TOML serializer silently loses Guid, decimal, TimeSpan and Uri properties, and throws on char instead of returning false #165

What's wrong

TomlSerializationProvider hands the model to Tomlyn 0.19.0 with only IgnoreMissingProperties = true:

• the options: Essentials.SerializationProviders.Toml/TomlSerializationProvider.cs:29
• the write: Toml.FromModel(obj, ModelOptions) at :58

There are no ConvertToToml / ConvertToModel hooks. Tomlyn's model writer does not treat several common .NET scalars as TOML values:

1. Guid, decimal, TimeSpan and Uri come back zeroed, and no error is raised. A model { Name="n", Id=<guid>, Price=12.34m, Timeout=30s, When=<DateTimeOffset>, Kind=K.B, U=new Uri("https://x.y/") } serializes to:

name = "n"
when = 2026-01-02T03:04:05Z
kind = "B"
[id]
[price]
[timeout]
[u]
segments = ["/"]

TrySerialize returns true. Reading it back gives Id = Guid.Empty, Price = 0, Timeout = 00:00:00, U = null.

The provider's own IsScalar (:219-228) lists Guid, decimal and TimeSpan as scalars, so it assumes Tomlyn writes them as values. It does not.

2. A char property makes TrySerialize throw. Tomlyn throws InvalidOperationException: Invalid primitive System.Char from ModelToTomlTransform.WritePrimitive. The catch list at :62-73 only covers TomlException, IOException and ObjectDisposedException. So:

• TrySerialize throws instead of returning false, which breaks the contract in Essentials/ISerializationProvider.cs:29-35.
• TrySerializeAsync faults.

b283163 fixed the same gap for System.Text.Json ("Report System.Text.Json's InvalidOperationException as a failed serialize").

Why it matters

Any persistence provider configured with TOML (FileSystem, Temp, ConfigHome, DataHome over Shared/FileRecordStore.cs) does this:

1. It stores a record with a Guid id or a decimal amount and reports success.
2. RetrieveAsync later returns a non-null object with the fields reset to their defaults.
3. Because the object is non-null, the null guard does not fire.

The data is lost silently. Switching the serializer from JSON to TOML changes behaviour with no error.

The round-trip model in Essentials.Tests/SerializationRoundTripTests.cs:74-89 only has a string, an int, a list and a nested object, so nothing catches this.

This was reproduced against Tomlyn 0.19.0 with the provider's exact options at HEAD 7e2b3e2.

Suggested fix

• Add ConvertToToml / ConvertToModel to ModelOptions so that:
  • Guid, TimeSpan, Uri and char are written as strings and parsed back;
  • decimal is written as a string, or as a TOML float when lossless, and parsed back.
• Add catch (InvalidOperationException) to TrySerialize, and consider TargetInvocationException for throwing getters, so any type Tomlyn still rejects reports false rather than throwing.

Acceptance criteria

• A shared round-trip test over a model with Guid, decimal, TimeSpan, Uri, char, DateTimeOffset and an enum gives equal values for every serialization provider, TOML included.
• TrySerialize / TrySerializeAsync never throw for a type Tomlyn cannot represent. They return false.

Related: #69 (closed) fixed different TOML round-trip defects (multi-word names, lists, nested objects, records).