# Description of Changes
This is the implementation of issue #3191. This adds a Default attribute
to C# module fields.
**Note**: In C#, attribute arguments must be compile-time constants,
which means you can't directly use non-constant expressions like new
expressions, method calls, or dynamic values in attribute constructors.
(Ref:
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/attributes#2324-attribute-parameter-types)
For this reason, these default values are limited to primitive types,
enums, and strings.
This includes (shown as `C# Type` (`BSATN type`):
* `bool` (`Bool`)
* `sbyte` (`I8`)
* `byte` (`U8`)
* `short` (`I16`)
* `ushort` (`U16`)
* `int` (`I32`)
* `unit` (`U32`)
* `long` (`I64`)
* `ulong` (`U64`)
* `float` (`F32`)
* `double` (`F64`)
* `enum` (`Enum`)
* `string` (`String`)
* `null` (`RefOption`) <- Nullable type
Because of C# limitations, for nullable and complex data types, such as
a struct, can take use `[Default(null)]` to populate values with null
defaults. This allows things like structs to workaround the non-constant
expressions in attribute constructors limitation by allowing these
complex types to still be able to be added as new column tables.
The `int` type can also be in the form of Hex or Binary literals, such
as `[Default(0x2A)]` or `[Default(0b00101010)]`
Both Decimal (like `[Default(3.14m)]`) and Char (like `[Default('A')]`)
are unsupported types in BSATN and will still return `BSATN0001` errors.
# API and ABI breaking changes
Not API breaking.
This change only adds the `[Default(value)]` attribute logic.
Using the `[Default(value)]` attribute with older versions SpacetimeDB
C# modules will result in an error.
# Expected complexity level and risk
2
# Testing
Local testing of this requires use of CLI changes in #3278
- [x] Regression test of functionality added.
rekhoff