如何跳过Yamldotnet中的空藏品

Question

我试图找出如何跳过使用YamlDotNet序列化空集合。我已经尝试过定制的ChainedObjectGraphVisitor和IYamlTypeConverter。我刚开始使用YamlDotNet，这里有一些知识空白。

下面是我对访问者模式的实现，它导致了YamlDotNet.Core.YamlException “预期的标量、序列启动、映射-开始或别名，got”错误。我确实看到了一些MappingStart/MappingEnd的在线内容，但我不知道它如何适合我所要做的工作(消除大量空集合中的杂乱)。任何指向正确方向的指针都会受到赞赏。

实例化序列化程序：

var serializer = new YamlDotNet.Serialization.SerializerBuilder()
                .WithNamingConvention(new YamlDotNet.Serialization.NamingConventions.CamelCaseNamingConvention())
                .WithEmissionPhaseObjectGraphVisitor(args => new YamlIEnumerableSkipEmptyObjectGraphVisitor(args.InnerVisitor))
                .Build();

ChainedObjectGraphVisitor实现：

    public sealed class YamlIEnumerableSkipEmptyObjectGraphVisitor : ChainedObjectGraphVisitor
{
    public YamlIEnumerableSkipEmptyObjectGraphVisitor(IObjectGraphVisitor<IEmitter> nextVisitor)
        : base(nextVisitor)
    {
    }

    public override bool Enter(IObjectDescriptor value, IEmitter context)
    {
        bool retVal;

        if (typeof(System.Collections.IEnumerable).IsAssignableFrom(value.Value.GetType()))
        {   // We have a collection
            var enumerableObject = (System.Collections.IEnumerable)value.Value;
            if (enumerableObject.GetEnumerator().MoveNext()) // Returns true if the collection is not empty.
            {   // Serialize it as normal.
                retVal = base.Enter(value, context);
            }
            else
            {   // Skip this item.
                retVal = false;
            }
        }
        else
        {   // Not a collection, normal serialization.
            retVal = base.Enter(value, context);
        }

        return retVal;
    }
}

Answer 1

我相信答案也是用类似于Enter()方法中所做的逻辑覆盖基类中的EnterMapping()方法：

public override bool EnterMapping(IPropertyDescriptor key, IObjectDescriptor value, IEmitter context)
{
    bool retVal = false;

    if (value.Value == null)
        return retVal;

    if (typeof(System.Collections.IEnumerable).IsAssignableFrom(value.Value.GetType()))
    {   // We have a collection
        var enumerableObject = (System.Collections.IEnumerable)value.Value;
        if (enumerableObject.GetEnumerator().MoveNext()) // Returns true if the collection is not empty.
        {   // Don't skip this item - serialize it as normal.
            retVal = base.EnterMapping(key, value, context);
        }
        // Else we have an empty collection and the initialized return value of false is correct.
    }
    else
    {   // Not a collection, normal serialization.
        retVal = base.EnterMapping(key, value, context);
    }

    return retVal;
}

Answer 2

最后，我上了下面的课：

using System.Collections;
using YamlDotNet.Core;
using YamlDotNet.Serialization;
using YamlDotNet.Serialization.ObjectGraphVisitors;

sealed class YamlIEnumerableSkipEmptyObjectGraphVisitor : ChainedObjectGraphVisitor
{
    public YamlIEnumerableSkipEmptyObjectGraphVisitor(IObjectGraphVisitor<IEmitter> nextVisitor): base(nextVisitor)
    {
    }

    private bool IsEmptyCollection(IObjectDescriptor value)
    {
        if (value.Value == null)
            return true;

        if (typeof(IEnumerable).IsAssignableFrom(value.Value.GetType()))
            return !((IEnumerable)value.Value).GetEnumerator().MoveNext();

        return false;
    }

    public override bool Enter(IObjectDescriptor value, IEmitter context)
    {
        if (IsEmptyCollection(value))
            return false;

        return base.Enter(value, context);
    }

    public override bool EnterMapping(IPropertyDescriptor key, IObjectDescriptor value, IEmitter context)
    {
        if (IsEmptyCollection(value))
            return false;

        return base.EnterMapping(key, value, context);
    }
}

Answer 3

您可以指定DefaultValuesHandling

在序列化程序中：

var serializer = new SerializerBuilder()
    .ConfigureDefaultValuesHandling(DefaultValuesHandling.OmitEmptyCollections)
    .Build();   

或在字段/属性的属性YamlMember中：

public class MyDtoClass
{

    [YamlMember(DefaultValuesHandling = DefaultValuesHandling.OmitEmptyCollections)]
    public List<string> MyCollection;
}