Caching Intensive calculation in Getter properties

Question

In the following code:

public class SomeClass { // ... constructor and other stuff public in SomeProperty { get { return SomeHeayCalculation(); } } } 

I consider the class immutable, so every time I call SomeProperty , the same value is returned. My question is whether it is possible to calculate the value each time. Is there a built-in caching mechanism for such materials?

Answer 1

Yup - Lazy<T> , assuming you are using .NET 4:

 public class SomeClass { private readonly Lazy<Foo> foo = new Lazy<Foo>(SomeHeayCalculation); // ... constructor and other stuff public Foo SomeProperty { get { return foo.Value; } } } 

I assume that you are trying to avoid doing the calculation if the property never accesses. Otherwise, just follow it when building.

Note that properties are often understood to be “cheap” for pricing — and while you are doing it lazy so that later calls are cheap, it will still potentially be “heavy” enough for the first access to make the property inappropriate. Instead, consider the ComputeXyz method.

Answer 2

Just cache the calculation in a private variable like this:

 public class SomeClass { // ... constructor and other stuff private int? calculation = null; public int SomeProperty { get { if (!calculation.HasValue) calculation = SomeHeayCalculation(); return calculation.Value; } } } 

Answer 3

Besides what John suggested, you can use this template:

 public class SomeClass { // ... constructor and other stuff private Foo _foo; public Foo SomeProperty { get { return _foo ?? (_foo = SomeHeayCalculation()); } } } 

It's worth noting that this really breaks (reading: becomes less readable) for value types if you don't want to wrap them in Nullable<T> . In this case, you can stick with Lazy<T> if available.

Answer 4

Just save the flag to remember if calculations have been made.

 public class SomeClass { // ... constructor and other stuff private bool _propertyCalculated; private int _someProperty; public int SomeProperty { get { if (!_propertyCaculated) { _someProperty = SomeHeayCalculation(); _propertyCaculated = true; } return _someProperty; } } }