Parameters.AddWithValue: Parameter has already been defined - vb.net

Question

I try to add a parameters.addwithvalue.
Before change the code is like that..........
Private Sub ComboBox7_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox7.SelectedIndexChanged
Me.Cursor = Cursors.WaitCursor
MysqlConn.Close()
MysqlConn.Open()
COMMAND.CommandText = "select logo from licenses where name = '" & ComboBox7.Text & "'"
COMMAND.Connection = MysqlConn
Dim da As New MySqlDataAdapter(COMMAND)
Dim ds As New DataSet()
da.Fill(ds, "projectimages")
Dim c As Integer = ds.Tables(0).Rows.Count
If c > 0 Then
If IsDBNull(ds.Tables(0).Rows(c - 1)("logo")) = True Then
PictureBox6.Image = Nothing
Else
Dim bytBLOBData() As Byte = ds.Tables(0).Rows(c - 1)("logo")
Dim stmBLOBData As New MemoryStream(bytBLOBData)
PictureBox6.Image = Image.FromStream(stmBLOBData)
End If
End If
Me.Cursor = Cursors.Default
End Sub
Now what i try this to add paramatrers.addwithValue without succes:
Private Sub ComboBox7_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox7.SelectedIndexChanged
Me.Cursor = Cursors.WaitCursor
MysqlConn.Close()
MysqlConn.Open()
'COMMAND.CommandText = "select logo from licenses where name = '" & ComboBox7.Text & "'"
COMMAND.CommandText = "select logo from licenses where name = #ComboBox7Select"
COMMAND.Parameters.AddWithValue("#ComboBox7Select", If(String.IsNullOrEmpty(ComboBox7.Text), DBNull.Value, ComboBox7.Text))
COMMAND.Connection = MysqlConn
Dim da As New MySqlDataAdapter(COMMAND)
Dim ds As New DataSet()
da.Fill(ds, "projectimages")
Dim c As Integer = ds.Tables(0).Rows.Count
If c > 0 Then
If IsDBNull(ds.Tables(0).Rows(c - 1)("logo")) = True Then
PictureBox6.Image = Nothing
Else
Dim bytBLOBData() As Byte = ds.Tables(0).Rows(c - 1)("logo")
Dim stmBLOBData As New MemoryStream(bytBLOBData)
PictureBox6.Image = Image.FromStream(stmBLOBData)
End If
End If
Me.Cursor = Cursors.Default
End Sub
With error "Parameter '#ComboBox7Select' has already been defined."
What i do to change for work ??
Thanks you.

Answer 1

Don't store the MySqlConnection and MySqlCommand as fields in your class, don't reuse them at all. That's just a source of errors without any benefit. Create, initialize, use and dispose(Using-statement)them wherever you need them, so in this method.
You don't clear the parameters, that's why you get this error on second use.
So a simple COMMAND.Parameters.Clear() before you add it would solve the issue. But use the approach i have mentioned above:
Private Sub ComboBox7_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox7.SelectedIndexChanged
Dim ds As New DataSet()
Dim licenseNameValue As Object = DBNull.Value
If Not String.IsNullOrEmpty(ComboBox7.Text) Then licenseNameValue = ComboBox7.Text
Using mysqlConn As New MySqlConnection("ConnectionString...")
Using da As New MySqlDataAdapter("select logo from licenses where name = #licenseName", mysqlConn)
da.SelectCommand.CommandText = "select logo from licenses where name = #licenseName"
da.SelectCommand.Parameters.AddWithValue("#licenseName", licenseNameValue)
da.Fill(ds, "projectimages") ' you dont need to open/close the connection with DataAdapter.Fill
End Using
End Using
' ....
End Sub

Answer 2

The problem is that you are using a global variable COMMAND that you use each time the selected index changes in your combo. Either you initialize the command each time with:
COMMAND=New MySqlCommand()
Or you must clear the parameters:
COMMAND.Parameters.Clear()
COMMAND.Parameters.AddWithValue("#ComboBox7Select", If(String.IsNullOrEmpty(ComboBox7.Text), DBNull.Value, ComboBox7.Text))
But the best way is always create and dispose the MySql objects with the Using struct:
Using MysqlConn As New MySqlConnection(connString)
Using COMMAND As New MySqlCommand()
'your code
End Using
End Using