vb无法转换用户类型

Question

我有2个新班级

Public Class ColCust
    Inherits DataGridViewColumn

    Dim clsCellText As New CellCust
    Public Sub New()
        Me.CellTemplate = clsCellText
    End Sub
End Class

Public Class CellCust
    Inherits DataGridViewTextBoxCell
End Class

在主窗体上我有一个网格，我试图使用新类

With dg6
    .Columns.Clear()
    .Rows.Clear()
    For iCol As Integer = 0 To 4
        Dim col As New ColCust
        .Columns.Add(col)
    Next
    .Rows.Add(New String() {"1"})
    .Rows.Add(New String() {"2"})
    .Rows.Add(New String() {"3"})
    .Rows.Add(New String() {"4"})
    Me.Controls.Add(dg6)
End With

直到现在一切正常， 但在这里我得到了错误

  

无法投射类型的对象   ＆＃39; System.Windows.Forms.DataGridViewTextBoxColumn＆＃39;输入   ＆＃39; Test.ColCust＆＃39;

有什么问题？

Dim colText As DataGridViewTextBoxColumn
colText = dg6.Columns(0)