更清洁的方式做在C＃中的空检查？

问题描述：

假设，我有这个接口，

interface IContact
{
    IAddress address { get; set; }
}

interface IAddress
{
    string city { get; set; }
}

class Person : IPerson
{
    public IContact contact { get; set; }
}

class test
{
    private test()
    {
        var person = new Person();
        if (person.contact.address.city != null)
        {
            //this will never work if contact is itself null?
        }
    }
}

Person.Contact.Address.City！= NULL （这工作，检查城市为空或不是。）

Person.Contact.Address.City != null (This works to check if City is null or not.)

不过，这种检查失败，如果地址或联系方式或人本身就是空。

However, this check fails if Address or Contact or Person itself is null.

目前，一种解决办法我能想到的是这样的：

Currently, one solution I could think of was this:

if (Person != null && Person.Contact!=null && Person.Contact.Address!= null && Person.Contact.Address.City != null)

{ 
    // Do some stuff here..
}

是否有这样做的更清洁的方式？

Is there a cleaner way of doing this?

我真的不喜欢空检查正在做的（东西== NULL）。相反，有另一种很好的方式做一些事情，如 something.IsNull（）的方法？

I really don't like the null check being done as (something == null). Instead, is there another nice way to do something like the something.IsNull() method?

答

在一个通用的方法，你可以使用一个前pression树，并检查与扩展方法：

In a generic way, you may use an expression tree and check with an extension method:

if (!person.IsNull(p => p.contact.address.city))
{
    //Nothing is null
}

全部code：

Full code:

public class IsNullVisitor : ExpressionVisitor
{
    public bool IsNull { get; private set; }
    public object CurrentObject { get; set; }

    protected override Expression VisitMember(MemberExpression node)
    {
        base.VisitMember(node);
        if (CheckNull())
        {
            return node;
        }

        var member = (PropertyInfo)node.Member;
        CurrentObject = member.GetValue(CurrentObject,null);
        CheckNull();
        return node;
    }

    private bool CheckNull()
    {
        if (CurrentObject == null)
        {
            IsNull = true;
        }
        return IsNull;
    }
}

public static class Helper
{
    public static bool IsNull<T>(this T root,Expression<Func<T, object>> getter)
    {
        var visitor = new IsNullVisitor();
        visitor.CurrentObject = root;
        visitor.Visit(getter);
        return visitor.IsNull;
    }
}

class Program
{
    static void Main(string[] args)
    {
        Person nullPerson = null;
        var isNull_0 = nullPerson.IsNull(p => p.contact.address.city);
        var isNull_1 = new Person().IsNull(p => p.contact.address.city);
        var isNull_2 = new Person { contact = new Contact() }.IsNull(p => p.contact.address.city);
        var isNull_3 =  new Person { contact = new Contact { address = new Address() } }.IsNull(p => p.contact.address.city);
        var notnull = new Person { contact = new Contact { address = new Address { city = "LONDON" } } }.IsNull(p => p.contact.address.city);
    }
}

更清洁的方式做在C＃中的空检查？

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