"错误:并非所有代码路径都返回一个值."

Question

编译时,我的代码抛出了名义上的异常.我不明白为什么会发生这种情况,因为在广泛搜索之后,错误发生的原因似乎只有当条件存在且没有退出返回语句时,我认为我的代码是完全包容的.

bool CheckExisting()
{
    Account loginAcc = new Account();

    string path = Application.StartupPath.ToString() + "\\Customers";
    int fCount = Directory.GetFiles(path, "*.xml", SearchOption.AllDirectories).Length;
    for(int i = 0;i<fCount;i++)
    {
        String[] filePaths = Directory.GetFiles(Application.StartupPath + "\\Customers\\");
        XmlDocument xmlFile =new XmlDocument();
        xmlFile.Load(filePaths[i]);

        foreach(XmlNode node in xmlFile.SelectNodes("//Account"))
        {
            string firstName = node.SelectSingleNode("FirstName").InnerText;
            string lastName = node.SelectSingleNode("LastName").InnerText;
            string address1 = node.SelectSingleNode("Address1").InnerText;
            string address2 = node.SelectSingleNode("Address2").InnerText;
            string postCode = node.SelectSingleNode("Postcode").InnerText;
             string telePhone = node.SelectSingleNode("Telephone").InnerText;
            string mobile = node.SelectSingleNode("Mobile").InnerText;

            Account newAcc = new Account();

            newAcc.firstName = firstName;
            newAcc.lastName = lastName;
            newAcc.address1 = address1;
            newAcc.address2 = address2;
            newAcc.postCode = postCode;
            newAcc.telephone = telePhone;
            newAcc.mobile = mobile;

            loginAcc = newAcc;
        }

        if(txtFirstName.Text == loginAcc.firstName && txtLastName.Text == loginAcc.lastName)
        {
            return true;
        }
        else
        {
            return false;
        }
        return false;
    }
}

Answer 1

你的代码是有效的:

bool CheckExisting()
{
    // Some setup code

    for (int i = 0; i < fCount; i++)
    {
        // Code which isn't terribly relevant
        return ...;
    }
}

现在,C#5语言规范部分8.8.3讨论了for语句结束的可达性:

for如果至少满足下列条件之一,则可以访问语句的结束点:

• 该for语句包含break退出for语句的可访问语句.
• 该for语句是可达的,并且存在for-condition并且没有常量值true.

后一种情况在这种情况下是正确的,因此for语句的结尾是可达的......这就是方法的结束.具有非void返回类型的方法的结尾永远不可访问.

请注意,即使人类可以检测到您永远无法到达for语句的末尾,也是如此.例如:

bool Broken()
{
    for (int i = 0; i < 5; i++)
    {
        return true;
    }
    // This is still reachable!
}

我们知道循环将始终至少执行一次,但语言规则不会 - 因此语句的结尾是可达的,并且您得到编译时错误.