IL2104 IL2026 TRIM and JSON with WinUI3 and newer C#

 JSON: why is C# TRIM so horrible for no reason 

Trim analysis warning IL2026: SerializeExtra.Demonstrate_Bug_Program.Demonstrate_Bug_Main(): Using member 'System.Text.Json.JsonSerializer.Serialize(!!0, JsonSerializerOptions)' which has 'RequiresUnreferencedCodeAttribute' can break functionality when trimming application code. JSON serialization and deserialization might require types that cannot be statically analyzed. Use the overload that takes a JsonTypeInfo or JsonSerializerContext, or make sure all of the required types are preserved.

In the old days, C# had a simple JSON story: always use the Newtonsoft JSON library, helpfully packaged up in NuGet, and never use anything else, ever. But then I tried making an app with WinUI3 and the more recent C# (like version 10?), and now it completely sucks, and it's ten times harder than it should be. And it's all Microsoft's fault

C# introduced a new concept: TRIM. This is a link time option. It's on by default for Release mode and off by default for Debug mode. What it does it go through your app and removes all of the "unneeded" code. The problem is that it means that the normal way Newtonsoft's JSON works doesn't work; it will just plain crash.

Note the horrific discovery mechanism for this, by the way: there you are, coding happily in a shiny new app, using shiny new features, and a switch you didn't set causes weird-ass warnings when you compile, and then your app crashes, but only when you're almost done and are now testing in Release mode.

The only solution is to switch to the System.Text.Json.JsonSerializer and Deserializer classes. But even these are completely broken with the new switch, and delightfully (sarcasm) the official Microsoft documentation guides you right into the absolute wrong way to use them.

Worse, the C# classes also guide you into the wrong way. Those shiny new templated classes? Those won't work at all.

Luckily, I've researched it so you can just jump right into the pit of success.

• When you make your SourceGenerationContext from JsonSerializerContext, always add a TypeInfoPropertyName option 
• When you call Serialize of Deserialize, you must use the non-templated versions. Pass in your object (or string, for deserialize), the typeof(yourclass) and your SourceGenerationContext.Default object. 
• Never new up a SourceGenerationContext object. When you do, it won't have the JsonSourceGenerationOptions that the Microsoft docs tell you to use
• Always specify a [JsonSourceGenerationOptions(WriteIndented = true)]. If you wanted a compressed serialization format, you'd have used Protocol Buffers, not JSON. When you use JSON, it's because your users want a human readable result, not some one-line nonsense.

See my complete project on Github for more details. The About file is particularly helpful. Look at the WeatherForecast_Fix1.cs file for full details.