【推荐】2019 Java 开发者跳槽指南.pdf(吐血整理) >>> 
我正在用C＃编写一个程序，该程序需要重复访问1个图像文件。 在大多数情况下，它都可以工作，但是如果我的计算机运行速度很快，它将在尝试将文件保存回文件系统之前尝试访问该文件，并抛出错误： “文件正在被另一个进程使用” 。
我想找到一种解决方法，但是我所有的Google搜索都只能通过使用异常处理来创建检查。 这与我的宗教信仰背道而驰，所以我想知道是否有人能做得更好？
#1楼
使用此命令检查文件是否被锁定：
using System.IO;
using System.Runtime.InteropServices;
internal static class Helper
{
const int ERROR_SHARING_VIOLATION = 32;
const int ERROR_LOCK_VIOLATION = 33;
private static bool IsFileLocked(Exception exception)
{
int errorCode = Marshal.GetHRForException(exception) & ((1 << 16) - 1);
return errorCode == ERROR_SHARING_VIOLATION || errorCode == ERROR_LOCK_VIOLATION;
}
internal static bool CanReadFile(string filePath)
{
//Try-Catch so we dont crash the program and can check the exception
try {
//The "using" is important because FileStream implements IDisposable and
//"using" will avoid a heap exhaustion situation when too many handles
//are left undisposed.
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None)) {
if (fileStream != null) fileStream.Close(); //This line is me being overly cautious, fileStream will never be null unless an exception occurs... and I know the "using" does it but its helpful to be explicit - especially when we encounter errors - at least for me anyway!
}
}
catch (IOException ex) {
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex)) {
// do something, eg File.Copy or present the user with a MsgBox - I do not recommend Killing the process that is locking the file
return false;
}
}
finally
{ }
return true;
}
}
出于性能原因，我建议您以相同的操作读取文件内容。 这里有些例子：
public static byte[] ReadFileBytes(string filePath)
{
byte[] buffer = null;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
sum += count; // sum is a buffer offset for next reading
fileStream.Close(); //This is not needed, just me being paranoid and explicitly releasing resources ASAP
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{
}
return buffer;
}
public static string ReadFileTextWithEncoding(string filePath)
{
string fileContents = string.Empty;
byte[] buffer;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
{
sum += count; // sum is a buffer offset for next reading
}
fileStream.Close(); //Again - this is not needed, just me being paranoid and explicitly releasing resources ASAP
//Depending on the encoding you wish to use - I'll leave that up to you
fileContents = System.Text.Encoding.Default.GetString(buffer);
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{ }
return fileContents;
}
public static string ReadFileTextNoEncoding(string filePath)
{
string fileContents = string.Empty;
byte[] buffer;
try
{
using (FileStream fileStream = File.Open(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.None))
{
int length = (int)fileStream.Length; // get file length
buffer = new byte[length]; // create buffer
int count; // actual number of bytes read
int sum = 0; // total number of bytes read
// read until Read method returns 0 (end of the stream has been reached)
while ((count = fileStream.Read(buffer, sum, length - sum)) > 0)
{
sum += count; // sum is a buffer offset for next reading
}
fileStream.Close(); //Again - this is not needed, just me being paranoid and explicitly releasing resources ASAP
char[] chars = new char[buffer.Length / sizeof(char) + 1];
System.Buffer.BlockCopy(buffer, 0, chars, 0, buffer.Length);
fileContents = new string(chars);
}
}
catch (IOException ex)
{
//THE FUNKY MAGIC - TO SEE IF THIS FILE REALLY IS LOCKED!!!
if (IsFileLocked(ex))
{
// do something?
}
}
catch (Exception ex)
{
}
finally
{
}
return fileContents;
}
自己尝试一下：
byte[] output1 = Helper.ReadFileBytes(@"c:\temp\test.txt");
string output2 = Helper.ReadFileTextWithEncoding(@"c:\temp\test.txt");
string output3 = Helper.ReadFileTextNoEncoding(@"c:\temp\test.txt");
#2楼
以我的经验，您通常要这样做，然后“保护”文件以做一些花哨的事情，然后使用“受保护”的文件。 如果只有这样一个文件要使用，可以使用Jeremy Thompson的答案中解释的技巧。 但是，如果您尝试对许多文件执行此操作（例如，当您编写安装程序时），则可能会遭受很多伤害。
解决这个问题的一种非常优雅的方法是使用以下事实：如果文件系统中的文件之一正在使用，则您的文件系统将不允许您更改其名称。 将文件夹保留在同一文件系统中，它将像超级按钮一样工作。
请注意，您应该意识到可以利用此漏洞的明显方法。 毕竟，文件不会被锁定。 另外，请注意还有其他原因可能导致“ Move操作失败。 显然，正确的错误处理（MSDN）可以为您提供帮助。
var originalFolder = @"c:\myHugeCollectionOfFiles"; // your folder name here
var someFolder = Path.Combine(originalFolder, "..", Guid.NewGuid().ToString("N"));
try
{
Directory.Move(originalFolder, someFolder);
// Use files
}
catch // TODO: proper exception handling
{
// Inform user, take action
}
finally
{
Directory.Move(someFolder, originalFolder);
}
对于单个文件，我会坚持Jeremy Thompson发表的锁定建议。
#3楼
只需按预期使用异常即可。 接受该文件正在使用中，然后重试，直到操作完成。 这也是最有效的方法，因为您在执行操作之前不会浪费任何时间检查状态。
例如，使用以下功能
TimeoutFileAction(() => { System.IO.File.etc...; return null; } );
可重用的方法在2秒后超时
private T TimeoutFileAction<T>(Func<T> func)
{
var started = DateTime.UtcNow;
while ((DateTime.UtcNow - started).TotalMilliseconds < 2000)
{
try
{
return func();
}
catch (System.IO.IOException exception)
{
//ignore, or log somewhere if you want to
}
}
return default(T);
}
#4楼
上面接受的答案会遇到以下问题：如果已打开文件以FileShare.Read模式写入文件，或者该文件具有“只读”属性，则代码将不起作用。 修改后的解决方案最可靠地工作，需要牢记两点（对于公认的解决方案也是如此）：
- 它不适用于以写共享模式打开的文件
- 这没有考虑线程问题，因此您需要将其锁定或单独处理线程问题。
牢记以上几点，这将检查文件是被锁定以进行写入还是被锁定以防止读取 ：
public static bool FileLocked(string FileName)
{
FileStream fs = null;
try
{
// NOTE: This doesn't handle situations where file is opened for writing by another process but put into write shared mode, it will not throw an exception and won't show it as write locked
fs = File.Open(FileName, FileMode.Open, FileAccess.ReadWrite, FileShare.None); // If we can't open file for reading and writing then it's locked by another process for writing
}
catch (UnauthorizedAccessException) // https://msdn.microsoft.com/en-us/library/y973b725(v=vs.110).aspx
{
// This is because the file is Read-Only and we tried to open in ReadWrite mode, now try to open in Read only mode
try
{
fs = File.Open(FileName, FileMode.Open, FileAccess.Read, FileShare.None);
}
catch (Exception)
{
return true; // This file has been locked, we can't even open it to read
}
}
catch (Exception)
{
return true; // This file has been locked
}
finally
{
if (fs != null)
fs.Close();
}
return false;
}
#5楼
您可以返回一个任务，该任务将在流可用时立即为您提供。 这是一个简化的解决方案，但这是一个很好的起点。 这是线程安全的。
private async Task<Stream> GetStreamAsync()
{
try
{
return new FileStream("sample.mp3", FileMode.Open, FileAccess.Write);
}
catch (IOException)
{
await Task.Delay(TimeSpan.FromSeconds(1));
return await GetStreamAsync();
}
}
您可以照常使用此流：
using (var stream = await FileStreamGetter.GetStreamAsync())
{
Console.WriteLine(stream.Length);
}
来源：oschina
链接：https://my.oschina.net/u/3797416/blog/3144803