DocumentEditorSample.cs

var mscorlib = MetadataReference.CreateFromAssembly(typeof(object).Assembly);
var workspace = new AdhocWorkspace();
var projectId = ProjectId.CreateNewId();
var versionStamp = VersionStamp.Create();
var projectInfo = ProjectInfo.Create(projectId, versionStamp, "NewProject", "projName", LanguageNames.CSharp);
var newProject = workspace.AddProject(projectInfo);
var sourceText = SourceText.From(@"
class C
{
    void M()
    {
        char key = Console.ReadKey();
        if (key == 'A')
        {
            Console.WriteLine(""You pressed A"");
        }
        else
        {
            Console.WriteLine(""You didn't press A"");
        }
    }
}");
var document = workspace.AddDocument(newProject.Id, "NewFile.cs", sourceText);
var syntaxRoot = await document.GetSyntaxRootAsync();
var ifStatement = syntaxRoot.DescendantNodes().OfType<IfStatementSyntax>().Single();

var conditionWasTrueInvocation =
SyntaxFactory.ExpressionStatement(
    SyntaxFactory.InvocationExpression(SyntaxFactory.IdentifierName("LogConditionWasTrue"))
    .WithArgumentList(
                    SyntaxFactory.ArgumentList()
                    .WithOpenParenToken(
                        SyntaxFactory.Token(
                            SyntaxKind.OpenParenToken))
                    .WithCloseParenToken(
                        SyntaxFactory.Token(
                            SyntaxKind.CloseParenToken))))
            .WithSemicolonToken(
                SyntaxFactory.Token(
                    SyntaxKind.SemicolonToken));

var conditionWasFalseInvocation =
SyntaxFactory.ExpressionStatement(
    SyntaxFactory.InvocationExpression(SyntaxFactory.IdentifierName("LogConditionWasFalse"))
    .WithArgumentList(
                    SyntaxFactory.ArgumentList()
                    .WithOpenParenToken(
                        SyntaxFactory.Token(
                            SyntaxKind.OpenParenToken))
                    .WithCloseParenToken(
                        SyntaxFactory.Token(
                            SyntaxKind.CloseParenToken))))
            .WithSemicolonToken(
                SyntaxFactory.Token(
                    SyntaxKind.SemicolonToken));

//Finally... create the document editor
var documentEditor = await DocumentEditor.CreateAsync(document);
//Insert LogConditionWasTrue() before the Console.WriteLine()
documentEditor.InsertBefore(ifStatement.Statement.ChildNodes().Single(), conditionWasTrueInvocation);
//Insert LogConditionWasFalse() after the Console.WriteLine()
documentEditor.InsertAfter(ifStatement.Else.Statement.ChildNodes().Single(), conditionWasFalseInvocation);

var newDocument = documentEditor.GetChangedDocument();