Refactor (#94)

CliFx.Analyzers/CommandMustImplementInterfaceAnalyzer.cs

@@ -0,0 +1,53 @@
+using System.Linq;
+using CliFx.Analyzers.ObjectModel;
+using CliFx.Analyzers.Utils.Extensions;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Diagnostics;
+
+namespace CliFx.Analyzers
+{
+    [DiagnosticAnalyzer(LanguageNames.CSharp)]
+    public class CommandMustImplementInterfaceAnalyzer : AnalyzerBase
+    {
+        public CommandMustImplementInterfaceAnalyzer()
+            : base(
+                $"Commands must implement `{SymbolNames.CliFxCommandInterface}` interface",
+                $"This type must implement `{SymbolNames.CliFxCommandInterface}` interface in order to be a valid command.")
+        {
+        }
+
+        private void Analyze(SyntaxNodeAnalysisContext context)
+        {
+            if (context.Node is not ClassDeclarationSyntax classDeclaration)
+                return;
+
+            var type = context.SemanticModel.GetDeclaredSymbol(classDeclaration);
+            if (type is null)
+                return;
+
+            var hasCommandAttribute = type
+                .GetAttributes()
+                .Select(a => a.AttributeClass)
+                .Any(s => s.DisplayNameMatches(SymbolNames.CliFxCommandAttribute));
+
+            var implementsCommandInterface = type
+                .AllInterfaces
+                .Any(s => s.DisplayNameMatches(SymbolNames.CliFxCommandInterface));
+
+            // If the attribute is present, but the interface is not implemented,
+            // it's very likely a user error.
+            if (hasCommandAttribute && !implementsCommandInterface)
+            {
+                context.ReportDiagnostic(CreateDiagnostic(classDeclaration.GetLocation()));
+            }
+        }
+
+        public override void Initialize(AnalysisContext context)
+        {
+            base.Initialize(context);
+            context.RegisterSyntaxNodeAction(Analyze, SyntaxKind.ClassDeclaration);
+        }
+    }
+}