public override async Task RegisterCodeFixesAsync(CodeFixContext context)
{
var root = await context.Document.GetSyntaxRootAsync(context.CancellationToken).ConfigureAwait(false);
var method = context.GetFirstNodeWithDiagnostic <MethodDeclarationSyntax>(root);
// Extracting semantic model and precodition block
var semanticModel = await context.Document.GetSemanticModelAsync(context.CancellationToken).ConfigureAwait(false);
var preconditionBlock = PreconditionsBlock.GetPreconditions(method, semanticModel);
Contract.Assert(preconditionBlock.Preconditions.Count != 0, "Precondition block should have at least one statement!");
// Caching all precondition. This will help to remove or leave them in different methods.
var preconditionStatements = preconditionBlock.Preconditions.Select(p => p.IfThrowStaement).ToImmutableHashSet();
// Extracting new method: it should contains all statements from the original method
// but without preconditions.
var extractedMethodBody = method.Body.Statements.Where(s => !preconditionStatements.Contains(s));
// Clonning original method by changing it's body and changing the visibility
var extractedMethod =
method.WithStatements(extractedMethodBody)
.WithIdentifier(Identifier($"Do{method.Identifier.Text}"))
.WithVisibilityModifier(VisibilityModifier.Private);
// Updating original method: removing everything except preconditions
var updatedMethodBody = method.Body.Statements.Where(s => preconditionStatements.Contains(s)).ToList();
// Creating an invocation for extracted method
var originalMethodCallExpression = CreateMethodCallExpression(extractedMethod, method.ParameterList.AsArguments());
updatedMethodBody.Add(SyntaxFactory.ReturnStatement(originalMethodCallExpression));
// Removing 'async'
var updatedMethod =
method.WithStatements(updatedMethodBody)
.WithoutModifiers(t => t.IsKind(SyntaxKind.AsyncKeyword));
// Заменяем метод парой узлов: новым методом и выделенным методом
var newRoot = root.ReplaceNode(method, new[] { updatedMethod, extractedMethod });
var codeAction = CodeAction.Create(FixText, ct => Task.FromResult(context.Document.WithSyntaxRoot(newRoot)));
context.RegisterCodeFix(codeAction, context.Diagnostics.First());
}