Code contracts on auto-implemented properties

Question

Is there any way to put contracts on automatically implemented properties in .NET? (And how if the answer is 'Yes')?

(I assume using .NET code contracts from DevLabs)

Answer 1

Yes, this is possible - all that is needed is to add your contract condition to the [ContractInvariantMethod] method in your class, which then adds the equivalent Requires precondition to the automatic setter, and a post condition Ensures is added to the get. From section 2.3.1 of the Reference

As the example illustrates, invariants on auto-properties turn into:

1. A precondition for the setter
2. A postcondition for the getter
3. An invariant for the underlying backing field

And by example:

public int MyProperty { get; private set ;}

[ContractInvariantMethod]
private void ObjectInvariant ()
{
  Contract.Invariant ( this.MyProperty >= 0 );
}

"Is equivalent to the following code:"

private int backingFieldForMyProperty;
public int MyProperty 
{
  get 
  {
    Contract.Ensures(Contract.Result<int>() >= 0);
    return this.backingFieldForMyProperty;
  }

  private set 
  {
    Contract.Requires(value >= 0);
    this.backingFieldForMyProperty = value;
  }
}

[ContractInvariantMethod]
private void ObjectInvariant () 
{
  Contract.Invariant ( this.backingFieldForMyProperty >= 0 );
...

Answer 2

I'm thinking not, but you could easily write a snippet that would do this. If you go this route, here is a free snippet editor that will make the task very easy.

Answer 3

Thanks Porges.

My mistake was that I actually used ReleaseRequires option, which, indeed, deals only with generic version of the method, Requires<T>.

Invariant which is put on an auto-implemented property is really turned into a Requires precondition, but it's not generic - that's why it didn't work using this option.

What to do:

• VARIANT 1. Consider using code snippets and lovely Requires<T> instead of auto-implemented properties - that enables us to use exceptions of desired type.

• VARIANT 2. Change the option ReleaseRequires to Preconditions in the Code Contracts' options and feel free to write invariants on auto-properties - the rewriter tool will automatically change them into the Requires. However, they will be non-generic - that means, in case of contract broken, a ContractException will be thrown and there is no way to change this behaviour.

Thanks everyone for the help!