the exception would be thrown in this case is because I am attempting to set the PropertyOfSomeO bject property to something but have not instantiated x yet.

The following code will NOT throw the exception because I have instantiated x before attempting to use it:

Dim x As SomeObject
x = new SomeObject()
x.PropertyOfSomeObject = "a value"

I could have simplified that to:

Dim x As New SomeObject
x.PropertyOfSomeObject = "a value"

In your praticular case you have declared an array of string's s on line 4 and have not instantiated it before you attempt to access it on line 17.

Consider instantiating this array in the Page Load Event.

-Frinny

(PS. using MsgBox in a web application will not display a message to the user accessing the web application through a web browser)