[WPF]為什麼使用SaveFileDialog建立檔案需要刪除許可權？
阿新 • • 發佈：2020-04-07
## 1. 問題
好像很少人會遇到這種需求。假設有一個資料夾，使用者有幾乎所有許可權，但沒有刪除的許可權，如下圖所示：
這時候使用SaveFileDialog在這個資料夾裡建立檔案居然會報如下錯誤：
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，但檔案還是會創建出來，不過這是個空檔案。不僅WPF，普通的記事本也會有這個問題，SaveFileDialog會建立一個空檔案，記事本則沒有被儲存。具體可以看以下GIF：
## 2. 問題原因
其實當SaveFileDialog關閉前，對話方塊會建立一個測試檔案，用於檢查檔名、檔案許可權等，然後又刪除它。所以如果有檔案的建立許可權，而沒有檔案的刪除許可權，在建立測試檔案後就沒辦法刪除這個測試檔案，這時候就會報錯，而測試檔案留了下來。
有沒有發現`SaveFileDialog`中有一個屬性Options？
``` CS
//
// 摘要:
// 獲取 Win32 通用檔案對話方塊標誌，檔案對話方塊使用這些標誌來進行初始化。
//
// 返回結果:
// 一個包含 Win32 通用檔案對話方塊標誌的 System.Int32，檔案對話方塊使用這些標誌來進行初始化。
protected int Options { get; }
```
本來應該可以設定一個`NOTESTFILECREATE`的標誌位，但WPF中這個屬性是隻讀的，所以WPF的SaveFileDialog肯定會建立測試檔案。
## 3. 解決方案
SaveFileDialog本身只是Win32 API的封裝，我們可以參考SaveFileDialog的原始碼，偽裝一個呼叫方法差不多的MySaveFileDialog，然後自己封裝`GetSaveFileName`這個API。程式碼大致如下：
``` CS
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().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;
}
///
/// Converts the given filter string to the format required in an OPENFILENAME_I
/// structure.
///