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Handling dynamic property types (that is, properties of type object ) in Windows Communication Foundation (WCF) is complicated. For example, you must specify serializers and provide KnownType attributes.

Protocol Buffer (Protobuf) provides two simpler options for dealing with values that might be of more than one type. The Any type can represent any known Protobuf message type. And you can use the oneof keyword to specify that only one of a range of fields can be set in any message.

Any is one of Protobuf's "well-known types": a collection of useful, reusable message types with implementations in all supported languages. To use the Any type, you must import the google/protobuf/any.proto definition.

syntax = "proto3";
import "google/protobuf/any.proto";
message Stock {
    // Stock-specific data
message Currency {
    // Currency-specific data
message ChangeNotification {
    int32 id = 1;
    google.protobuf.Any instrument = 2;
In the C# code, the Any class provides methods for setting the field, extracting the message, and checking the type.
public void FormatChangeNotification(ChangeNotification change)
    if (change.Instrument.Is(Stock.Descriptor))
        FormatStock(change.Instrument.Unpack<Stock>());
    else if (change.Instrument.Is(Currency.Descriptor))
        FormatCurrency(change.Instrument.Unpack<Currency>());
        throw new ArgumentException("Unknown instrument type");
Protobuf's internal reflection code uses the Descriptor static field on each generated type to resolve Any field types. There's also a TryUnpack<T> method, but that creates an uninitialized instance of T even when it fails. It's better to use the Is method as shown earlier.
Oneof
Oneof fields are a language feature: the compiler handles the oneof keyword when it generates the message class. Using oneof to specify the ChangeNotification message might look like this:
message Stock {
    // Stock-specific data
message Currency {
    // Currency-specific data
message ChangeNotification {
  int32 id = 1;
  oneof instrument {
    Stock stock = 2;
    Currency currency = 3;
Fields within the oneof set must have unique field numbers in the overall message declaration.
When you use oneof, the generated C# code includes an enum that specifies which of the fields has been set. You can test the enum to find which field is set. Fields that aren't set return null or the default value, rather than throwing an exception.
public void FormatChangeNotification(ChangeNotification change)
    switch (change.InstrumentCase)
        case ChangeNotification.InstrumentOneofCase.None:
            return;
        case ChangeNotification.InstrumentOneofCase.Stock:
            FormatStock(change.Stock);
            break;
        case ChangeNotification.InstrumentOneofCase.Currency:
            FormatCurrency(change.Currency);
            break;
        default:
            throw new ArgumentException("Unknown instrument type");
Setting any field that's part of a oneof set will automatically clear any other fields in the set. You can't use repeated with oneof. Instead, you can create a nested message with either the repeated field or the oneof set to work around this limitation.
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