Allow ProblemDetails \ ValidationProblemDetails to be more testable

Fixes https://github.com/dotnet/aspnetcore/issues/15166

src/Mvc/Mvc.Core/src/ControllerBase.cs

@@ -1886,13 +1886,29 @@ namespace Microsoft.AspNetCore.Mvc
             string title = null,
             string type = null)
         {
-            var problemDetails = ProblemDetailsFactory.CreateProblemDetails(
+            ProblemDetails problemDetails;
-                HttpContext,
+            if (ProblemDetailsFactory == null)
-                statusCode: statusCode ?? 500,
+            {
-                title: title,
+                // ProblemDetailsFactory may be null in unit testing scenarios. Improvise to make this more testable.
-                type: type,
+                problemDetails = new ProblemDetails
-                detail: detail,
+                {
-                instance: instance);
+                    Detail = detail,
+                    Instance = instance,
+                    Status = statusCode ?? 500,
+                    Title = title,
+                    Type = type,
+                };
+            }
+            else
+            {
+                problemDetails = ProblemDetailsFactory.CreateProblemDetails(
+                    HttpContext,
+                    statusCode: statusCode ?? 500,
+                    title: title,
+                    type: type,
+                    detail: detail,
+                    instance: instance);
+            }
 
             return new ObjectResult(problemDetails)
             {
@@ -1958,14 +1974,30 @@ namespace Microsoft.AspNetCore.Mvc
         {
             modelStateDictionary ??= ModelState;
 
-            var validationProblem = ProblemDetailsFactory.CreateValidationProblemDetails(
+            ValidationProblemDetails validationProblem;
-                HttpContext,
+            if (ProblemDetailsFactory == null)
-                modelStateDictionary,
+            {
-                statusCode: statusCode,
+                // ProblemDetailsFactory may be null in unit testing scenarios. Improvise to make this more testable.
-                title: title,
+                validationProblem = new ValidationProblemDetails(modelStateDictionary)
-                type: type,
+                {
-                detail: detail,
+                    Detail = detail,
-                instance: instance);
+                    Instance = instance,
+                    Status = statusCode,
+                    Title = title,
+                    Type = type,
+                };
+            }
+            else
+            {
+                validationProblem = ProblemDetailsFactory?.CreateValidationProblemDetails(
+                    HttpContext,
+                    modelStateDictionary,
+                    statusCode: statusCode,
+                    title: title,
+                    type: type,
+                    detail: detail,
+                    instance: instance);
+            }
 
             if (validationProblem.Status == 400)
             {

src/Mvc/Mvc.ViewFeatures/test/ControllerUnitTestabilityTests.cs

@@ -373,7 +373,7 @@ namespace Microsoft.AspNetCore.Mvc
             Assert.Equal(routeName, acceptedAtRouteResult.RouteName);
             Assert.Single(acceptedAtRouteResult.RouteValues);
             Assert.Equal("sample", acceptedAtRouteResult.RouteValues["route"]);
-            Assert.Same(value,acceptedAtRouteResult.Value);
+            Assert.Same(value, acceptedAtRouteResult.Value);
 
             // Arrange
             controller = new TestabilityController();
@@ -682,6 +682,42 @@ namespace Microsoft.AspNetCore.Mvc
             Assert.Equal(new { Arg1 = "Hi", Arg2 = "There" }, result.Arguments);
         }
 
+        [Fact]
+        public void Problem_Works()
+        {
+            // Arrange
+            var detail = "Some random error";
+            var controller = new TestabilityController();
+
+            // Act
+            var result = controller.Problem(detail);
+
+            // Assert
+            var badRequest = Assert.IsType<ObjectResult>(result);
+            var problemDetails = Assert.IsType<ProblemDetails>(badRequest.Value);
+            Assert.Equal(detail, problemDetails.Detail);
+        }
+
+        [Fact]
+        public void ValidationProblem_Works()
+        {
+            // Arrange
+            var detail = "Some random error";
+            var controller = new TestabilityController();
+
+            // Act
+            controller.ModelState.AddModelError("some-key", "some-error");
+            var result = controller.ValidationProblem(detail);
+
+            // Assert
+            var badRequest = Assert.IsType<ObjectResult>(result);
+            var validationProblemDetails = Assert.IsType<ValidationProblemDetails>(badRequest.Value);
+            Assert.Equal(detail, validationProblemDetails.Detail);
+            var error = Assert.Single(validationProblemDetails.Errors);
+            Assert.Equal("some-key", error.Key);
+            Assert.Equal(new[] { "some-error" }, error.Value);
+        }
+
         public static IEnumerable<object[]> TestabilityViewTestData
         {
             get