Serialization-CustomConverters.md

Custom Type Converters for DynamoDB

The serialization logic can be extended by defining custom type converters either by implementing the IDynamoAttributeConverter interface or deriving from the ADynamoAttributeConverter base class.

The converter must implement the CanConvert(), the ToAttributeValue(), and one of the deserialization methods:

• FromBool() to convert from AttributeValue.BOOL
• FromBinary() to convert from AttributeValue.B
• FromNumber() to convert from AttributeValue.NS
• FromString() to convert from AttributeValue.S
• FromList() to convert from AttributeValue.L
• FromMap() to convert from AttributeValue.M
• FromBinarySet() to convert from AttributeValue.BS
• FromNumberSet() to convert from AttributeValue.NS
• FromStringSet() to convert from AttributeValue.SS

Optionally, the converter can also implement GetDefaultValue() to control the default value used to initialize a property that was not deserialized.

Sample Custom Converter: DynamoTimeSpanConverter

The following converter serializes TimeSpan from and to an AttributeValue instance.

class DynamoTimeSpanConverter : ADynamoAttributeConverter {

    //--- Methods ---
    public override bool CanConvert(Type typeToConvert) => typeToConvert == typeof(TimeSpan);

    public override AttributeValue ToAttributeValue(object value, Type targetType, DynamoSerializerOptions options)
        => new AttributeValue {
            N = ((TimeSpan)value).TotalSeconds.ToString(CultureInfo.InvariantCulture)
        };

    public override object FromNumber(string value, Type targetType, DynamoSerializerOptions options) {
        if(!double.TryParse(value, System.Globalization.NumberStyles.Float | System.Globalization.NumberStyles.AllowThousands, CultureInfo.InvariantCulture, out var parsedValue)) {
            throw new DynamoSerializationException("invalid value for TimeSpan");
        }
        return TimeSpan.FromSeconds(parsedValue);
    }
}

The custom converter must then be added to the serialization options to become available.

var document = DynamoSerializer.Serialize(item, new DynamoSerializerOptions {
    Converters = {
        new DynamoTimeSpanConverter()
    }
});