interface IFoo
{
    Task<IEnumerable<Bar>> BarsAsync { get; }
}

interface IFoo
{
    IEnumerable<Bar> Bars { get; }
}

internal class ExpensiveFoo : IFoo
{
    private readonly Lazy<IEnumerable<Bar>> bars;
    public IEnumerable<Bar> Bars { get { return bars.Value; } }
    public ExpensiveFoo() { bars = new Lazy<IEnumerable<Bar>>(GetBars); }
    private IEnumerable<Bar> GetBars() { … /* some expensive computation */ }
}

interface IFoo
{
    Task<IEnumerable<Bar>> BarsAsync { get; }
}

internal class ExpensiveFoo : IFoo
{
    private Lazy<Task<IEnumerable<Bar>>> bars;
    public Task<IEnumerable<Bar>> BarsAsync { get { return bars.Value; } }
    public ExpensiveFoo() { bars = new Lazy<Task<IEnumerable<Bar>>>(GetBarsAsync); }
    private async Task<IEnumerable<Bar>> GetBarsAsync() { … };
}

internal class CheapFoo : IFoo
{
    public Task<IEnumerable<Bar>> BarsAsync { get { return Task.FromResult(bars) } }
    private IEnumerable<Bar> bars;
    public CheapFoo(IEnumerable<Bar> bars) { this.bars = bars; }
}

Foo foo = …;
foreach (Bar bar in await foo.BarsAsync) …