Structure to Byte array

[agent]

I'm looking into the possibility of persisting data by putting it into a structure and then copying the structure (via the marshal class) to a byte array which is then output to the file.

I'm using the following code:

Imports System.Runtime.InteropServices
 
Public Class Form1
 
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Dim a As test
        Dim b() As Byte
 
 
        a.a = "testing"
 
        ReDim b(Marshal.SizeOf(a))
 
        Dim ptr As GCHandle = GCHandle.Alloc(b, GCHandleType.Pinned)
 
        Marshal.StructureToPtr(a, ptr.AddrOfPinnedObject, True)
    End Sub
End Class
 
<StructLayout(LayoutKind.Sequential)> _
Public Structure test
    Public a As String
End Structure

What I'm getting, however, is b()'s length is set to 5 and only the first three elements are use and contain different values each time. Why isn't the actual contents of the structure contained here? Is there a correct way of doing this? Am I not understanding what GCHandle.Alloc and Marshal.StructureToPtr do?