C#: Improve auto builder logic to detect Sdk reference.

Author: michaelnebel

csharp/autobuilder/Semmle.Autobuild.Shared/Project.cs

@@ -3,7 +3,6 @@
 using System.IO;
 using System.Linq;
 using System.Xml;
-using Semmle.Util.Logging;
 
 namespace Semmle.Autobuild.Shared
 {
@@ -26,6 +25,36 @@ public class Project<TAutobuildOptions> : ProjectOrSolution<TAutobuildOptions> w
         private readonly Lazy<List<Project<TAutobuildOptions>>> includedProjectsLazy;
         public override IEnumerable<IProjectOrSolution> IncludedProjects => includedProjectsLazy.Value;
 
+        /// <summary>
+        /// According to https://learn.microsoft.com/en-us/visualstudio/msbuild/how-to-use-project-sdk?view=vs-2022#reference-a-project-sdk
+        /// there are three ways to reference a project SDK:
+        ///  1. As an attribute on the <Project/>.
+        ///  2. As a top level element of <Project>.
+        ///  3. As an attribute on an <Import> element.
+        ///
+        /// Returns true, if the Sdk attribute is used, otherwise false.
+        /// </summary>
+        private static bool UsesSdk(XmlElement xml)
+        {
+            // Case 1. and 2.
+            if (xml.HasAttribute("Sdk") || xml.ChildNodes.OfType<XmlElement>().Any(e => e.Name == "Sdk"))
+            {
+                return true;
+            }
+
+            var test = xml.GetElementsByTagName("Import");
+            // Case 3.
+            foreach (XmlElement e in xml.GetElementsByTagName("Import"))
+            {
+                if (e.HasAttribute("Sdk"))
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         public Project(Autobuilder<TAutobuildOptions> builder, string path) : base(builder, path)
         {
             ToolsVersion = new Version();
@@ -49,7 +78,7 @@ public Project(Autobuilder<TAutobuildOptions> builder, string path) : base(build
 
             if (root?.Name == "Project")
             {
-                if (root.HasAttribute("Sdk"))
+                if (UsesSdk(root))
                 {
                     DotNetProject = true;
                     return;