Passing parameter into a Task.Factory.StartNew

问题:

Given the following code:

string injectedString = "Read string out of HttpContext";
Task.Factory.StartNew(() =>
 {
    MyClass myClass = new MyClass();
    myClass.Method(injectedString);
 }

Is this the best way to pass the string into the Task/Thread?

My concerns with this method are:

• Would the garbage collector know when the string became out of context and clean it up correctly?
• Is there a better way to inject dependencies into a Task breaking the link to the object in the main thread?

This is in a Asp.Net webservice, if it matters and is a fire and forget type thread, I'm not waiting for any kind of response.

My string is actually going to get read out of the HttpContext, which is one reason why I'm injecting it in this way (Thread does not have access to the calling threads HtppContext)

回答1:

Your lambda will be hoisted out into a compiler generated class. The injectedString variable will become a field of that class.

So, it will be garbage collected when the generated class is out of scope (which is basically at the very end of your lambda), and the GC decides to perform a collection.

In response to your comment:

There is no duplication. The compiler turns this:

 string injectedString = "Read string out of HttpContext";
 Task.Factory.StartNew(() =>
 {
    MyClass myClass = new MyClass();
    myClass.Method(injectedString);
 }

Into this:

CompilerGeneratedClass c1 = new CompilerGeneratedClass();
c1.injectedString = "Read string out of HttpContext";
// call delegate here.

Remember also: Strings are interned in the CLR. Even if the code was duplicated.. string literals will be interned in a pool. You would essentially only have a native WORD sized reference duplicated that pointed at the string (string literals only..)

回答2:

You should probably use the Task.Factory.StartNew(Action<object> action, object state) overload to pass state into your new task.

Task.Factory.StartNew((object myState) => {
    var i = (int)myState;

    //Do calculations...
    var x = i + 10; 
}, 10);

回答3:

If you're concerned about injectedString may be "garbage collected" before myClass.Method(injectedString); runs ?

Answer is no. You don't need to worry about that, because injectedString is no more a local variable when it is closed in the lambda. It will become a field in new compiler generated class.

If you're concerned about will the garbage collector collect it at right time? Answer is yes, It will do when the instance of that class goes out of scope and no refernces to it in managed code. That would happen certainly after Task gets completed and gets out of scope.