AWS DynamoDB's .NET SDK handles primitive types and collections out of the box. Anything outside those defaults—enums you want stored as strings, value objects without parameterless constructors—requires a custom converter. This piece walks through the mechanics, shows where to wire the converter, and flags the trade-offs worth weighing before you ship.
At a glance
- The .NET DynamoDB SDK supports common types automatically
- Complex .NET classes with default constructors serialize as JSON objects
- Enums default to their underlying integer value
- IPropertyConverter overrides how any type maps to DynamoDB
- Apply converters per property or globally on the DynamoDBContext
When default behavior breaks down
Two scenarios trip up most developers. First, enums. A property like WeatherType.Sunny stores as its integer value. Read the row back later and you see a number, not a name. Reorder the enum or drop a value and the meaning shifts silently.
Second, value objects. Any class without a parameterless constructor cannot be instantiated by the deserializer. The SDK needs explicit instructions on how to rebuild it.
How IPropertyConverter works
The interface has two methods: FromEntry converts a DynamoDBEntry back to your .NET type, and ToEntry does the reverse.
For enums stored as strings, FromEntry uses Enum.Parse and ToEntry returns Primitive(value.ToString()). Apply the DynamoDBProperty attribute on the property and the SDK handles the rest.
Alternatively, register the converter on DynamoDBContext's ConverterCache. Every property of that type gets converted automatically—no attribute duplication. Trade-off: explicitness for brevity.
Our analysis: the real cost and benefit
Custom converters pay off when readability matters more than raw storage efficiency. Seeing "Sunny" instead of "1" in the DynamoDB console during a 2 a.m. Incident is the practical argument.
The hidden cost is maintenance. Each converter is another piece of code to test and version. A buggy converter can corrupt data silently on writes or reads. Treat converters like schema migrations: write a round-trip test that asserts the shape on both ends before deploying.
Also note this article is sponsored by AWS. The technique works, but validate it against your own team's conventions and tolerance for vendor-specific patterns.
Decision checklist: per-property vs. Global converter
| Scenario | Recommended approach |
|---|---|
| One property of a type, no duplicates planned | DynamoDBProperty attribute |
| Multiple properties of same type across one item | DynamoDBProperty attribute |
| Same type across multiple tables or items | Global on DynamoDBContext |
| Type has no default constructor | Custom converter required |
| Type needs human-readable storage (enums, dates) | Custom converter recommended |
| Prototype or throwaway code | Skip converters, use defaults |
Worked example: persisting a forecast record
Given a WeatherForecast class with a WeatherType enum and a Wind value object. The Wind class has a default constructor, so it serializes as a JSON object automatically. Apply the DynamoDBProperty attribute with DynamoEnumStringConverter on the enum property. Read the item back and you get "Sunny" in the console, not its numeric value.
Add a second WeatherType property elsewhere and you face the choice: duplicate the attribute or register the converter globally on the context. Most teams prefer global registration to prevent drift as the model grows.
What to watch
AWS could simplify the converter surface in future releases—fluent registration or annotation-free conventions would reduce boilerplate. For now, if human-readable rows in DynamoDB matter to your debugging workflow, write the converter. Budget time for round-trip testing before you consider it done.
Key takeaways
- Use IPropertyConverter for any type without a default mapping
- Apply converters per property for clarity or globally to reduce repetition
- Always round-trip test converters through read and write operations
- Prefer global registration when the same type appears across multiple items
Frequently asked questions
When do I need a custom converter in DynamoDB for .NET?
When your type is not a primitive, collection, or a complex class with a default constructor. Enums are a common case if you want strings instead of integers.
Can I store enums as strings?
Yes. A custom converter can return the enum's ToString() on the way in and use Enum.Parse on the way out.
Does the converter apply to all properties of a type?
Only if you register it on the DynamoDBContext. Otherwise it applies only to properties marked with the DynamoDBProperty attribute.
Do complex types always need a converter?
No. .NET classes with a default constructor are serialized as JSON objects automatically.