class SomeClass
{
    private Task<ValueType> foo = null;
    private readonly SemaphoreSlim sem = new SemaphoreSlim();

    /* ... */

    Task<ValueType> getFoo()
    {
        var foo = this.foo;
        if (!foo) {
            return asyncInitFoo();
        }

        return foo;
    }

    async Task<ValueType> asyncInitFoo()
    {
        await sem.WaitAsync();
        try
        {
            if (foo == null) {
                foo = Task.FromResult(/* ... */);
            }

            return foo;
        }
        finally
        {
            sem.Release();
        }
    }

    /* ... */
}