Add polyfill for Path.Join(...)

PolyShim.Tests/NetCore30/PathTests.cs

@@ -41,4 +41,21 @@ public void TrimEndingDirectorySeparator_Test()
         Path.TrimEndingDirectorySeparator("/a/b/c").Should().Be("/a/b/c");
         Path.TrimEndingDirectorySeparator(string.Empty).Should().Be(string.Empty);
     }
+
+    [Fact]
+    public void Join_Test()
+    {
+        // Act & assert
+
+        // For some reason, dropping the array literals leads to nullability errors on .NET Framework
+        // ReSharper disable RedundantExplicitParamsArrayCreation
+        Path.Join(["C:/Program Files/", "Utilities/SystemUtilities"])
+            .Should()
+            .Be("C:/Program Files/Utilities/SystemUtilities");
+        Path.Join(["C:/", "/Program Files"]).Should().Be("C://Program Files");
+        Path.Join(["C:/Users/Public/Documents/", "C:/Users/User1/Documents/Financial/"])
+            .Should()
+            .Be("C:/Users/Public/Documents/C:/Users/User1/Documents/Financial/");
+        // ReSharper restore RedundantExplicitParamsArrayCreation
+    }
 }

PolyShim/NetCore30/Path.cs

@@ -6,33 +6,73 @@
 // ReSharper disable PartialTypeWithSinglePart
 
 using System.IO;
+using System.Linq;
+using System.Text;
 
 internal static partial class PolyfillExtensions
 {
     extension(Path)
     {
-        // https://learn.microsoft.com/dotnet/api/system.io.path.endsindirectoryseparator#system-io-path-endsindirectoryseparator(system-string)
-        public static bool EndsInDirectorySeparator(string? path)
-        {
-            if (string.IsNullOrEmpty(path))
-                return false;
-
-            var lastChar = path![^1];
-
+        private static bool IsDirectorySeparator(char c) =>
 #if !NETSTANDARD || NETSTANDARD1_3_OR_GREATER
-            return lastChar == Path.DirectorySeparatorChar
-                || lastChar == Path.AltDirectorySeparatorChar;
+            c == Path.DirectorySeparatorChar || c == Path.AltDirectorySeparatorChar;
 #else
-            return lastChar is '\\' or '/';
+            c is '\\' or '/';
 #endif
-        }
+
+        // https://learn.microsoft.com/dotnet/api/system.io.path.endsindirectoryseparator#system-io-path-endsindirectoryseparator(system-string)
+        public static bool EndsInDirectorySeparator(string? path) =>
+            !string.IsNullOrEmpty(path) && Path.IsDirectorySeparator(path![^1]);
 
         // https://learn.microsoft.com/dotnet/api/system.io.path.trimendingdirectoryseparator#system-io-path-trimendingdirectoryseparator(system-string)
         public static string TrimEndingDirectorySeparator(string path) =>
             EndsInDirectorySeparator(path)
             && !Path.GetPathRoot(path).Equals(path, System.StringComparison.Ordinal)
                 ? path[..^1]
                 : path;
+
+        // https://learn.microsoft.com/dotnet/api/system.io.path.join#system-io-path-join(system-string())
+        public static string Join(params string?[] paths)
+        {
+            if (paths.Length <= 0)
+                return string.Empty;
+
+            var maxSize = paths.Sum(path => path?.Length ?? 0);
+            maxSize += paths.Length - 1;
+
+            var builder = new StringBuilder(maxSize);
+
+            foreach (var path in paths)
+            {
+                if (string.IsNullOrEmpty(path))
+                    continue;
+
+                if (builder.Length == 0)
+                {
+                    builder.Append(path);
+                }
+                else
+                {
+                    if (
+                        !Path.IsDirectorySeparator(builder[^1])
+                        && !Path.IsDirectorySeparator(path![0])
+                    )
+                    {
+                        builder.Append(
+#if !NETSTANDARD || NETSTANDARD1_3_OR_GREATER
+                            Path.DirectorySeparatorChar
+#else
+                            '/'
+#endif
+                        );
+                    }
+
+                    builder.Append(path);
+                }
+            }
+
+            return builder.ToString();
+        }
     }
 }
 #endif