In the Bond C# manual, it notes the following:
These following changes will break wire compatibility and are not recommended:
- Adding or removing required fields
- Incompatible change of field types (any type change not covered above); e.g.: int32 to string, string to wstring
- ...
But it doesn't explain why. The use case here is that I'm using Bond that connects a C# application with a C++ backend. The field is currently a string. I want to change it to a wstring. The manual notes that C# strings can handle C++ strings and C++ wstrings. Therefore, why I can't I just change the field type from string to wstring? Why does this break wire compat?
In Bond's binary formats, strings are UTF8 encoded (no BOM) and wstrings are UTF16-LE encoded. If you were to switch a field from string to wstring, the reading side would try to interpret UTF8 data as UTF16-LE data. These two encodings are not compatible with each other, hence a field type change from string to wstring is a breaking change.
Note that the manual says "For example C# string can represent either Bond type string or wstring." It does not say anything about C++ types. When working with Bond across C# and C++, there are three type systems: Bond's, C#'s, and C++'s.
If on the C++ side, you want to use something akin to std::wstring to store the field in memory, take a look as using Custom type mapping with the string concept.