Dasher is a fast, lightweight, cross-platform serialisation tool.

Use it to get data objects from one application to another, and to reliably control behaviour as contracts evolve independently.

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In inter-application communication, message contracts evolve and problematic incompatibilities can arise. In the worst case, these failures are silent and untracked. Dasher gives you control over what happens as versions change, in a way that's very natural to C# developers. You can be very strict about what you receive, or lenient.

API

The core API is very simple:

// serialise to stream
new Serialiser<Holiday>().Serialise(stream, christmas);

// deserialise from stream
var christmas = new Deserialiser<Holiday>().Deserialise(stream);

Dasher messages are regular CLR types (POCO), with no need for a base class or attributes. They can (and should) be immutable as well:

public sealed class Holiday
{
    public string Name { get; }
    public DateTime Date { get; }

    public Holiday(string name, DateTime date)
    {
        Name = name;
        Date = date;
    }
}

Supported types

Both serialiser and deserialiser support the core built-in types of byte, sbyte, short, ushort, int, uint, long, ulong, float, double, decimal, string, char, as well as byte[], ArraySegment<byte>, DateTime, DateTimeOffset, TimeSpan, Guid, IntPtr, Version, Nullable<T>, IReadOnlyList<T>, IReadOnlyDictionary<TKey, TValue>, Tuple<...>, ValueTuple<...> and enum types.

Dasher also provides and supports types:

• Dasher.Union<...> allows a value to be one of a fixed number of known types
• Dasher.Empty represents a value with no fields or state that may gracefully support addition of fields in future versions

More details on Union and Empty types can be found later in this document.

Types not listed so far are treated as complex types. Complex types have their properties serialised, and are deserialised according to their constructor parameters.

If custom treatment for specific types is required, the ITypeProvider interface allows bespoke code to be emitted for serialisation and deserialisation at runtime.

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Contract Evolution

Over time, the serialiser and/or deserialiser may need to modify the contracts of the messages they exchange.

Dasher gives the receiver the final say in whether it will accept a given message or not. Therefore, Dasher's Deserialiser class governs, through user configuration, how different messages are interpreted.

Let's look at some examples.

Handing unexpected fields

The most common modifications to contracts are the addition and removal of fields. In this example, the serialised message contains a field that the deserialiser does not expect.

This may be because the serialiser added the field, or because the deserialiser removed it. In practice, both situations are the same.

Imagine a deserialiser for this class:

public sealed class Reindeer
{
    public string Name { get; }
    public int Position { get; }

    public Reindeer(string name, int position)
    {
        Name = name;
        Position = position;
    }
}

Consider the payload to deserialise as:

{ "Name": "Dasher", "Position": 1, "HasRedNose": false }

Clearly HasRedNose is an unexpected field. There are two possible behaviours: ignore the field, or throw an exception. Which occurs depends upon how the Deserialiser is instantiated.

// return new Reindeer("Dasher", 1)
new Deserialiser<Reindeer>(UnexpectedFieldBehaviour.Ignore).Deserialise(...);

// throw new DeserialisationException("Unexpected field \"HasRedNose\".", typeof(Reindeer))
new Deserialiser<Reindeer>(UnexpectedFieldBehaviour.Throw).Deserialise(...);

When no UnexpectedFieldBehaviour is specified in the deserialiser's constructor, the default behaviour is to throw:

// throws as above
new Deserialiser<Reindeer>().Deserialise(...);

Handling missing fields

In the previous example we considered the case where the serialiser provided a field that was not expected by the deserialiser. Now we consider the opposite case, where the deserialiser requires a field that the serialiser has not provided.

This may be because the deserialiser added the field, or because the serialiser removed it. In practice, both situations can be treated in the same way.

Missing a required field

public sealed class Reindeer
{
    public string Name { get; }
    public int Position { get; }

    public Reindeer(string name, int position)
    {
        Name = name;
        Position = position;
    }
}

Consider the payload to deserialise as:

{ "Name": "Dancer" }

The Position field is not specified, and the programmer who declared the Reindeer type requires such a field to be passed to the constructor.

In this case, attempts to deserialise the payload result in an exception:

// throw new DeserialisationException("Missing required field \"Position\".", typeof(Reindeer))
new Deserialiser<Reindeer>().Deserialise(...);

Missing an optional field

Often it's desirable to make a field optional by providing a default value to use when no value is specified. This enables backwards-compatible modifications to contracts, allowing serialisers to be updated independently of deserialisers, at some later time.

public sealed class Reindeer
{
    public string Name { get; }
    public int Position { get; }
    public bool HasRedNose { get; }

    public Reindeer(string name, int position, bool hasRedNose = false)
    {
        Name = name;
        Position = position;
        HasRedNose = hasRedNose;
    }
}

Consider the payload to deserialise as:

{ "Name": "Prancer", "Position": 3 }

No value is provided for HasRedNose in the message, however the constructor parameter includes a default value of false. In such a case, Dasher uses the default value of a field when the field is omitted from the payload.

// return new Reindeer("Prancer", 3, false)
new Deserialiser<Reindeer>(UnexpectedFieldBehaviour.Ignore).Deserialise(...);

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Union Types

Dasher is strict about the types it deals with. This allows great control over message contract versions, but sometime you don't know exactly which type you will need. Union types allow flexibility, but in a controlled fashion.

Dasher provides a Union<T1,T2,...> type. This allows a given field's type to be one of a set of known types.

Let's look at some examples. Firstly, construction of a union instance:

// implicit conversion
Union<int, string> union1 = 1;
Union<int, string> union2 = "Hello";

// alternatively, explicit construction
var union1 = Union<int, string>.Create(1);
var union2 = Union<int, string>.Create("Hello");

There are a few ways to consume unions:

// consuming a union
union1.Type   // int
union1.Value  // 1 (boxed as object)

// perform an action based on type
union1.Match(
  i => Console.WriteLine($"Union holds an int: {i}"),
  s => Console.WriteLine($"Union holds a string: {s}"));

// mapping based on type
var num = union1.Match(
  i => i * 2,
  s => s.Length);

The Match methods offer great performance as they won't box value types and don't involve any lookup based on type (beyond a single vtable lookup).

A class could have a property of type Union<int, string> and be successfully serialised and deserialised by Dasher. That property can contain either of these types.

This allows a controlled form of polymorphism (with base type requirement), and allows for heterogeneous lists/dictionaries. For example:

IReadOnlyList<Union<AddItemRequest, RemoveItemRequest>> Requests { get; }

Unions are fully composable, so the above to could be inverted if you knew all items in the list would have the same type:

Union<IReadOnlyList<AddItemRequest>, IReadOnlyList<RemoveItemRequest>> Requests { get; }

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Empty Type

Empty is useful in contracts when a value is not currently required, but may be one day in future.

Consider a message that indicates some kind of notification. The message itself might not contain any fields, as it is enough to simply observe the empty message. However one day, you may wish to add one or more optional fields and support backwards compatibility. In such a case, use Empty today and introduce a complex or union type at some later point.

It is expected that Empty would be most useful as a top-level type (i.e. Serialiser<Empty>) or used in conjunction with generic wrapper types (e.g. Serialiser<MyEnvelope<Empty>>>).

The Dasher.Empty CLR type itself cannot be instantiated or subclassed. At runtime, the value will be null.

Empty values may be deserialised to the following types:

• A complex type for which all constructor parameters have default values, where an instance is instantiated with those defaults
• A union type, where the resulting value is null
• A nullable nullable complex struct, where the resulting value is null

If a Deserialiser is constructed with option UnexpectedFieldBehaviour.Ignore, then non-empty values received for an Empty type are discarded. However if UnexpectedFieldBehaviour.Throw is used (the default setting) then a DeserialisationException is raised.

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Encoding

You don't need to know how Dasher encodes messages in order to use it successfully, but a deeper understanding is never a bad thing. Thankfully it's not too complex.

On the wire, messages are encoded using MsgPack, which is an efficient binary serialisation format. If you're familiar with JSON, you can think of it in much the same way. MsgPack messages are quick to pack and unpack, and the resulting byte stream is very concise (much more so than JSON or XML for example).

Using our example from above, this object:

new Holiday("Christmas", new DateTime(2015, 12, 25));

Would logically be encoded as (using JSON for readability):

{ "Name": "Christmas", "Date": 635865984000000000 }

And physically encoded as (the actual byte stream):

82 A4 4E 61 6D 65 A9 43 68 72 69 73 74 6D 61 73 A4 44 61 74 65 D3 08 D3 0C BE 55 13 00 00
|  |___________/  |__________________________/  |___________/  |_______________________/
|  |              |                             |              |
|  \ "Name"       \ "Christmas"                 \ "Date"       \ 64-bit integer
|
\ map of two key/value pairs

MsgPack specifies the encoding of types sbyte, byte, short, ushort, int, uint, long, ulong, float, double, bool, string, and byte[].

Dasher encodes the following types using the following MsgPack formats:

• Array (other than byte[]) as array
• ArraySegment<byte> as binary (byte array)
• char as 1-character string
• DateTime as 64-bit integer
• DateTimeOffset as two-element array of 64-bit integer (ticks) and 16-bit integer (offset)
• Decimal as string
• Empty as zero-sized map
• Enum as string
• Guid as byte array
• IntPtr as 64-bit integer
• IReadOnlyDictionary<T, K> as map from T to K (recur on this list)
• IReadOnlyList<T> as array of T (recor on this list)
• Nullable<T> as either null or T (recur on this list)
• TimeSpan as 64-bit integer
• Tuple<> and ValueTuple<> as array of values (recur on this list)
• Union<> as two-element array: type identifier as string, value (recur on this list)
• Version as string
• Finally, if none of the above apply, class and struct as heterogeneous MsgPack map from name to value

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License

Copyright 2015-2017 Drew Noakes

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.