c# – CodeContracts：可能在空引用上调用方法

第二次更新：下面概述的方法是currently the recommended solution.

可能的解决方法是创建一个属性,例如确保要保留的不变量. (当然,您需要假设它们才能确保被证明.)完成此操作后,您可以使用该属性,并且应该正确地证明所有不变量.

以下是使用此方法的示例：

class Foo
{
    private static readonly Foo instance = new Foo();
    private readonly string bar;

    public static Foo Instance
    // workaround for not being able to put invariants on static fields
    {
        get
        {
            Contract.Ensures(Contract.Result<Foo>() != null);
            Contract.Ensures(Contract.Result<Foo>().bar != null);

            Contract.Assume(instance.bar != null);
            return instance;
        }
    }

    public Foo()
    {
        Contract.Ensures(bar != null);
        bar = "Hello world!";
    }

    public static int BarLength()
    {
        Contract.Assert(Instance != null);
        Contract.Assert(Instance.bar != null);
        // both of these are proven ok

        return Instance.bar.Length;
    }
}