https://learn.microsoft.com/ja-jp/dotnet/api/system.windows.forms.listview.checkedindices?view=netframework-4.8.1&viewFallbackFrom=windowsdesktop-10.0
ListView.CheckedIndices Property (System.Windows.Forms) | Microsoft Learn
Gets the indexes of the currently checked items in the control.
property systemwindows formslistviewmicrosoftlearn