diff --git a/csharp/autobuilder/Semmle.Autobuild.Shared/Project.cs b/csharp/autobuilder/Semmle.Autobuild.Shared/Project.cs
index 4a6acb127e3..9c2902f8973 100644
--- a/csharp/autobuilder/Semmle.Autobuild.Shared/Project.cs
+++ b/csharp/autobuilder/Semmle.Autobuild.Shared/Project.cs
@@ -108,7 +108,8 @@ namespace Semmle.Autobuild.Shared
                         }
 
                         var includePath = builder.Actions.PathJoin(include.Value.Split('\\', StringSplitOptions.RemoveEmptyEntries));
-                        ret.Add(new Project<TAutobuildOptions>(builder, builder.Actions.PathJoin(DirectoryName, includePath)));
+                        var path = Path.IsPathRooted(includePath) ? includePath : builder.Actions.PathJoin(DirectoryName, includePath);
+                        ret.Add(new Project<TAutobuildOptions>(builder, path));
                     }
                     return ret;
                 });
diff --git a/csharp/extractor/Semmle.Extraction.CSharp/Extractor/CsProjFile.cs b/csharp/extractor/Semmle.Extraction.CSharp/Extractor/CsProjFile.cs
index d130e4002ff..844f16086c6 100644
--- a/csharp/extractor/Semmle.Extraction.CSharp/Extractor/CsProjFile.cs
+++ b/csharp/extractor/Semmle.Extraction.CSharp/Extractor/CsProjFile.cs
@@ -159,7 +159,11 @@ namespace Semmle.Extraction.CSharp
                 return null;
             }
 
-            return Path.GetFullPath(Path.Join(projDir?.FullName ?? string.Empty, Path.DirectorySeparatorChar == '/' ? file.Replace("\\", "/") : file));
+            var normalized = Path.DirectorySeparatorChar == '/' ? file.Replace("\\", "/") : file;
+            var path = projDir is not null && !Path.IsPathRooted(normalized)
+                ? Path.Join(projDir.FullName, normalized)
+                : normalized;
+            return Path.GetFullPath(path);
         }
 
         private readonly string[] references;