c# - Cached property vs Lazy<T>

Question

In .NET 4 the following snippet with a cached property can also be written using the System.Lazy<T> class. I measured the performance of both approaches and it's pretty much the same. Is there any real benefit or magic for why I should use one over the other?

Cached Property

public static class Brushes
{
    private static LinearGradientBrush _myBrush;

    public static LinearGradientBrush MyBrush
    {
        get
        {
            if (_myBrush == null)
            {
                var linearGradientBrush = new LinearGradientBrush { ...};
                linearGradientBrush.GradientStops.Add( ... );
                linearGradientBrush.GradientStops.Add( ... );

                _myBrush = linearGradientBrush;
            }

            return _myBrush;
        }
    }
}

Lazy<T>

public static class Brushes
{
    private static readonly Lazy<LinearGradientBrush> _myBrush =
        new Lazy<LinearGradientBrush>(() =>
            {
                var linearGradientBrush = new LinearGradientBrush { ...};
                linearGradientBrush.GradientStops.Add( ... );
                linearGradientBrush.GradientStops.Add( ... );

                return linearGradientBrush;
            }
        );

    public static LinearGradientBrush MyBrush
    {
        get { return _myBrush.Value; }
    }
}

Answer 1

I would use Lazy<T> in general:

• It's thread-safe (may not be an issue in this case, but would be in others)
• It makes it obvious what's going on just by the name
• It allows null to be a valid value

Note that you don't have to use a lambda expression for the delegate. For example, here's an approach which may be slightly cleaner:

public static class Brushes
{
    private static readonly Lazy<LinearGradientBrush> _myBrush =
        new Lazy<LinearGradientBrush>(CreateMyBrush);

    private static LinearGradientBrush CreateMyBrush()
    {
        var linearGradientBrush = new LinearGradientBrush { ...};
        linearGradientBrush.GradientStops.Add( ... );
        linearGradientBrush.GradientStops.Add( ... );

        return linearGradientBrush;
    }

    public static LinearGradientBrush MyBrush
    {
        get { return _myBrush.Value; }
    }
}

This is particularly handy when the creation process gets complicated with loops etc. Note that by the looks of it, you could use a collection initializer for GradientStops in your creation code.

Another option is not to do this lazily, of course... unless you have several such properties in your class and you only want to create the relevant objects on a one-by-one basis, you could rely on lazy class initialization for many situations.

As noted in DoubleDown's answer, there's no way of resetting this to force recomputation (unless you make the Lazy<T> field not readonly) - but I've very rarely found that to be important.