SkinTextBox的样式

错误        CS0030        Cannot convert type 'CCWin.SkinControl.SkinTextBox' to 'System.Windows.Forms.TextBox'       
源语句：
  PassKey pass = new PassKey(this.Left + skinTextBox_txtPwd.Left - 25, this.Top + skinTextBox_txtPwd.Bottom,(TextBox)skinTextBox_txtPwd);