C#•2y ago

✅ Extracting an object from a Utf8JsonReader

I'm currently writing my own JsonConverter, and I've got the writer bit working fine. (simplified example below)

public override void Write(Utf8JsonWriter writer, MyClass value, JsonSerializerOptions options)
{
    writer.WriteString("name", value.Name);
    writer.WriteString("value", value.Value);
}

public override void Write(Utf8JsonWriter writer, MyClass value, JsonSerializerOptions options)
{
    writer.WriteString("name", value.Name);
    writer.WriteString("value", value.Value);
}

However, I'm struggling a bit to do the opposite of this in Read() I have the Utf8JsonReader object in Read(), but the only thing that I can figure out is how to get a direct value from the reader, and can't get values by key from it. What am I missing here? Thanks

4 Replies

HughOP•2y ago

I think I've figured this one out. It's a little less straight-forward than I'd hoped... I ended up with this:

public override MyClass? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
    if(reader.TokenType != JsonTokenType.StartObject)
    {
        return null;
    }

    reader.Read();

    string propertyName?;
    string propertyValue?;

    while(reader.TokenType != JsonTokenType.EndObject)
    {
        string jsonPropertyName = reader.GetString() ?? "";
        reader.Read();
        string jsonPropertyValue = reader.GetString() ?? "";
        reader.Read();

        switch(jsonPropertyName)
        {
            case "name":
                propertyName = jsonPropertyValue;
                break;
            case "value":
                propertyValue = jsonPropertyValue;
                break;
        }
    }

    if(propertyName is not null && propertyValue is not null)
    {
        return new MyClass(propertyName, propertyValue);
    }

    return null;
}

public override MyClass? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
    if(reader.TokenType != JsonTokenType.StartObject)
    {
        return null;
    }

    reader.Read();

    string propertyName?;
    string propertyValue?;

    while(reader.TokenType != JsonTokenType.EndObject)
    {
        string jsonPropertyName = reader.GetString() ?? "";
        reader.Read();
        string jsonPropertyValue = reader.GetString() ?? "";
        reader.Read();

        switch(jsonPropertyName)
        {
            case "name":
                propertyName = jsonPropertyValue;
                break;
            case "value":
                propertyValue = jsonPropertyValue;
                break;
        }
    }

    if(propertyName is not null && propertyValue is not null)
    {
        return new MyClass(propertyName, propertyValue);
    }

    return null;
}

If I'm over-complicating this, I'd appreciate any pointers here

canton7•2y ago

Looks roughly equivalent to the example here: https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/converters-how-to?pivots=dotnet-7-0#sample-factory-pattern-converter (you're not using a factory of course, but I don't think you need to)

How to write custom converters for JSON serialization - .NET

Learn how to create custom converters for the JSON serialization classes that are provided in the System.Text.Json namespace.

canton7•2y ago

Although your serializer doesn't write a { but your deserializer expects one...

HughOP•2y ago

Thanks - I'll have a look at that page

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✅ Extracting an object from a Utf8JsonReader