The ConnectionString property has not been initialized error with VB.NET - vb.net

Question

I am trying to fill a datagrid with data from access, but every time I run this program i get an error saying ConnectionString property has not been initialized i have tried everything i know. Can someone please help
Private Sub RefreshData()
cnn = New OleDb.OleDbConnection
cnn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\My_db.accdb"
If Not cnn.State = ConnectionState.Open Then
' open connection '
cnn.Open()
End If
Dim da As New OleDb.OleDbDataAdapter()
Dim dt As New DataTable
'fill datatable'
da.Fill(dt)
Me.DataGridView1.DataSource = dt
' close connection'
cnn.Close()
End Sub
Private Sub BindGrid()
If Not cnn.State = ConnectionState.Open Then
' open connection '
cnn.Open()
End If
Dim cmd As New OleDb.OleDbCommand
cmd.Connection = cnn
cmd.CommandText = "SELECT * FROM Training log WHERE Runner Name='" & Profile.UsernameTextBox.Text & "'"
cmd.ExecuteNonQuery()
Me.RefreshData()
cnn.Close()
End Sub

Answer 1

It looks like the OleDbCommand is never set to use the OleDbConnection object, and the DataAdapter is never set to use the command. Try this, which also fixes several other items that do not conform to commonly accepted practice:
'Put this module in a separate file
'Any and *ALL* code that talks directly to the DB should go in this module, and use the style of the RefreshData() method below.
Public Module DataLayer
Private Property ConnectionString() As String
Get
Return "Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\My_db.accdb"
End Get
End Property
'I left the name alone so you could match it up to your original code,
' but a better name would be something like "TrainingLogByRunner()"
Public Function RefreshData(ByVal RunnerName As String) As DataTable
Dim dt As New DataTable
Using cnn As New OleDb.OleDbConnection(ConnectionString), _
cmd As New OleDb.OleDbCommand("SELECT * FROM [Training log] WHERE [Runner Name]= ?", cnn), _
da As new OleDb.OleDbDataAdapter(cmd)
'**NEVER** use string concatentation to substitute this kind of value into a query!
'Had to guess at column type/length here
cmd.Parameters.Add("?", OleDbType.VarWChar, 40).Value = RunnerName
'No need to call Open() for the connection...
' the DataAdapter.Fill() method will manage opening/closing the connection
da.Fill(dt)
End Using
Return dt
End Function
End Module
Private Sub BindGrid()
Me.DataGridView1.DataSource = DataLayer.RefreshData(Profile.UsernameTextBox.Text)
End Sub