Add Analyzer rule to warn of `FirstOrDefault` on list of structs

Common.ruleset

@@ -28,6 +28,9 @@
 		<!-- Don't call typeof(T).ToString(), use nameof operator or typeof(T).FullName -->
 		<Rule Id="BHI1103" Action="Error" />
 
+		<!-- Call to FirstOrDefault when elements are of a value type; FirstOrNull may have been intended -->
+		<Rule Id="BHI3100" Action="Error" />
+
 		<!-- Throw NotImplementedException from methods/props marked [FeatureNotImplemented] -->
 		<Rule Id="BHI3300" Action="Error" />
 	</Rules>

ExternalProjects/BizHawk.Analyzer/FirstOrDefaultOnStructAnalyzer.cs

@@ -0,0 +1,51 @@
+namespace BizHawk.Analyzers;
+
+using System.Collections.Immutable;
+using System.Linq;
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Microsoft.CodeAnalysis.Operations;
+
+[DiagnosticAnalyzer(LanguageNames.CSharp)]
+public sealed class FirstOrDefaultOnStructAnalyzer : DiagnosticAnalyzer
+{
+	private static readonly DiagnosticDescriptor DiagUseFirstOrNull = new(
+		id: "BHI3100",
+		title: "Call to FirstOrDefault when elements are of a value type; FirstOrNull may have been intended",
+		messageFormat: "Call to FirstOrDefault when elements are of a value type; did you mean FirstOrNull?",
+		category: "Usage",
+		defaultSeverity: DiagnosticSeverity.Warning,
+		isEnabledByDefault: true);
+
+	public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get; } = ImmutableArray.Create(DiagUseFirstOrNull);
+
+	public override void Initialize(AnalysisContext context)
+	{
+		context.ConfigureGeneratedCodeAnalysis(GeneratedCodeAnalysisFlags.None);
+		context.EnableConcurrentExecution();
+		context.RegisterCompilationStartAction(initContext =>
+		{
+			if (initContext.Compilation.GetTypeByMetadataName("BizHawk.Common.CollectionExtensions.CollectionExtensions") is null) return; // project does not have BizHawk.Common dependency
+			var linqExtClassSym = initContext.Compilation.GetTypeByMetadataName("System.Linq.Enumerable")!;
+			IMethodSymbol? firstOrDefaultNoPredSym = null;
+			IMethodSymbol? firstOrDefaultWithPredSym = null;
+			foreach (var sym in linqExtClassSym.GetMembers("FirstOrDefault").Cast<IMethodSymbol>())
+			{
+				if (sym.Parameters.Length is 2) firstOrDefaultWithPredSym = sym;
+				else firstOrDefaultNoPredSym = sym;
+			}
+			initContext.RegisterOperationAction(
+				oac =>
+				{
+					var operation = (IInvocationOperation) oac.Operation;
+					var calledSym = operation.TargetMethod.ConstructedFrom;
+					if (!(firstOrDefaultWithPredSym!.Matches(calledSym) || firstOrDefaultNoPredSym!.Matches(calledSym))) return;
+					var receiverExprType = (INamedTypeSymbol) operation.SemanticModel.GetTypeInfo((CSharpSyntaxNode) operation.Arguments[0].Syntax)!.ConvertedType!;
+					if (receiverExprType.TypeArguments[0].IsValueType) oac.ReportDiagnostic(Diagnostic.Create(DiagUseFirstOrNull, operation.Syntax.GetLocation()));
+				},
+				OperationKind.Invocation);
+		});
+	}
+}

src/BizHawk.Client.EmuHawk/config/HotkeyConfig.cs

@@ -6,6 +6,7 @@ using System.Windows.Forms;
 
 using BizHawk.Client.Common;
 using BizHawk.Common;
+using BizHawk.Common.CollectionExtensions;
 
 namespace BizHawk.Client.EmuHawk
 {
@@ -197,7 +198,7 @@ namespace BizHawk.Client.EmuHawk
 		{
 			if (e.IsPressed(Keys.Enter) || e.IsPressed(Keys.Tab))
 			{
-				var k = HotkeyInfo.AllHotkeys.FirstOrDefault(kvp => string.Compare(kvp.Value.DisplayName, SearchBox.Text, true) is 0).Key;
+				var k = HotkeyInfo.AllHotkeys.FirstOrNull(kvp => string.Compare(kvp.Value.DisplayName, SearchBox.Text, true) is 0)?.Key;
 
 				// Found
 				if (k is not null)

src/BizHawk.Client.EmuHawk/tools/HexEditor/HexEditor.cs

@@ -16,6 +16,7 @@ using BizHawk.Emulation.Common;
 using BizHawk.Client.Common;
 using BizHawk.Client.EmuHawk.Properties;
 using BizHawk.Client.EmuHawk.ToolExtensions;
+using BizHawk.Common.CollectionExtensions;
 
 namespace BizHawk.Client.EmuHawk
 {
@@ -242,13 +243,13 @@ namespace BizHawk.Client.EmuHawk
 		{
 			if (_textTable.Any())
 			{
-				var byteArr = new List<byte>();
-				foreach (var chr in str)
+				var byteArr = new byte[str.Length];
+				for (var i = 0; i < str.Length; i++)
 				{
-					byteArr.Add((byte)_textTable.FirstOrDefault(kvp => kvp.Value == chr).Key);
+					var c = str[i];
+					byteArr[i] = (byte) (_textTable.FirstOrNull(kvp => kvp.Value == c)?.Key ?? 0);
 				}
-
-				return byteArr.ToArray();
+				return byteArr;
 			}
 
 			return str.Select(Convert.ToByte).ToArray();

src/BizHawk.Common/HawkFile/HawkFile.cs

@@ -3,6 +3,8 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 
+using BizHawk.Common.CollectionExtensions;
+
 namespace BizHawk.Common
 {
 	/// <summary>
@@ -257,7 +259,8 @@ namespace BizHawk.Common
 		}
 
 		/// <summary>finds an ArchiveItem with the specified name (path) within the archive; returns null if it doesnt exist</summary>
-		public HawkArchiveFileItem? FindArchiveMember(string? name) => ArchiveItems.FirstOrDefault(ai => ai.Name == name);
+		public HawkArchiveFileItem? FindArchiveMember(string? name)
+			=> ArchiveItems.FirstOrNull(ai => ai.Name == name);
 
 		/// <returns>a stream for the currently bound file</returns>
 		/// <exception cref="InvalidOperationException">no stream bound (haven't called <see cref="BindArchiveMember(int)"/> or overload)</exception>

src/BizHawk.Emulation.DiscSystem/DiscFormats/CUE/CUE_Compile.cs

@@ -4,6 +4,7 @@ using System.Collections.Generic;
 using System.Linq;
 
 using BizHawk.Common;
+using BizHawk.Common.CollectionExtensions;
 
 //this would be a good place for structural validation
 //after this step, we won't want to have to do stuff like that (it will gunk up already sticky code)
@@ -109,10 +110,10 @@ namespace BizHawk.Emulation.DiscSystem.CUE
 
 		public override string ToString()
 		{
-			var idx = Indexes.FirstOrDefault(cci => cci.Number == 1);
-			if (idx.Number != 1) return $"T#{Number:D2} NO INDEX 1";
+			var idx = Indexes.FirstOrNull(static cci => cci.Number is 1);
+			if (idx is null) return $"T#{Number:D2} NO INDEX 1";
 			var indexlist = string.Join("|", Indexes);
-			return $"T#{Number:D2} {BlobIndex}:{idx.FileMSF} ({indexlist})";
+			return $"T#{Number:D2} {BlobIndex}:{idx.Value.FileMSF} ({indexlist})";
 		}
 	}