CodeContracts：数组访问可能超出上限

Question

I'm getting these warnings from CodeContracts:

Array access might be above the upper bound. Did you meant 0 instead of 1?
Array access might be above the upper bound. Did you meant 1 instead of 2?
Array access might be above the upper bound. Did you meant 2 instead of 3?
Array access might be above the upper bound. Did you meant 3 instead of 4?

On this line of code:

private readonly string[] _addr;

public string AddressLine1
{
    get
    {
        return _addr[0] ?? _addr[1] ?? _addr[2] ?? _addr[3];
    }
}

public string AddressLine2
{
    get
    {
        return _addr[1] ?? _addr[2] ?? _addr[3];
    }
}

public string AddressLine3
{
    get
    {
        return _addr[2] ?? _addr[3];
    }
}

How do I tell the Contracts analyzer that these indices are guaranteed inside bounds? _addr is initialized in the constructor to string[4] .

Answer 1

I was able to get rid of these warnings by adding a method to that class that defined the invariants:

[ContractInvariantMethod]
private void AddressInvariants()
{
    Contract.Invariant(_addr.Length == 4);
}

However, I think there's also a bug in your code.

If _addr[0] == null and _addr[1] != null , then AddressLine1 and AddressLine2 return the same value. This seems like a bug.

You can fix this easily enough (and remove the need to specify contract invariants) by using something along the lines of what @ryanyuyu mentioned:

public string AddressLine1
{
    get
    {
        // Use the first non-null element.
        return _addr.Where(x => x != null).FirstOrDefault();
    }
}

public string AddressLine2
{
    get
    {
        // Use the second non-null element.
        return _addr.Where(x => x != null).Skip(1).FirstOrDefault();
    }
}

public string AddressLine3
{
    get
    {
        // Use the third non-null element.
        return _addr.Where(x => x != null).Skip(2).FirstOrDefault();
    }
}

Answer 2

ContractInvariantMethod works when _addr is a class member. But Contract.Assert() works with local variables too.

static void MyParse(string foo)
{
    string[] split = foo.Split(',');
    Contract.Assert(split.Length == 4);

    string a = split[0];
    string b = split[1];
    string c = split[2];
    string d = split[3];
}

Answer 3

I don't know about the Contracts analyzer, but your code could be cleaner. You are basically repeating code that just finds the first non-null string (and if the last element is null, you are returning that regardless). I'm a fan of using LINQ .FirstOrDefault which will let you find the first element that matches a condition (in your case not null). If no such element is found, it returns the default value (for a String it's null )

return _addr.FirstOrDefault(str => str != null);

See it in action at this .NET Fiddle .