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How it works...

.NET Compiler Platform SDK is a wrapper project that redirects us to fetch the project templates for analyzer + CodeFix projects for C# and Visual Basic. Creating a new project from these templates creates a fully functional analyzer project which has a default analyzer, unit tests, and a VSIX project:

CSharpAnalyzers: Core analyzer project that contains the default analyzer implementation that reports a diagnostic for all type names that contain any lowercase letters.
CSharpAnalyzers.Test: Analyzer unit test project that contains a couple of analyzer and code fixer unit tests and test helpers.
CSharpAnalyzers.Vsix: The VSIX project that packages the analyzer into a VSIX. This is the start-up project in the solution.

Clicking on F5 to start debugging the solution builds and deploys the analyzer to the Visual Studio extension hive and then starts a new Visual Studio instance from this hive. Our analyzer is enabled by default for all C# projects created in this VS instance.

Let's expand a bit more on the diagnostic analyzer source code defined in DiagnosticAnalyzers.cs. It contains a type named CSharpAnalyzersAnalyzer, which derives from DiagnosticAnalyzer. DiagnosticAnalyzer is an abstract type with the following two abstract members:

SupportedDiagnostics property: Analyzer must define one or more supported diagnostic descriptors. Descriptors describe the metadata for the diagnostics that an analyzer can report in analyzer actions. It contains fields such as the diagnostic ID, message format, title, description, hyperlink to documentation for the diagnostic, and so on. Can be used to create and report diagnostics:

private static DiagnosticDescriptor Rule = new DiagnosticDescriptor(DiagnosticId, Title, MessageFormat, Category, DiagnosticSeverity.Warning, isEnabledByDefault: true, description: Description);

public override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics { get { return ImmutableArray.Create(Rule); } }

Initialize method: Diagnostic analyzers must implement the Initialize method to register analyzer action callbacks for a specific code entity kind of interest, which is named type symbols for the default analyzer. The initialize method is invoked once for the analyzer lifetime to allow analyzer initialization and registration of actions.

 public override void Initialize(AnalysisContext context)
 {
  context.RegisterSymbolAction(AnalyzeSymbol, SymbolKind.NamedType);
 }

 private static void AnalyzeSymbol(SymbolAnalysisContext context)
 {
   ...
 }

Invoke AnalysisContext.EnableConcurrentExecution() in the Initialize method if your analyzer can handle action callbacks from multiple threads simultaneously -- this enables the analyzer driver to execute the analyzer more efficiently on a machine with multiple cores. Additionally, also invoke AnalysisContext.ConfigureGeneratedCodeAnalysis() in theInitialize method to configure whether or not the analyzer wants to analyze and/or report diagnostics in generated code.

Analyzer actions are invoked for every code entity of interest in a user s source code. Additionally, as the user edits code and a new compilation is created, action callbacks are continuously invoked for entities defined in the new compilation during code editing. The error list makes sure that it only reports the diagnostics from the active compilation.

Use http://source.roslyn.io for rich semantic search and navigation of Roslyn source code, which is open sourced at https://github.com/dotnet/roslyn.git. For example, you can look at the definition and references for DiagnosticAnalyzer using the query URL http://source.roslyn.io/#q=DiagnosticAnalyzer.