How to get return value from call back method in c#

Question

I passed "GetFilesRevisions_Results" method for GetAsync which will handle the result for it.But I want to return a integer from "GetFilesRevisions_Results" method.How I can achieve this ? Thanks in advance!

private int GetVersionNumber(string i_sFileName)
{
    #region Get latest version no.
    int nVerNo = 0;
   // RequestResult result;
    try
    {

    OAuthUtility.GetAsync
       (
       "https://api.dropboxapi.com/1/revisions/auto/",
           new HttpParameterCollection
                        {
                           { "path", i_sFileName },
                           { "access_token", accessToken },
                           { "rev_limit", 1 }
                        },
          callback:  GetFilesRevisions_Results ??? How I can access return variable 
       );
    }
    catch
    { 

    }
    return nVerNo;
    #endregion
}

private int GetFilesRevisions_Results(RequestResult result)
{
    int nVerNo = 0;

    if (result.StatusCode == 200)
    {
        dynamic dynJson = JsonConvert.DeserializeObject(Convert.ToString(result));
        foreach (var item in dynJson)
        {
            nVerNo = Convert.ToInt32(item.rev);
        }

    }
    else
    {
        throw new Exception("Failed to get revisions of files");
    }

    return  nVerNo;
}
#endregion Get version Number

score 2 · Answer 1 · answered Dec 21 '15 at 14:35

There's no way for you to know when your callback will be invoked, thus a return value is not the proper way to get your int.

You may use an event with an int argument and invoke it from within GetFilesRevisions_Results, just before the return. You will then be able to use your integer value from any of this event listeners.

score 1 · Answer 2 · answered Dec 21 '15 at 15:00

You probably want to use a wait handle:

AutoResetEvent waitHandle = new AutoResetEvent(false);
int nVerNoGlobalTempHolder = 0;

private int GetVersionNumber(string i_sFileName)
{
    #region Get latest version no.
    for (int i = 0; i < 10 && nVerNoGlobalTempHolder != 0; i++)
    {
       //Someone is waiting for this callback already...
       //Do something like:
       Thread.Sleep(500);
    }
    If (nVerNoGlobalTempHolder == 0) throw new Exception("timeout");

   // RequestResult result;
    try
    {

    OAuthUtility.GetAsync
       (
       "https://api.dropboxapi.com/1/revisions/auto/",
           new HttpParameterCollection
                        {
                           { "path", i_sFileName },
                           { "access_token", accessToken },
                           { "rev_limit", 1 }
                        },
          callback:  GetFilesRevisions_Results ??? How I can access return variable 
       );
    }
    catch
    { 

    }
    waitHandle.WaitOne();
    int nVerNo =nVerNoGlobalTempHolder;

    nVerNoGlobalTempHolder = 0;//Reset this in case you have multiple thread calling it
    return nVerNo;    
}


private int GetFilesRevisions_Results(RequestResult result)
{

    if (result.StatusCode == 200)
    {
        dynamic dynJson = JsonConvert.DeserializeObject(Convert.ToString(result));
        foreach (var item in dynJson)
        {
            nVerNoGlobalTempHolder = Convert.ToInt32(item.rev);
        }

    }
    else
    {
        throw new Exception("Failed to get revisions of files");
    }
    WaitHandle.Set();

}

This also implements very basic syncing in case more than one thread calls it. If you you dont need that, remove the the for loop at the start

If what you need is to handle the value from within `GetVersionNumber`, you will have to use a global scoped synchronization object. You may use a [captured variable](http://stackoverflow.com/questions/148669/local-variables-with-delegates) instead, by making your `GetFilesRevisions_Results` a delegate, but that would still be similar to a global variable. — aegar, Dec 22 '15 at 08:44

How to get return value from call back method in c#

2 Answers2