OCTET STRING and Encoding

This page shows you how OCTET STRING type works internally.


If you notice, both SNMP data type OCTET STRING and Opaque can be viewed as byte array type in .NET. Maybe you wonder why OCTET STRING is not viewed as System.String, the reason is simple. System.String does have an important property that distinguishes itself from a byte array - encoding. It seems when SNMP was designed, it does not provide a default encoding for OCTET STRING explicitly (and sometimes it is even used just like a byte array when you store MAC address bytes in it).

Therefore, a problem has been there for a long time, “which encoding should be used there if an OctetString instance comes”. In very old revision of #SNMP Library, the answer was ASCII and there was no way in them to change it to Unicode or any other encodings. Note that it is recommended you use ASCII if possible.

In more recent builds of #SNMP Library you can explicitly state which encoding you prefer.

Important Methods

As a reference at first, there are new methods and properties in OctetString class you should pay attention to,

  1. The overloading constructor public OctetString(string str, Encoding encoding). You may provide a specific Encoding so that the internal bytes are what you expect after initialization.

  2. The overloading method public string ToString(Encoding encoding). According to Encoding provided, the internal bytes are decoded to a correct string.

  3. The instance property public Encoding Encoding. This is a read only property that illustrates what encoding is assigned to this OctetString instance.

  4. The static property public static Encoding DefaultEncoding. This is a property to define default encoding used for all OctetString instance if no encoding is assigned via the constructor described in item 1. The default value of this property is Encoding.ASCII.

  5. The overloading constructor public OctetString(string str). OctetString.DefaultEncoding is used to generate internal bytes from str.

  6. The overloading method public string ToString(). OctetString.Encoding is used to generate a string from internal bytes.


Now let’s review a few common scenarios and discuss what you may do to customize OctetString’s behaviors.

Default ASCII Way

If you don’t touch the properties and methods related to Encoding, then you are in a pure ASCII way. It should behavior the same as old releases of #SNMP (0.5, 1.0 and 1.1).

Default Encoding.* Way

If you change DefaultEncoding to any other Encoding types, and don’t touch the properties and methods related to Encoding later, you are in a pure Encoding.* way.

Basic Hybrid Way

If some of your devices require ASCII while others require Unicode (aka UTF-16), then you may find it a little bit difficult. My suggestions are,

Whenever you need to construct an OctetString object, specify its encoding explicitly. Whenever you need to use an OctetString object, check its Encoding before calling ToString(). If the Encoding is not correct, call ToString(Encoding) explicitly.

Last Words

I understand the current implementation is not perfect yet. When #SNMP parses incoming packets, it always use DefaultEncoding to construct OctetString objects from raw bytes. This is not optimal for hybrid cases. Hope a better way can be found soon.