rough draft of YamlDotNet-based config backend

The problem seems to be with nullable value type deserialization,
due to aaubry/YamlDotNet#360.

See yaml-nullable-scratch.linq.

Config.cs

@@ -2,9 +2,12 @@
 using System.Diagnostics;
 using System.Drawing;
 using System.IO;
+using System.Text;
 using System.Threading;
+using YamlDotNet.Serialization;
 
 namespace VidDraw {
+    /// <summary>Partial or complete YAML-backed configuration data.</summary>
     internal record Config(Codec? Codec, Color? Color) {
         public Config() : this(null, null) { }
 
@@ -34,7 +37,6 @@ private static string ConfigDir
         private static Mutex Mutex { get; } =
             Sync.CreateMutex($"APPDATA::{ConfigFilename}");
 
-
         private static string GetConfigPath()
             => Path.Combine(ConfigDir, ConfigFilename);
 
@@ -45,27 +47,48 @@ private static string GetConfigPath()
             try {
                 // If it already exists, this does nothing and succeeds.
                 Directory.CreateDirectory(dir);
-            } catch (SystemException ex)
-                    when (ex is IOException or UnauthorizedAccessException) {
+            } catch (SystemException ex) when (IsNonFatal(ex)) {
                 Debug.Print($"Can't create config dir: {ex.Message}");
                 return null;
             }
 
             return Path.Combine(dir, ConfigFilename);
         }
 
+        private static string TrySlurpConfig()
+        {
+            try {
+                return File.ReadAllText(GetConfigPath(), Encoding.UTF8);
+            } catch (SystemException ex) when (IsNonFatal(ex)) {
+                Debug.Print($"Can't read config file: {ex.Message}");
+                return string.Empty;
+            }
+        }
+
+        private static bool IsNonFatal(SystemException ex)
+            => ex is IOException or UnauthorizedAccessException;
+
         private Config PatchedBy(Config delta)
             => new(delta.Codec ?? Codec,
                    delta.Color ?? Color);
 
-        private static Config TryRead()
-        {
+        // FIXME: Cache Serializer and Deserializer instances thread-locally
+        // instead of constructing them every time in TryRead and TryWrite.
 
-        }
+        private static Config TryRead()
+            => new DeserializerBuilder()
+                .IgnoreUnmatchedProperties()
+                .Build()
+                .Deserialize<Config>(TrySlurpConfig())
+            ?? new();
 
         private void TryWrite()
         {
-
+            if (TryCreateConfigPath() is string path) {
+                File.WriteAllText(path: path,
+                                  contents: new Serializer().Serialize(this),
+                                  encoding: Encoding.UTF8);
+            }
         }
     }
 }

yaml-nullable-scratch.linq

@@ -0,0 +1,113 @@
+<Query Kind="Statements">
+  <NuGetReference Version="11.1.1">YamlDotNet</NuGetReference>
+</Query>
+
+using System.Drawing;
+using YamlDotNet.Serialization;
+
+//new Config { Color = Color.Red }.TrySave();
+//Config.TryLoad().Dump();
+//new Config { Codec = Codec.H264 }.TrySave();
+
+
+// After running this:
+new Config {
+    Color = Color.Red,
+    Codec = Codec.H264,
+}.TrySave();
+
+// This fails due to https://github.com/aaubry/YamlDotNet/issues/360:
+Config.TryLoad().Dump();
+
+/// <summary>Video stream encoding selections.</summary>
+internal enum Codec : uint {
+    Raw,
+    Uncompressed,
+    MotionJpeg,
+    H264,
+}
+
+/// <summary>Partial or complete YAML-backed configuration data.</summary>
+internal record Config(Codec? Codec, Color? Color) {
+    public Config() : this(null, null) { }
+
+    internal static Config TryLoad()
+    {
+        //using var @lock = new Lock(Mutex);
+        return TryRead();
+    }
+
+    internal void TrySave()
+    {
+        //using var @lock = new Lock(Mutex);
+        TryRead().PatchedBy(this).TryWrite();
+    }
+
+    private const string ProgramName = "VidDraw-scratch";
+
+    private const string ConfigFilename = ProgramName + ".yml";
+
+    private static string AppDataDir
+        => Environment.GetFolderPath(
+            Environment.SpecialFolder.ApplicationData);
+
+    private static string ConfigDir
+        => Path.Combine(AppDataDir, ProgramName);
+
+    //private static Mutex Mutex { get; } =
+    //    Sync.CreateMutex($"APPDATA::{ConfigFilename}");
+
+    private static string GetConfigPath()
+        => Path.Combine(ConfigDir, ConfigFilename);
+
+    private static string? TryCreateConfigPath()
+    {
+        var dir = ConfigDir;
+
+        try {
+            // If it already exists, this does nothing and succeeds.
+            Directory.CreateDirectory(dir);
+        } catch (SystemException ex) when (IsNonFatal(ex)) {
+            Debug.Print($"Can't create config dir: {ex.Message}");
+            return null;
+        }
+
+        return Path.Combine(dir, ConfigFilename);
+    }
+
+    private static string TrySlurpConfig()
+    {
+        try {
+            return File.ReadAllText(GetConfigPath(), Encoding.UTF8);
+        } catch (SystemException ex) when (IsNonFatal(ex)) {
+            Debug.Print($"Can't read config file: {ex.Message}");
+            return string.Empty;
+        }
+    }
+
+    private static bool IsNonFatal(SystemException ex)
+        => ex is IOException or UnauthorizedAccessException;
+
+    private Config PatchedBy(Config delta)
+        => new(delta.Codec ?? Codec,
+               delta.Color ?? Color);
+
+    // FIXME: Cache Serializer and Deserializer instances thread-locally
+    // instead of constructing them every time in TryRead and TryWrite.
+
+    private static Config TryRead()
+        => new DeserializerBuilder()
+            //.IgnoreUnmatchedProperties() // Commented to reveal bug 360 (see above).
+            .Build()
+            .Deserialize<Config>(TrySlurpConfig())
+        ?? new();
+
+    private void TryWrite()
+    {
+        if (TryCreateConfigPath() is string path) {
+            File.WriteAllText(path: path,
+                              contents: new Serializer().Serialize(this),
+                              encoding: Encoding.UTF8);
+        }
+    }
+}