[WPF]為什麼使用SaveFileDialog創建文件需要刪除許可權？
- 2020 年 4 月 7 日
- 筆記
1. 問題
好像很少人會遇到這種需求。假設有一個文件夾，用戶有幾乎所有許可權，但沒有刪除的許可權，如下圖所示：
這時候使用SaveFileDialog在這個文件夾里創建文件居然會報如下錯誤：
這哪裡是網路位置了，我又哪裡去找個管理員？更奇怪的是，雖然報錯了，但文件還是會創建出來，不過這是個空文件。不僅WPF，普通的記事本也會有這個問題，SaveFileDialog會創建一個空文件，記事本則沒有被保存。具體可以看以下GIF：
2. 問題原因
其實當SaveFileDialog關閉前，對話框會創建一個測試文件，用於檢查文件名、文件許可權等，然後又刪除它。所以如果有文件的創建許可權，而沒有文件的刪除許可權，在創建測試文件後就沒辦法刪除這個測試文件，這時候就會報錯，而測試文件留了下來。
有沒有發現SaveFileDialog中有一個屬性Options？
// // 摘要: // 獲取 Win32 通用文件對話框標誌，文件對話框使用這些標誌來進行初始化。 // // 返回結果: // 一個包含 Win32 通用文件對話框標誌的 System.Int32，文件對話框使用這些標誌來進行初始化。 protected int Options { get; } 本來應該可以設置一個NOTESTFILECREATE的標誌位，但WPF中這個屬性是只讀的，所以WPF的SaveFileDialog肯定會創建測試文件。
3. 解決方案
SaveFileDialog本身只是Win32 API的封裝，我們可以參考SaveFileDialog的源碼，偽裝一個調用方法差不多的MySaveFileDialog，然後自己封裝GetSaveFileName這個API。程式碼大致如下：
internal class FOS { public const int OVERWRITEPROMPT = 0x00000002; public const int STRICTFILETYPES = 0x00000004; public const int NOCHANGEDIR = 0x00000008; public const int PICKFOLDERS = 0x00000020; public const int FORCEFILESYSTEM = 0x00000040; public const int ALLNONSTORAGEITEMS = 0x00000080; public const int NOVALIDATE = 0x00000100; public const int ALLOWMULTISELECT = 0x00000200; public const int PATHMUSTEXIST = 0x00000800; public const int FILEMUSTEXIST = 0x00001000; public const int CREATEPROMPT = 0x00002000; public const int SHAREAWARE = 0x00004000; public const int NOREADONLYRETURN = 0x00008000; public const int NOTESTFILECREATE = 0x00010000; public const int HIDEMRUPLACES = 0x00020000; public const int HIDEPINNEDPLACES = 0x00040000; public const int NODEREFERENCELINKS = 0x00100000; public const int DONTADDTORECENT = 0x02000000; public const int FORCESHOWHIDDEN = 0x10000000; public const int DEFAULTNOMINIMODE = 0x20000000; public const int FORCEPREVIEWPANEON = 0x40000000; } [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)] public class OpenFileName { internal int structSize = 0; internal IntPtr hwndOwner = IntPtr.Zero; internal IntPtr hInstance = IntPtr.Zero; internal string filter = null; internal string custFilter = null; internal int custFilterMax = 0; internal int filterIndex = 0; internal string file = null; internal int maxFile = 0; internal string fileTitle = null; internal int maxFileTitle = 0; internal string initialDir = null; internal string title = null; internal int flags = 0; internal short fileOffset = 0; internal short fileExtMax = 0; internal string defExt = null; internal int custData = 0; internal IntPtr pHook = IntPtr.Zero; internal string template = null; } public class LibWrap { // Declare a managed prototype for the unmanaged function. [DllImport("Comdlg32.dll", SetLastError = true, ThrowOnUnmappableChar = true, CharSet = CharSet.Auto)] public static extern bool GetSaveFileName([In, Out] OpenFileName ofn); } public bool? ShowDialog() { var openFileName = new OpenFileName(); Window window = Application.Current.Windows.OfType<Window>().Where(w => w.IsActive).FirstOrDefault(); if (window != null) { var wih = new WindowInteropHelper(window); IntPtr hWnd = wih.Handle; openFileName.hwndOwner = hWnd; } openFileName.structSize = Marshal.SizeOf(openFileName); openFileName.filter = MakeFilterString(Filter); openFileName.filterIndex = FilterIndex; openFileName.fileTitle = new string(new char[64]); openFileName.maxFileTitle = openFileName.fileTitle.Length; openFileName.initialDir = InitialDirectory; openFileName.title = Title; openFileName.defExt = DefaultExt; openFileName.structSize = Marshal.SizeOf(openFileName); openFileName.flags |= FOS.NOTESTFILECREATE | FOS.OVERWRITEPROMPT; if (RestoreDirectory) openFileName.flags |= FOS.NOCHANGEDIR; // lpstrFile // Pointer to a buffer used to store filenames. When initializing the // dialog, this name is used as an initial value in the File Name edit // control. When files are selected and the function returns, the buffer // contains the full path to every file selected. char[] chars = new char[FILEBUFSIZE]; for (int i = 0; i < FileName.Length; i++) { chars[i] = FileName[i]; } openFileName.file = new string(chars); // nMaxFile // Size of the lpstrFile buffer in number of Unicode characters. openFileName.maxFile = FILEBUFSIZE; if (LibWrap.GetSaveFileName(openFileName)) { FileName = openFileName.file; return true; } return false; } /// <summary> /// Converts the given filter string to the format required in an OPENFILENAME_I /// structure. /// </summary> private static string MakeFilterString(string s, bool dereferenceLinks = true) { if (string.IsNullOrEmpty(s)) { // Workaround for VSWhidbey bug #95338 (carried over from Microsoft implementation) // Apparently, when filter is null, the common dialogs in Windows XP will not dereference // links properly. The work around is to provide a default filter; " |*.*" is used to // avoid localization issues from description text. // // This behavior is now documented in MSDN on the OPENFILENAME structure, so I don't // expect it to change anytime soon. if (dereferenceLinks && System.Environment.OSVersion.Version.Major >= 5) { s = " |*.*"; } else { // Even if we don't need the bug workaround, change empty // strings into null strings. return null; } } StringBuilder nullSeparatedFilter = new StringBuilder(s); // Replace the vertical bar with a null to conform to the Windows // filter string format requirements nullSeparatedFilter.Replace('|', ' '); // Append two nulls at the end nullSeparatedFilter.Append(' '); nullSeparatedFilter.Append(' '); // Return the results as a string. return nullSeparatedFilter.ToString(); } 注意其中的這句：
openFileName.flags |= FOS.NOTESTFILECREATE | FOS.OVERWRITEPROMPT; 因為我的需求就是不創建TestFile，所以我直接這麼寫而不是提供可選項。一個更好的方法是給WPF提ISSUE，我已經這麼做了：
Make SaveFileDialog support NOTESTFILECREATE.
但看來我等不到有人處理的這天，如果再有這種需求，還是將就著用我的這個自創的SaveFileDialog吧：
4. 參考
Common Item Dialog (Windows) Microsoft Docs
GetSaveFileNameA function (commdlg.h) – Win32 apps Microsoft Docs
OPENFILENAMEW (commdlg.h) – Win32 apps Microsoft Docs
