无法编辑datagrid单元格

Question

我已阅读有关此主题的所有帖子，但似乎没有解决我的情况。我试图将一些简单的数据加载到数据网格中，然后允许用户编辑数据。我遇到的问题是网格不允许编辑单元格。我使用的是wpf和VB。这是创建网格的XAML：

<DataGrid
    x:Name="dgrMain"
    CanUserAddRows="True"
    CanUserDeleteRows="True"
    AutoGenerateColumns="False"
    SelectionMode="Single"
    SelectionUnit="CellOrRowHeader"
    IsReadOnly="False"
    ItemsSource="{Binding}">
    <DataGrid.Columns>
        <DataGridTextColumn
            Header="Col1"
            Binding="{Binding Path=Col1}"
            IsReadOnly="False" />
        <DataGridTextColumn
            Header="Col2"
            Binding="{Binding Path=Col2}"
            IsReadOnly="False" />
        <DataGridTextColumn
            Header="Col3"
            Binding="{Binding Path=Col3}"
            IsReadOnly="False" />
    </DataGrid.Columns>
</DataGrid

以下是加载数据的代码：

Private Sub MainWindow_Loaded(sender As Object, e As RoutedEventArgs) Handles Me.Loaded
    Dim dt As DataTable = New DataTable("OriginalData")
    Dim col As DataColumn
    col = New DataColumn
    col.DataType = Type.GetType("System.Int32")
    col.ColumnName = "Col1"
    col.ReadOnly = False
    dt.Columns.Add(col)
    col = New DataColumn
    col.DataType = Type.GetType("System.String")
    col.ColumnName = "Col2"
    col.ReadOnly = False
    dt.Columns.Add(col)
    col = New DataColumn
    col.DataType = Type.GetType("System.String")
    col.ColumnName = "Col3"
    col.ReadOnly = False
    dt.Columns.Add(col)
    Dim dr As DataRow
    For I As Int32 = 0 To 9
        dr = dt.NewRow
        dr("Col1") = I + 1
        dr("Col2") = Chr(I + 65)
        dr("Col3") = Chr(122 - I)
        dt.Rows.Add(dr)
    Next
    dgrMain.DataContext = dt.DefaultView
End Sub

OnCellBeginEdit事件触发，但最终用户无法编辑单元格内容。为什么呢？