确保对象不变？

Question

有没有办法使用代码契约来确保/检查方法不会更改对象的任何成员，类似于 C++ const 方法或将所有成员设置为 readonly？

即比以下更简单的方法：

Contract.Ensures(this.member1 == Contract.OldValue(this.member1));
Contract.Ensures(this.member2 == Contract.OldValue(this.member2));
Contract.Ensures(this.member3 == Contract.OldValue(this.member3));
Contract.Ensures(this.member4 == Contract.OldValue(this.member4));
Contract.Ensures(this.member5 == Contract.OldValue(this.member5));

或同样使用Contract.EnsuresOnThrow。

Answer 1

所以本质上你想检查方法是Pure。 official documentation 表示它尚不支持（参见第 5.4 节）：

5.4 Purity

All methods called within a contract must be pure: that is, they must not update
any pre-existing state. (A pure method is allowed to modify objects that have been
created after entry into the pure method.) Code Contract tools currently assume
the following things are pure:

* Methods marked [Pure] (If a type is marked [Pure], then that applies to all of
  its methods.) The pure attribute is dened in the contract library. (Section 4.3)

* Property getters.

* Operators (static methods whose names start with op , have one or two parameters
  and a non-void return type).

* Any method whose fully qualified name begins with
  System.Diagnostics.Contracts.Contract, System.String, System.IO.Path, or
  System.Type.

* Any invoked delegate, provided that the delegate type itself is attributed with
  [Pure]. The existing delegate types System.Predicate<T> and System.Comparison<T>
  are considered pure.

In the future, there will be a purity checker that will enforce these assumptions.

Answer 2

如果你的类实现了

ICloneable

和

IEquatable<YourClassType>

作为值相等（不是引用相等），你可以这样写：

Contract.Ensures( 
    this.Equals ( Contract.OldValue ( (YourClassType)this.Clone() )
);