c# - Multiple callers waiting on long running GET, any way to make them all wait on the same return? -

i running wcf rest service , have discovered bit of bottleneck. current code boils down below:

private static list<widget> widgets;    public async task<list<searchresult>> search(string term) {     if(widgets == null) {         // call takes 60 seconds         widgets = await getwidgets();     }     return searchutil.search(term, widgets); } 

the problem many requests can enter if check , call long running operation. instead, want additional incoming requests wait on original call complete , 1 call getwidgets() made. how can achieve stop firing off many requests when dictionary empty?

as small aside, safe assume list/dictionary remain populated entire time service alive? or empty reason? best way handle type of situation (i'm guessing other mechanism cache)?

thanks!

if understand correctly, want cache never expires , populated first time requests value. should pretty straightforward build in generic way:

class nonexpiringlazyloadingcache<t> {     private readonly func<task<t>> _factory;     private task<t> _retrievaltask;     private readonly object _lockobject = new object();      public nonexpiringlazyloadingcache(func<task<t>> factory)     {         this._factory = factory;     }      public async task<t> getvalue()     {         lock (this._lockobject)             if (this._retrievaltask == null)                 this._retrievaltask = this._factory();          await this._retrievaltask;         return this._retrievaltask.result;     } } 

key things note task<t> await return when task completed, thread safe; task guaranteed executed once.

usage:

 private static nonexpiringlazyloadingcache<list<widget>> cache = new nonexpiringlazyloadingcache<list<widget>>(getwidgets);  ...  var widgets = await cache.getvalue(); 

as duration of object's lifetime, depends on wcf service hosted. stays long process stays alive, iis hosted services value cleared when application pool recycled.