Question

I have a code contract expresses as this – it validates that entity to be stored is not null and is valid for persistence. It works. Fab.

[ContractClassFor(typeof(IRepository<,>))]
internal abstract class ContractsForIRepository<T, TId> : IRepository<T, TId> where T : IEntity
{
    private ContractsForIRepository()
    {

    } 

    public T Persist(T entity)
    {
        Contract.Requires<InvalidEntityException>(entity != null, "Entity is null");
        Contract.Requires<InvalidEntityException>(entity.IsValidForPersistence(), "Entity not valid for persistence");
        return default(T);
    }

}

However, I would like the exception to be more useful – as anyone receiving the message will want to know what entity was invalid and what is looked like. All of the entities override ToString(), so I wanted to include this in the error message:

Contract.Requires<InvalidEntityException>(entity.IsValidForPersistence(), "Entity not valid for persistence " + entity.ToString());

I have included ToString to be explicit – it would be called implicitly if I omitted it, but I think it makes my question clearer.

The problem is, this isn’t allowed by code contracts and I get the following message.

User message to contract call can only be string literal, or a static field, or static property that is at least internally visible.

Is there any way to include specific data in the exception message?

Answer 1

According the documentation:

2.10 Overloads on Contract Methods

All of the contract methods have overloads that take a string in
addition to the boolean condition:

Contract.Requires(x != null, "If x is null, then the missiles are red!");

The user-supplied string will be displayed whenever the contract is
violated at runtime. Currently, it must be a compile-time constant.

So, what you are asking is not possible.