RoslynAnalysisTests.cs

using System.Collections.Generic;
using System.Linq;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using NUnit.Framework;
using static Microsoft.CodeAnalysis.CSharp.SyntaxKind;

namespace Tests
{
    public class RoslynAnalysisTests
    {
        const string BadFile1 = "class A {}";
        const string BadFile2 = "class B { class C {} }";
        const string GoodFile1 = "namespace N { class A {} }";
        const string GoodFile2 = "namespace N { class X { class Y {} } }";

        [Test]
        public void TestAllFiles()
        {
            string[] files = { BadFile1, GoodFile1, BadFile2, GoodFile2 };

            var badFiles =
                files
                    .Select(file => CSharpSyntaxTree.ParseText(file))
                    .SelectMany(AllClasses)
                    .Where(OutsideOfNamespace);

            CollectionAssert.IsEmpty(badFiles);
        }

        static bool OutsideOfNamespace(ClassDeclarationSyntax classNode) => classNode.Ancestors().All(NotANamespace);
        static bool NotANamespace(SyntaxNode node) => node.Kind() != NamespaceDeclaration;
        static IEnumerable<ClassDeclarationSyntax> AllClasses(SyntaxTree syntaxTree) => AllNodes(syntaxTree).OfType<ClassDeclarationSyntax>();
        static IEnumerable<SyntaxNode> AllNodes(SyntaxTree syntaxTree) => syntaxTree.GetRoot().DescendantNodesAndSelf();
    }
}