CODEWITHSUNDEEP
c#.netcode-contracts
ronag
ronag

Reputation: 51253

Ensures that object is unchanged?

Is there any way using Code Contracts to ensure/check that a method does not change any members of an object, similarly to C++ const methods or setting all the members to readonly?

i.e. a simpler way than the following:

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));

or the same using Contract.EnsuresOnThrow.

Upvotes: 2

Views: 295

Answers (2)

ddur
ddur

Reputation: 75

If your class implements

ICloneable

and

IEquatable<YourClassType>

as value equality (not reference equality), than you can write:

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

Upvotes: 0

Mightymuke
Mightymuke

Reputation: 5144

So essentially you want to check that the method is Pure. The official documentation indicates that its not yet supported (see section 5.4):

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.

Upvotes: 2

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