Question

I have my code that is reading values from a database and then putting them into an array. I want to copy the values of this array to another array, however when I try, it keeps on throwing an exception,

Object reference not set to an instance of an object.

I don’t seem to see what is wrong. How can I fix this problem?

My code is here:

    ' Read values from a database and assign them to an array
    Dim counter As Integer = 0
    Dim reader As MySqlDataReader
    reader = cmd.ExecuteReader()
    Dim dataarray As string()

    While reader.Read()
        Dim datatoAdd As string = reader.GetValue(1) & ", " & _
                                  reader.GetValue(2) & ", " & _
                                  reader.GetValue(3) & ", " & _
                                  reader.GetValue(6) & ", " & _
                                  reader.GetValue(7) & ", " & _
                                  reader.GetValue(8) & ", " & _
                                  reader.GetValue(9) & ", " & _
                                  reader.GetValue(10) & ", " & _
                                  reader.GetValue(11)

        dataarray(counter) = datatoAdd
    End While

    reader.Close()
    connection.Close()
    MessageBox.Show('Data added successfully)

Catch ex As Exception
    MessageBox.Show(ex.Message)

Answer 1

Your statement

Dim dataarray As string()

just declared dataarray — you defined that dataarray can point to an array of string. Currently, it does not point anywhere. You have to create a new array and assign it to dataarray.

The easiest way to do so is to specify the size of the array in the declaration:

Dim dataarray(UPPER_BOUND) As String

This will create an array with indexes 0 to UPPER_BOUND (i.e., with UPPER_BOUND+1 elements). (If you don’t know the upper bound yet, don’t use an array, but use a List(Of String) instead.)

More details:

• MSDN: Arrays in Visual Basic

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EDIT: You state in the comments that you don’t know the size of the array. In that case you shouldn’t be using an array at all. Arrays are by definition fixed-size data structures. (Yes, there’s ReDim Preserve, but it’s still not a good idea.)

Instead, use a resizable data structure such as List(Of String). If you really need an array, you can convert it into an array afterwards.