Private Sub AddNullableStringParameter( ByVal SQL as OleDbCommand, ByVal _
  columnName as String, ByVal varName as string )
if varname = "" then
  SQL.CommandText = SQL.CommandText & string.Format( " ([{0}] IS NULL)" & _
    "",columnName )
else
  SQL.CommandText = SQL.CommandText & string.Format( " ([{0}] = ?) ",columnName _
    )
  SQL.Parameters.AddWithValue(columnName, VarName )
End If
 
.....
  Dim SQLResult As Object
  Dim connStr As String = "Provider=Microsoft.Jet.OLEDB.4.0;" & _
    "DataSource=c:\test.mdb"
  Using dbConnection As OleDbConnection = New OleDbConnection(connStr)
    Dim SQL As New OleDbCommand
 
    dbConnection.Open()
    SQL.Connection = dbConnection
 
    SQL.CommandText = "SELECT COUNT([id]) FROM Tabelle Where"  
    AddNullableStringParameter( SQL , "Name", VarName )
 
    SQLResult = SQL.ExecuteScalar
  End Using 
  MsgBox(SQLResult.ToString)