private async Task<Solution> MoveToNewFile(Document document, ClassDeclarationSyntax classDeclaration, CancellationToken cancellationToken)
{
var className = classDeclaration.Identifier.Text;
var syntaxRoot = await document.GetSyntaxRootAsync();
var newNamespace = classDeclaration
.FirstAncestorOrSelf<NamespaceDeclarationSyntax>()
.WithMembers(SyntaxFactory.List(new[] { (MemberDeclarationSyntax)classDeclaration }));
var compilationUnit = SyntaxFactory.CompilationUnit()
.WithMembers(SyntaxFactory.List(new[] { (MemberDeclarationSyntax)newNamespace }))
.WithUsings(SyntaxFactory.List(syntaxRoot.DescendantNodes().OfType<UsingDirectiveSyntax>()))
.WithAdditionalAnnotations(Formatter.Annotation);
var newDocument = document.Project.AddDocument(className + ".cs", string.Empty, folders: document.Folders).WithSyntaxRoot(compilationUnit);
newDocument = newDocument.Project
.GetDocument(document.Id)
.WithSyntaxRoot(syntaxRoot.RemoveNode(classDeclaration, SyntaxRemoveOptions.KeepNoTrivia));
return newDocument.Project.Solution;
}