防止同时读取和写入文件

Question

I have a process that needs to read and write to a file. The application has a specific order to its reads and writes and I want to preserve this order. What I would like to do is implement something that lets the first operation start and makes the second operation wait until the first is done with a first come first served like of queue to access the file. From what I have read file locking seems like it might be what I am looking for but I have not been able to find a very good example. Can anyone provide one?

Currently I am using a TextReader/Writer with .Synchronized but this is not doing what I hoped it would.

Sorry if this is a very basic question, threading gives me a headache :S

Answer 1

It should be as simple as this:

public static readonly object LockObj = new object();

public void AnOperation()
{
    lock (LockObj)
    {
        using (var fs = File.Open("yourfile.bin"))
        {
            // do something with file
        }
    }
}

public void SomeOperation()
{
    lock (LockObj)
    {
        using (var fs = File.Open("yourfile.bin"))
        {
            // do something else with file
        }
    }
}

Basically, define a lock object, then whenever you need to do something with your file, make sure you get a lock using the C# lock keyword. On reaching the lock statement, execution will block indefinitely until a lock has been obtained.

There are other constructs you can use for locking, but I find the lock keyword to be the most straightforward.

Answer 2

If you're using a current version of the .Net Framework, you can benefit from Task.ContinueWith .

If your units of work are logically always, "read some, then write some", the following expresses that intent succinctly and should scale:

string path = "file.dat";

// Start a reader task
var task = Task.Factory.StartNew(() => ReadFromFile(path));

// Continue with a writer task
task.ContinueWith(tt => WriteToFile(path));

// We're guaranteed that the read will occur before the write
// and that the write will occur once the read completes.
// We also can check the antecedent task's result (tt.Result in our
// example) for any special error logic we need.