Implementing Custom JsonWriter (JSON.net)

问题

I have been working on implementing this a custom JSON writer by following this sample

I'm overriding the WriteValue methods so that instead of printing the value of a json, I print the Type.ToString() and conceal the real value of the item.

I'm having trouble with this method:

private void WriteValueElement(string elementName, JTokenType type)
{
    _writer.WriteStartElement(elementName);
    _writer.WriteAttributeString("type", type.ToString());
}

And particularly with this statement _writer.WriteStartElement(elementName); I just can't figure out where this WriteStartElement is. This method signature doesn't exist in the baseClass, but also nothing in the sample

回答1:

If you want to do this by subclassing JsonTextWriter or JsonWriter, you're going to need to override all the WriteValue methods and remap the value being written. There are many methods with this name, and since Newtonsoft could in theory add new methods in later versions (for BigNumber, for instance) this could become a maintenance nuisance.

A simpler solution might be to serialize to JToken then remap the values like so:

public static class JsonExtensions
{
    public static IList<JToken> ChildrenTokenList(this JContainer container)
    {
        return container;
    }

    public static IEnumerable<JToken> DescendantsAndSelf(this JToken node)
    {
        // Small wrapper adding this method to all JToken types.
        if (node == null)
            return Enumerable.Empty<JToken>();
        var container = node as JContainer;
        if (container != null)
            return container.DescendantsAndSelf();
        else
            return new[] { node };
    }

    public static JToken MapValues(this JToken root, Func<JValue, JValue> map)
    {
        if (map == null || root == null)
            throw new ArgumentNullException();
        if (root is JValue)
            return map((JValue)root);
        else
        {
            root = root.DeepClone();
            foreach (var container in root.DescendantsAndSelf().OfType<JContainer>())
            {
                var children = container.ChildrenTokenList();
                for (int i = 0, n = children.Count; i < n; i++)
                    if (children[i] is JValue)
                        children[i] = map((JValue)children[i]);
            }
            return root;
        }
    }

    public static JToken MapValuesToValueTypes(this JToken root)
    {
        return root.MapValues(v => (v.Type == JTokenType.Null ? (JValue)null : (JValue)v.Type.ToString()));
    }
}

If you insist on subclassing the writer, a prototype implementation would look like:

public class ValueTypeJsonWriter : JsonTextWriter
{
    public ValueTypeJsonWriter(TextWriter textWriter)
        : base(textWriter)
    {
    }

    private void WriteValueType(object value)
    {
        if (value == null)
            base.WriteNull();
        else
        {
            var token = JToken.FromObject(value);
            base.WriteValue(token.Type.ToString());
        }
    }

    public override void WriteValue(object value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(bool value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(bool? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(byte value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(byte? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(byte[] value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(char value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(char? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(DateTime value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(DateTime? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(DateTimeOffset value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(DateTimeOffset? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(decimal value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(decimal? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(double value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(double? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(float value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(float? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(Guid value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(Guid? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(int value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(int? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(long value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(long? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(sbyte value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(sbyte? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(short value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(short? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(string value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(TimeSpan value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(TimeSpan? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(uint value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(uint? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(ulong value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(ulong? value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(Uri value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(ushort value)
    {
        WriteValueType(value);
    }

    public override void WriteValue(ushort? value)
    {
        WriteValueType(value);
    }
}

Prototype fiddle.

来源：https://stackoverflow.com/questions/32612392/implementing-custom-jsonwriter-json-net