Length Calculated Fields “Length” Calculated Fields are a special type of CalculatedFields that have build in support in the MessageSerializer. Many protocols have messages which feature a Length byte that is used to indicate how many bytes are remaining in a message and a Length Calculated Field can be used to calculate and fill in the Length in your message. Length Calculated Fields use the CalculatedLength and CalculatedLengthResult attributes, along with the CalculatorLength calculator to specify how the length will be calculated and then do the actual calculation. CalculatedLength and CalculatedLengthResult inherit directly from the CalculatedField and CalculatedFieldResult attributes (you can find their definitions under CalculatedFields). CalculatedLength and CalculatedLengthResult don’t modify anything about how CalculatedField and CalculatedFieldResult work except for they use different defaults: Defaults that change: Name = “Length” DefaultStart = NextField - this means that the bytes that are included in the length calculation should start at the field after the length field Calculator = typeof(CalculatorLength<>) Priority = 1000 Verify = false Defaults that stay the same: DefaultEnd = EndOfMessage - this means the length will be calculated with all the fields up to the last one in the message Start and End = Unspecified - this means that the DefaultStart and DefaultEnd will be used for Start and End Exclude = false - this means that all the fields between Start and End will be included. The CalculatorLength calculation just computes how many bytes there are in the fields that are to be included in the length field. However, it is also treated specially by the MessageSerializer as it has a GetVaryingLengthFieldLength function that determines what the length of a variable length field in a message is. Otherwise when deserializing a message the TypeSerializer for the varying length field wouldn’t know how many bytes the field takes up. Note: By default the PropertyRuleLength will consider any property named Length to be the length field for a message using the default CalculatedLengthResult properties. Example To actually use a Length calculated field let’s say you have a message with the following definition: [1 byte Message Type] [1 byte MessageLength] [2 bytes SomeRandomFieldNotIncludedInLength] [4 bytes SomeNumber] [4 bytes SomeOtherNumber] [vary SomeStringOfVaryingLength] [1 byte SomeOtherFieldNotIncludedInTheLength] The “fields” are somewhat wordy to make it clear what they are being used for but here are a couple notes anyways: We want the MessageLength field to be the length field for the message and have that length be automatically calculated The fields included in the length are SomeNumber, SomeOtherNumber, SomeNullTerminatedStringOfVaryingLength The way you could define this message in your code is as follows: public class TestCalculatedLengthMessage : IMessageSerializable { public byte MessageType { get; set; } [CalculatedLengthResult] public byte MessageLength { get; set; } [CalculatedLength(Exclude = true)] public ushort SomeRandomFieldNotIncludedInLength { get; set; } public uint SomeNumber { get; set; } public uint SomeOtherNumber { get; set; } public string SomeStringOfVaryingLength { get; set; } [CalculatedLength(End = Position.PreviousField)] public byte SomeOtherFieldNotIncludedInTheLength { get; set; } } Some things to take note of: The MessageLength field was not named Length since by default it would automatically be considered the CalculatedLengthResult field which would defeat the purpose of this example The CalculatedLengthResult attribute on the MessageLength field indicates that MessageLength is the length field of the message. By default, specifying the CalculatedLengthResult attribute without any properties being changed means that the length would be calculated starting with the next field (in this case SomeRandomFieldNotIncludedInLength) and go to the last field in the message (SomeOtherFieldNotIncludedInTheLength) To exclude the SomeRandomFieldNotIncludedInLength from the length calculation the CalculatedLength attribute was used with Exclude set to true. Since the way this message is structured we could also say that the length calculation should start with SomeNumber it would also have been valid to instead use one of the following: Put [CalculatedLength(Start = Position.NextField)] on the SomeRandomFieldNotIncludedInLength property instead of what we did use Put [CalculatedLength(Start = Position.ThisField)] on the SomeNumber property and then there wouldn’t be a need for a CalculatedLength attribute on SomeRandomFieldNotIncludedInLength To exclude the SomeOtherFieldNotIncludedInTheLength from the length calculation, [CalculatedLength(End = Position.PreviousField)] was used to indicate that the last field to include in the length calculation should be the field before SomeOtherFieldNotIncludedInTheLength (SomeStringOfVaryingLength). The following would also be valid: Put [CalculatedLength(End = Position.ThisField)] on SomeStringOfVaryingLength and then the CalculatedLength attribute wouldn’t be necessary on SomeOtherFieldNotIncludedInTheLength Put [CalculatedLength(Exclude = true)] on SomeOtherFieldNotIncludedInTheLength instead of what we did put If this message was more normal and all the fields after the MessageLength field should have been included in the length then only the CalculatedLengthResult attribute would have been needed.