C＃中的代码合同问题

Question

I am facing a problem that I can not figure out.

Say I have two methods: public void Method1(object obj) in ViewModel class and public void Method2(object obj) in Model class.

Method2 gets called from Method1 using the instance of Model class(say, objM is the object of Model class and a member of ViewModel class).

class ViewModel
{
public void Methods1(object obj)
{
     if (!(
                        (      (false == this.HasSal)
                            && (typeof(Class1) == obj.GetType())
                          )
                    ||
                        (      (true == this.HasSal)
                            && (typeof(Class2) == obj.GetType())
                        )
                   ) 
                )
            {
                throw new ArgumentException("invalid obj");
            }
            Contract.EndContractBlock();
            objM.Method2(obj);
            .....
} 
}

class Model
{
public void Method2(object obj)
{
 Contract.Requires(
                    (      (false == this.HasSal)
                        && (typeof(Class1) == obj.GetType())
                    )
                ||
                    (      (true == this.HasSal)
                        && (typeof(Class2) == obj.GetType())
                    )

            );
    .....
    }
}

Now whenever I try to build the code, Visual studio produces following warning

Code contracts: Requires unproven
(
                    (      (false == this.HasSal)
                        && (typeof(Class1) == obj.GetType())
                    )
                ||
                    (      (true == this.HasSal)
                        && (typeof(Class2) == obj.GetType())
                    )

            )

Please suggest.

Answer 1

As rtrokzzz have given link to another SO question .NET 4 Code Contracts: “requires unproven: source != null” in comments

Solution for you is to add Contract.Ensures in Method1() and Method2()

Please note: I have not used Code Contracts but I believe from my understanding that code will be as

Contract.Ensures(obj != null);

Update

Contract.Requires(obj != null);

Refer: How to avoid “source !=null” when using Code Contracts and Linq To Sql?

Answer 2

I don't believe that the static checker will ever be able to verify your contract, because the type of obj isn't known until run-time - there is no guarantee that only objects of type Class1 or Class2 will be passed to Method1 .

It might be possible to prove this by adding additional contracts to methods that call Method1 . If you include that code, I might be able to suggest a way to satisfy the static checker.

EDIT: Actually, there's another problem too. If HasSal is a public-setter property, then I'm not sure your contract can be verified - there's always a possibility that another thread could change the value of HasSal in between Method1 being called and the method body being executed.