Why Am I Getting an 'Unknown Wire-Type 6' Exception During protobuf-net Deserialization?

Troubleshooting unknown line type exceptions in protobuf-net deserialization

In protobuf-net, a ProtoException is encountered during the deserialization process, and the message is "unknown wire-type 6", indicating that there is an unexpected field type in the input data. To solve this problem, we need to understand the concept of line types and identify the underlying causes.

Understanding line types

The line type is a 3-bit flag in the field header, which specifies the subsequent data format. This is critical for handling unexpected data in the stream. Each line type value corresponds to a specific data type:

• 0: variable length integer (base-128 encoding)
• 1: 64-bit integer or double precision floating point number
• 2: Length prefix (string, byte array, packed array, sub-object)
• 3: Start group (deprecated)
• 4: End group (deprecated)
• 5: 32-bit integer or single-precision floating point number

Debugging issues

To pinpoint the problem field causing the unknown line type exception:

1. Validate input data: Make sure the input data is real protobuf data and not another format (e.g., JSON, XML).
2. Check for file truncation: If serializing to a file, verify that the file is not truncated before writing. Untruncated files may contain residual data, corrupting the protobuf stream.
3. Verify object compatibility: Double check that the deserialized object type matches the type used for serialization. Deserializing to an incorrect type can cause this exception.

Other Tips

• Use tools like WireShark to inspect the binary stream and identify the wire type of the offending field.
• Set the ProtoSyntaxDefault option to protobuf 3, which has a stricter line format and prevents unknown line type issues.
• Consider enabling the ThrowProtoExceptionOnUnknownWireTypes option to flag unknown wire types during deserialization.

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