Add friendly error message to UrlHelper for when there is no IRouter (#8145)

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

@@ -32,7 +32,21 @@ namespace Microsoft.AspNetCore.Mvc.Routing
         /// Gets the top-level <see cref="IRouter"/> associated with the current request. Generally an
         /// <see cref="IRouteCollection"/> implementation.
         /// </summary>
-        protected IRouter Router => ActionContext.RouteData.Routers[0];
+        protected IRouter Router
+        {
+            get
+            {
+                var routers = ActionContext.RouteData.Routers;
+                if (routers.Count == 0)
+                {
+                    throw new InvalidOperationException("Could not find an IRouter associated with the ActionContext. "
+                        + "If your application is using endpoint routing then you can get a IUrlHelperFactory with "
+                        + "dependency injection and use it to create a UrlHelper, or use Microsoft.AspNetCore.Routing.LinkGenerator.");
+                }
+
+                return routers[0];
+            }
+        }
 
         /// <inheritdoc />
         public override string Action(UrlActionContext actionContext)

src/Mvc/Mvc.Core/test/Routing/UrlHelperTestBase.cs

@@ -964,6 +964,27 @@ namespace Microsoft.AspNetCore.Mvc.Routing
             Assert.Equal("http://localhost/app/home/contact", url);
         }
 
+        [Fact]
+        public void NoRouter_ErrorsWithFriendlyErrorMessage()
+        {
+            // Arrange
+            var urlHelper = new UrlHelper(new ActionContext
+            {
+                RouteData = new RouteData(new RouteValueDictionary()),
+                HttpContext = new DefaultHttpContext()
+            });
+
+            // Act
+            var ex = Assert.Throws<InvalidOperationException>(() => urlHelper.ActionLink("contact", "home"));
+
+            // Assert
+            var expectedMessage = "Could not find an IRouter associated with the ActionContext. "
+                + "If your application is using endpoint routing then you can get a IUrlHelperFactory with "
+                + "dependency injection and use it to create a UrlHelper, or use Microsoft.AspNetCore.Routing.LinkGenerator.";
+
+            Assert.Equal(expectedMessage, ex.Message);
+        }
+
         protected abstract IServiceProvider CreateServices();
 
         protected abstract IUrlHelper CreateUrlHelper(ActionContext actionContext);