Description:

Observed 12 test failures in MongoDB.Driver.Tests, specifically within the test class:
`MongoDB.Driver.Core.WireProtocol.Messages.Encoders.BinaryEncoders.CommandMessageBinaryEncoderTests`

These tests were failing on Big Endian architectures (e.g., s390x) due to mismatches in how 32-bit integers were being interpreted during encoding and decoding.

At first glance, it appeared that the failures stem from incorrect serialization/deserialization logic in the source. However, a deeper debugging of the implementation suggests otherwise.

The CommandMessageBinaryEncoder writes and reads integer values using a BsonStream, which in this context is implemented by BsonStreamAdapter. This adapter wraps a standard Stream and overrides the relevant methods to ensure wire protocol compliance.
WriteInt32, ReadInt32

Root Cause — Platform-Dependent Test Assertions:

The issue lies in the test code, which uses:
 
BitConverter.ToInt32(...)
BitConverter.GetBytes(...)
 
These APIs are inherently platform-endian, consequently, the test expectations did not hold when run on s390x.
we can use  BinaryPrimitives.ReadInt32LittleEndian and BinaryPrimitives.WriteInt32LittleEndian instead

 
It appears that the tests may be making assumptions about host endianness, whereas the source code explicitly writes in Little Endian as per wire protocol—so my understanding is that adjusting the tests, rather than the production code, seems like the right direction here.
 
Tests Affected:

• WriteMessage_should_write_messageLength

• WriteMessage_should_write_flags

• WriteMessage_should_write_responseTo

• WriteMessage_should_write_requestId

• WriteMessage_should_write_expected_opcode

• ReadMessage_should_throw_when_flags_is_invalid