[WPF]为什么使用SaveFileDialog创建文件需要删除权限？

1. 问题

好像很少人会遇到这种需求。假设有一个文件夹，用户有几乎所有权限，但没有删除的权限，如下图所示：

这时候使用SaveFileDialog在这个文件夹里创建文件居然会报如下错误：

这哪里是网络位置了，我又哪里去找个管理员？更奇怪的是，虽然报错了，但文件还是会创建出来，不过这是个空文件。不仅WPF，普通的记事本也会有这个问题，SaveFileDialog会创建一个空文件，记事本则没有被保存。具体可以看以下GIF：

2. 问题原因

其实当SaveFileDialog关闭前，对话框会创建一个测试文件，用于检查文件名、文件权限等，然后又删除它。所以如果有文件的创建权限，而没有文件的删除权限，在创建测试文件后就没办法删除这个测试文件，这时候就会报错，而测试文件留了下来。

有没有发现SaveFileDialog中有一个属性Options？

//

// 摘要:

//     获取 Win32 通用文件对话框标志，文件对话框使用这些标志来进行初始化。

//

// 返回结果:

//     一个包含 Win32 通用文件对话框标志的 System.Int32，文件对话框使用这些标志来进行初始化。

protected int Options { get; }

本来应该可以设置一个NOTESTFILECREATE的标志位，但WPF中这个属性是只读的，所以WPF的SaveFileDialog肯定会创建测试文件。

3. 解决方案

SaveFileDialog本身只是Win32 API的封装，我们可以参考SaveFileDialog的源码，伪装一个调用方法差不多的MySaveFileDialog，然后自己封装GetSaveFileName这个API。代码大致如下：

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('|', '\0');

    // Append two nulls at the end

    nullSeparatedFilter.Append('\0');

    nullSeparatedFilter.Append('\0');

    // Return the results as a string.

    return nullSeparatedFilter.ToString();

}

注意其中的这句：

openFileName.flags |= FOS.NOTESTFILECREATE | FOS.OVERWRITEPROMPT;

因为我的需求就是不创建TestFile，所以我直接这么写而不是提供可选项。一个更好的方法是给WPF提ISSUE，我已经这么做了：

Make SaveFileDialog support NOTESTFILECREATE.

但看来我等不到有人处理的这天，如果再有这种需求，还是将就着用我的这个自创的SaveFileDialog吧：

CustomSaveFileDialog

4. 参考

Common Item Dialog (Windows) Microsoft Docs

GetSaveFileNameA function (commdlg.h) - Win32 apps Microsoft Docs

OPENFILENAMEW (commdlg.h) - Win32 apps Microsoft Docs

巴特西

[WPF]为什么使用SaveFileDialog创建文件需要删除权限？

1. 问题

2. 问题原因

3. 解决方案

4. 参考

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