public override void Initialize(AnalysisContext context)
{
    // Each time the compiler encounters a method declaration, call the AnalyzeNode method. 
    context.RegisterSyntaxNodeAction(AnalyzeNode, SyntaxKind.MethodDeclaration);
}

private void AnalyzeNode(SyntaxNodeAnalysisContext context)
{
    // Upcast the node to the type that corresponds to the specific SyntaxKind we registered: 
    var methodDeclarationSyntax = (MethodDeclarationSyntax) context.Node;
    
    // We can now do syntactic ("textual") analysis, such as checking if the method has a public modifier:
    if (methodDeclarationSyntax.Modifiers.Any(SyntaxKind.PublicKeyword))
    {
        // ...
    }
    
    // We can also do semantic analysis, which gives us far deeper inspection opportunities than just 
    // looking at the syntax.
    var methodDeclarationSymbol = context.SemanticModel.GetDeclaredSymbol(methodDeclarationSyntax);
    
    // For example, we can check if it's an extension method: 
    if (methodDeclarationSymbol.IsExtensionMethod)
    {
        // ...
    }
    
    // Or we can get its attributes:
    var methodAttributes = methodDeclarationSymbol.GetAttributes();
    
    // Or anything else that the compiler can possibly know.

    // If we find a problem, we report it like this. The Descriptor here refers to one of the descriptors
    // we passed to the SupportedDiagnostics property above.
    if (weFoundAnError) 
    {
        context.ReportDiagnostic(Diagnostic.Create(Descriptor, methodDeclaration.GetLocation()));
    }
}