Purpose of PureAttribute on parameter

Question

I understand that the PureAttribute is used to mark something (class, method, delegate etc.) as making no visible changes, but I can see from the following definition that it can be applied to method parameters:

[AttributeUsageAttribute(AttributeTargets.Class|AttributeTargets.Constructor|AttributeTargets.Method|AttributeTargets.Property|AttributeTargets.Event|AttributeTargets.Parameter|AttributeTargets.Delegate, AllowMultiple = false, Inherited = true)]
public sealed class PureAttribute : Attribute

What is the purpose of this attribute being applied to a parameter, such as in the following:

public void SomeMethod([Pure]SomeClass theParameter)
{
}

Does it imply that SomeMethod should not use anything on theParameter which is not marked as [Pure], meaning we can ensure the instance of SomeClass visibly appears the same before and after the invocation of SomeMethod?

I have not seen the PureAttribute used in this way and was wondering if this is due to lack of support in code contracts or because of a misunderstanding of mine?

Answer 1

The PureAttribute, as you stated, indicates that a type or method is pure, that is, it does not make any visible state changes (taken straight from the MSDN article).

Maybe your SomeClass is not marked as pure because it can change the state, however it does not mean that everything in it is impure.

Maybe your SomeMethod doesn't use any of SomeClass impure methods, maybe it simply reads its properties (I'm assuming you're not performing impure action in property getters, otherwise you're evil), so its usage of SomeClass is pure.­­­­­