ReusableTask

A .NET Standard 2.0 compatible library which can be used to implement zero allocation async/await. This is conceptually similar to ValueTask<T>, except it's compatible with .NET 2.0 and has zero ongoing allocations once the cache initializes.

Sample usage:

public async ReusableTask InitializeAsync ()
{
    if (!Initialized) {
        await LongInitializationAsync ();
        Initialized = true;
    }
}

async ReusableTask LongInitializationAsync ()
{
    // Contact a database, load from a file, etc
}

public async ReusableTask<Cache> CreateCacheAsync ()
{
    if (!Initialized) {
        cache = await LongCacheCreationAsync ();
        Initialized = true;
    }
    return cache;
}

async ReusableTask<Cache> LongCacheCreationAsync ()
{
    // Contact a database, load from a file, etc..
    return new Cache (....);
}

The compiler generated async state machine for these methods is allocation-free, and results in no garbage collectable objects being created no matter how many times the methods are invoked.

The four things you cannot do with ValueTask you also cannot do with ReusableTask. The documentation can be read here in the remarks section, https://docs.microsoft.com/en-us/dotnet/api/system.threading.tasks.valuetask-1?view=netcore-3.0#remarks.

Unlike the documentation states for ValueTask, I would recommend that the default return value for any async method should be ReusableTask or ReusableTask<T>, unless benchmarking shows otherwise.