在 C# 中进行空检查的更简洁方法？

小编典典

在 C# 中进行空检查的更简洁方法？

all

假设，我有这个界面，

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（这可以检查 City 是否为空。）

但是，如果 Address 或 Contact 或 Person 本身为空，则此检查失败。

目前，我能想到的一种解决方案是：

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

{ 
    // Do some stuff here..
}

有更清洁的方法吗？

我真的不喜欢做null检查(something == null)。相反，是否有另一种好方法来做类似该something.IsNull()方法的事情？

阅读 70

2022-08-03

共1个答案

小编典典

通常，您可以使用表达式树并使用扩展方法进行检查：

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

完整代码：

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);
    }
}

2022-08-03