Reacting to Routing changes

src/Microsoft.AspNet.Mvc.Core/Routing/UrlHelper.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Text;
 using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Routing;
 
@@ -28,7 +29,7 @@ public UrlHelper(ActionContext actionContext)
             {
                 throw new ArgumentNullException(nameof(actionContext));
             }
-            
+
             ActionContext = actionContext;
         }
 
@@ -78,7 +79,7 @@ public virtual string Action(UrlActionContext actionContext)
                 valuesDictionary["controller"] = actionContext.Controller;
             }
 
-            var path = GeneratePathFromRoute(valuesDictionary);
+            var path = GeneratePathFromRoute(routeName: null, values: valuesDictionary);
             if (path == null)
             {
                 return null;
@@ -119,11 +120,6 @@ public virtual string RouteUrl(UrlRouteContext routeContext)
             return GenerateUrl(routeContext.Protocol, routeContext.Host, path, routeContext.Fragment);
         }
 
-        private string GeneratePathFromRoute(RouteValueDictionary values)
-        {
-            return GeneratePathFromRoute(routeName: null, values: values);
-        }
-
         /// <summary>
         /// Generates the absolute path of the url for the specified route values by
         /// using the specified route name.
@@ -142,15 +138,59 @@ protected virtual string GeneratePathFromRoute(string routeName, RouteValueDicti
 
             // VirtualPathData.VirtualPath returns string.Empty for null.
             Debug.Assert(pathData.VirtualPath != null);
-
-            var fullPath = HttpContext.Request.PathBase.Add(pathData.VirtualPath).Value;
-            if (fullPath.Length == 0)
+            var pathBase = HttpContext.Request.PathBase;
+            if (!pathBase.HasValue)
             {
-                return "/";
+                if (pathData.VirtualPath.Length == 0)
+                {
+                    return "/";
+                }
+                else
+                {
+                    return !pathData.VirtualPath.StartsWith("/", StringComparison.Ordinal)
+                        ? "/" + pathData.VirtualPath
+                        : pathData.VirtualPath;
+                }
             }
             else
             {
-                return fullPath;
+                if (pathData.VirtualPath.Length == 0)
+                {
+                    return pathBase;
+                }
+                else
+                {
+                    var requiresLeadingSlash = !pathData.VirtualPath.StartsWith("/", StringComparison.Ordinal);
+                    var virtualPathStartIndex = 0;
+                    if (pathBase.Value.EndsWith("/", StringComparison.Ordinal))
+                    {
+                        if (!requiresLeadingSlash)
+                        {
+                            // If the base path has a trailing slash and the virtual path string has a leading slash,
+                            // we need to trim one of them.
+                            virtualPathStartIndex = 1;
+
+                        }
+
+                        requiresLeadingSlash = false;
+                    }
+
+                    var virtualPathLength = pathData.VirtualPath.Length - virtualPathStartIndex;
+
+                    var builder = new StringBuilder(
+                        pathBase.Value.Length
+                        + (requiresLeadingSlash ? 1 : 0)
+                        + virtualPathLength);
+
+                    builder.Append(pathBase.Value);
+                    if (requiresLeadingSlash)
+                    {
+                        builder.Append("/");
+                    }
+                    builder.Append(pathData.VirtualPath, virtualPathStartIndex, virtualPathLength);
+
+                    return builder.ToString();
+                }
             }
         }
 
@@ -186,9 +226,7 @@ public virtual string Link(string routeName, object values)
 
         private string GenerateUrl(string protocol, string host, string path, string fragment)
         {
-            // We should have a robust and centrallized version of this code. See HttpAbstractions#28
             Debug.Assert(path != null);
-
             var url = path;
             if (!string.IsNullOrEmpty(fragment))
             {