我需要打开一个文件,但如果该文件当前不可用,我需要等待,直到它准备好。采取什么最好的方法?
场景
我正在使用文件作为应用程序数据的持久缓存机制。这些数据需要经常读取和反序列化(只写入一次,并偶尔删除)。我有一个清理进程在单独的线程上运行,确定哪些文件不再需要并将它们删除。文件的打开和读取可能会同时发生(很少,但可能发生),我希望该过程等待并尝试重新读取数据。
谢谢!
我不是try/catch IOException的忠实拥护者,因为:
您可以通过调用CreateFile并在最终返回句柄时返回流来避免使用异常:
public static System.IO.Stream WaitForExclusiveFileAccess(string filePath, int timeout)
{
IntPtr fHandle;
int errorCode;
DateTime start = DateTime.Now;
while(true)
{
fHandle = CreateFile(filePath, EFileAccess.GenericRead | EFileAccess.GenericWrite, EFileShare.None, IntPtr.Zero,
ECreationDisposition.OpenExisting, EFileAttributes.Normal, IntPtr.Zero);
if (fHandle != IntPtr.Zero && fHandle.ToInt64() != -1L)
return new System.IO.FileStream(fHandle, System.IO.FileAccess.ReadWrite, true);
errorCode = Marshal.GetLastWin32Error();
if (errorCode != ERROR_SHARING_VIOLATION)
break;
if (timeout >= 0 && (DateTime.Now - start).TotalMilliseconds > timeout)
break;
System.Threading.Thread.Sleep(100);
}
throw new System.IO.IOException(new System.ComponentModel.Win32Exception(errorCode).Message, errorCode);
}
#region Win32
const int ERROR_SHARING_VIOLATION = 32;
[Flags]
enum EFileAccess : uint
{
GenericRead = 0x80000000,
GenericWrite = 0x40000000
}
[Flags]
enum EFileShare : uint
{
None = 0x00000000,
}
enum ECreationDisposition : uint
{
OpenExisting = 3,
}
[Flags]
enum EFileAttributes : uint
{
Normal = 0x00000080,
}
[DllImport("kernel32.dll", EntryPoint = "CreateFileW", SetLastError = true, CharSet = CharSet.Unicode)]
static extern IntPtr CreateFile(
string lpFileName,
EFileAccess dwDesiredAccess,
EFileShare dwShareMode,
IntPtr lpSecurityAttributes,
ECreationDisposition dwCreationDisposition,
EFileAttributes dwFlagsAndAttributes,
IntPtr hTemplateFile);
#endregion
public static FileStream WaitForFileAccess(string filePath, FileMode fileMode, FileAccess access, FileShare share, TimeSpan timeout)
{
int errorCode;
DateTime start = DateTime.Now;
while (true)
{
SafeFileHandle fileHandle = CreateFile(filePath, ConvertFileAccess(access), ConvertFileShare(share), IntPtr.Zero,
ConvertFileMode(fileMode), EFileAttributes.Normal, IntPtr.Zero);
if (!fileHandle.IsInvalid)
{
return new FileStream(fileHandle, access);
}
errorCode = Marshal.GetLastWin32Error();
if (errorCode != ERROR_SHARING_VIOLATION)
{
break;
}
if ((DateTime.Now - start) > timeout)
{
return null; // timeout isn't an exception
}
Thread.Sleep(100);
}
throw new IOException(new Win32Exception(errorCode).Message, errorCode);
}
private static EFileAccess ConvertFileAccess(FileAccess access)
{
return access == FileAccess.ReadWrite ? EFileAccess.GenericRead | EFileAccess.GenericWrite : access == FileAccess.Read ? EFileAccess.GenericRead : EFileAccess.GenericWrite;
}
private static EFileShare ConvertFileShare(FileShare share)
{
return (EFileShare) ((uint) share);
}
private static ECreationDisposition ConvertFileMode(FileMode mode)
{
return mode == FileMode.Open ? ECreationDisposition.OpenExisting : mode == FileMode.OpenOrCreate ? ECreationDisposition.OpenAlways : (ECreationDisposition) (uint) mode;
}
[DllImport("kernel32.dll", EntryPoint = "CreateFileW", SetLastError = true, CharSet = CharSet.Unicode)]
private static extern SafeFileHandle CreateFile(
string lpFileName,
EFileAccess dwDesiredAccess,
EFileShare dwShareMode,
IntPtr lpSecurityAttributes,
ECreationDisposition dwCreationDisposition,
EFileAttributes dwFlagsAndAttributes,
IntPtr hTemplateFile);
FileStream f = null;
while (f == null)
{
try
{
f = new FileStream(...);
}
catch (IOException)
{
// wait a bit and try again
Thread.Sleep(5000);
}
}
当然,您可能不想无条件地捕获 IOException
。您可能想要捕获您知道如何处理的具体异常(例如,如果您遇到 DirectoryNotFoundException
,您可能不想再尝试)。I/O函数记录了它们预计抛出哪些异常以及在什么情况下抛出。
像所有“最佳方法是什么”的问题一样,这取决于您的需求。一些容易想到的选项:
您选择哪个取决于您如何处理它。