处理可能包含空值的属性链

Question

我有一些代码在长属性链的末尾提取一个值,其中任何一个属性都可以为null.

例如:

var value = prop1.prop2.prop3.prop4;

为了在prop1中处理null的可能性,我必须写:

var value = prop1 == null ? null : prop1.prop2.prop3.prop4;

为了在prop1和prop2中处理null的可能性,我必须写:

var value = prop1 == null 
            ? null 
            : prop1.prop2 == null ? null : prop1.prop2.prop3.prop4;

要么

var value = prop1 != null && prop1.prop2 != null 
            ? prop1.prop2.prop3.prop4 
            : null;

如果我想在prop1,prop2和prop3中处理null的可能性,或者甚至更长的属性链,那么代码开始变得非常疯狂.

必须有更好的方法来做到这一点.

如何处理属性链,以便在遇到null时返回null？

有点像?? 运营商会很棒.

Answer 1

更新

从C#6开始,现在使用零条件运算符将解决方案烘焙到语言中; ?.对于属性和?[n]索引器.

空条件运算符只允许您在接收方不为null时访问成员和元素,否则提供null结果:

int? length = customers?.Length; // null if customers is null
Customer first = customers?[0];  // null if customers is null

老答案

我看了一下那里的不同解决方案.他们中的一些使用链接多个扩展方法调用,我不喜欢,因为由于每个链添加的噪声量,它不是非常可读.

我决定使用仅涉及单个扩展方法调用的解决方案,因为它更具可读性.我没有测试性能,但在我的情况下,可读性比性能更重要.

我基于这个解决方案松散地创建了以下类

public static class NullHandling
{
    /// <summary>
    /// Returns the value specified by the expression or Null or the default value of the expression's type if any of the items in the expression
    /// return null. Use this method for handling long property chains where checking each intermdiate value for a null would be necessary.
    /// </summary>
    /// <typeparam name="TObject"></typeparam>
    /// <typeparam name="TResult"></typeparam>
    /// <param name="instance"></param>
    /// <param name="expression"></param>
    /// <returns></returns>
    public static TResult GetValueOrDefault<TObject, TResult>(this TObject instance, Expression<Func<TObject, TResult>> expression) 
        where TObject : class
    {
        var result = GetValue(instance, expression.Body);

        return result == null ? default(TResult) : (TResult) result;
    }

    private static object GetValue(object value, Expression expression)
    {
        object result;

        if (value == null) return null;

        switch (expression.NodeType)
        {
            case ExpressionType.Parameter:
                return value;

            case ExpressionType.MemberAccess:
                var memberExpression = (MemberExpression)expression;
                result = GetValue(value, memberExpression.Expression);

                return result == null ? null : GetValue(result, memberExpression.Member);

            case ExpressionType.Call:
                var methodCallExpression = (MethodCallExpression)expression;

                if (!SupportsMethod(methodCallExpression))
                    throw new NotSupportedException(methodCallExpression.Method + " is not supported");

                result = GetValue(value, methodCallExpression.Method.IsStatic
                                             ? methodCallExpression.Arguments[0]
                                             : methodCallExpression.Object);
                return result == null
                           ? null
                           : GetValue(result, methodCallExpression.Method);

            case ExpressionType.Convert:
                var unaryExpression = (UnaryExpression) expression;

                return Convert(GetValue(value, unaryExpression.Operand), unaryExpression.Type);

            default:
                throw new NotSupportedException("{0} not supported".FormatWith(expression.GetType()));
        }
    }

    private static object Convert(object value, Type type)
    {
        return Expression.Lambda(Expression.Convert(Expression.Constant(value), type)).Compile().DynamicInvoke();
    }

    private static object GetValue(object instance, MemberInfo memberInfo)
    {
        switch (memberInfo.MemberType)
        {
            case MemberTypes.Field:
                return ((FieldInfo)memberInfo).GetValue(instance);
            case MemberTypes.Method:
                return GetValue(instance, (MethodBase)memberInfo);
            case MemberTypes.Property:
                return GetValue(instance, (PropertyInfo)memberInfo);
            default:
                throw new NotSupportedException("{0} not supported".FormatWith(memberInfo.MemberType));
        }
    }

    private static object GetValue(object instance, PropertyInfo propertyInfo)
    {
        return propertyInfo.GetGetMethod(true).Invoke(instance, null);
    }

    private static object GetValue(object instance, MethodBase method)
    {
        return method.IsStatic
                   ? method.Invoke(null, new[] { instance })
                   : method.Invoke(instance, null);
    }

    private static bool SupportsMethod(MethodCallExpression methodCallExpression)
    {
        return (methodCallExpression.Method.IsStatic && methodCallExpression.Arguments.Count == 1) || (methodCallExpression.Arguments.Count == 0);
    }
}

这允许我写下面的内容:

var x = scholarshipApplication.GetValueOrDefault(sa => sa.Scholarship.CostScholarship.OfficialCurrentWorldRanking);

x将包含值的值scholarshipApplication.Scholarship.CostScholarship.OfficialCurrentWorldRanking或者null如果链中的任何属性在此过程中返回null.