HttpContext is null in a method called inside Parallel.For

问题

Posting this question after trying a lot. Doing normal for is not an option because we need to do a large amount of processing in very less time.

I have GetDataFor() inside which HttpContext.Current is used.

The code looks like this:

public void SomeMethod()
{
    var context = HttpContext.Current;

    Parallel.For(0, 100, i =>
    {
        var data = GetDataFor(i, context);
    });
}

public data GetDataFor(int i, HttpContext context)
{
    Uri requestUri = null;
    if (HttpContext.Current != null)
    {
        requestUri = HttpContext.Current.Request.Url;
        sCookie = string.Format("{0}", HttpContext.Current.Request.Headers["cookie"]);
    }
    else
    {
        requestUri = context.Request.Url;
    }

    //do something
    return data;
}

Everything works fine inside normal for loop. However, when I call it inside Parallel.For and pass HttpContext.Current, HttpContext.Current.Request, HttpContext.Current.Request.Url as method parameters:

HttpContext.Current cannot be serialized because it does not have a parameterless constructor

Passing HttpContextBase httpContext = null as parameter throws:

To be XML serializable, types which inherit from ICollection must have an implementation of Add(System.Object) at all levels of their inheritance hierarchy. System.Web.HttpApplicationStateBase does not implement Add(System.Object).

Tried making a property:

public string[] httpContextData
{
    get
    {
        string requestUrl = HttpContext.Current.Request.Url.ToString();
        string sCookie = string.Format("{0}", HttpContext.Current.Request.Headers["cookie"]);
        return new string[] { requestUrl, sCookie };
    }
}

and using in method:

var contextData = httpContextData;

which throws:

System.Uri cannot be serialized because it does not have a parameterless constructor

I did all this to send it's reference and state but unable to understand why the problem is not solving.

How do I use HttpContext.Current inside Parallel.For? What am I doing wrong here?

Btw, the needed stuff are:

HttpContext.Current.Request.Url and HttpContext.Current.Request.Headers["cookie"]

回答1:

HttpContext.Current is only available (not null) inside request-handling threads. Parallel.For creates multiple threads, none of which is has access to HttpContext.Current.

You have to pass all data that code in Parallel.For threads needs either through

local variables assigned before the loop or
TLocal instance used in Parallel.For<TLocal>.

In any event, code such as HttpContext.Current.... is out.

回答2:

The code is proprietary hence I'm only posting the relevant parts:

Since passing the following objects:

HttpContext.Current
HttpContext.Current.Request
HttpContext.Current.Request.Url

as params to GetDataFor was throwing so many errors.

Also, my needs were only

request url which can be re-generated by passing url as string to it's constructor
and a request header's value which is essentially a string

I only passed string to GetDataFor() method:

public void SomeMethod()
{
    string requestUrl = HttpContext.Current.Request.Url.ToString();
    string sCookie = string.Format("{0}", HttpContext.Current.Request.Headers["cookie"]);

    Parallel.For(0, 100, i =>
    {
        var data = GetDataFor(i,
    requestUrl: requestUrl,
    sCookie: sCookie);
    });
}

public data GetDataFor(int i, string requestUrl = null, string sCookie = null)
{
    Uri requestUri = null;
    if (HttpContext.Current != null)
    {
        requestUri = HttpContext.Current.Request.Url;
        sCookie = string.Format("{0}", HttpContext.Current.Request.Headers["cookie"]);
    }
    else
    {
        requestUri = new Uri(requestUrl);
    }

    //do something
    return data;
}

来源：https://stackoverflow.com/questions/35483658/httpcontext-is-null-in-a-method-called-inside-parallel-for

标签

asynchronous

parallel-processing

null

httpcontext