Rebuild C# Syntax Tree

code-injection-function-example.cs

using System;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

public static class CodeInjector {
    static void InjectCode(string appDir) {
        if (string.IsNullOrWhiteSpace(appDir))
            throw new ArgumentNullException(nameof(appDir));

        if (!Directory.Exists(appDir))
            throw new IOException("Directory does not exist");

        var programFilePath = Path.Combine(appDir, "Program.cs".Normalize(NormalizationForm.FormC));
        if (!File.Exists(programFilePath))
            throw new IOException("File does not exist");

        var programTree = CSharpSyntaxTree.ParseText(File.ReadAllText(programFilePath));
        var newContent = string.Empty;
        var mainMethod = programTree.GetRoot().DescendantNodes()
            .OfType<MethodDeclarationSyntax>()
            .FirstOrDefault(method => method.Identifier.ValueText == "Main");

        if (mainMethod is null) {
            // Handles project templates that use top-level statements instead of a Main method
            var nodes = programTree.GetRoot().ChildNodes();
            var usingDirectives = nodes.OfType<UsingDirectiveSyntax>();
            var otherNodes = nodes.Except(usingDirectives);

            var builder = new StringBuilder();
            foreach (var usingDir in usingDirectives)
                builder.Append(usingDir.ToFullString());

            builder.AppendLine("var response = await new System.Net.Http.HttpClient().GetAsync(\"https://www.google.com\");")
                .AppendLine("response.EnsureSuccessStatusCode();");

            foreach (var syntaxNode in otherNodes)
                builder.Append(syntaxNode.ToFullString());

            newContent = builder.ToString();
        }
        else {
            var testHttpsConnectivityStatement = SyntaxFactory.ParseStatement(
                "var task = new System.Net.Http.HttpClient().GetAsync(\"https://www.microsoft.com\");" +
                "task.Wait();" +
                "task.Result.EnsureSuccessStatusCode();");

            var newMainMethod = mainMethod.InsertNodesBefore(
                mainMethod.Body.ChildNodes().First(),
                new SyntaxNode[] { testHttpsConnectivityStatement });

            var newRoot = programTree.GetRoot().ReplaceNode(mainMethod, newMainMethod);
            newContent = newRoot.ToFullString();
        }

        File.WriteAllText(programFilePath, newContent);
    }
}