UseExceptionHandler throws if ExceptionHandlingPath not set (#417)

src/Microsoft.AspNetCore.Diagnostics/ExceptionHandler/ExceptionHandlerMiddleware.cs

@@ -30,12 +30,19 @@ namespace Microsoft.AspNetCore.Diagnostics
             _next = next;
             _options = options.Value;
             _logger = loggerFactory.CreateLogger<ExceptionHandlerMiddleware>();
-            if (_options.ExceptionHandler == null)
-            {
-                _options.ExceptionHandler = _next;
-            }
             _clearCacheHeadersDelegate = ClearCacheHeaders;
             _diagnosticSource = diagnosticSource;
+            if (_options.ExceptionHandler == null)
+            {
+                if (_options.ExceptionHandlingPath == null)
+                {
+                    throw new InvalidOperationException(Resources.FormatExceptionHandlerOptions_NotConfiguredCorrectly());
+                }
+                else
+                {
+                    _options.ExceptionHandler = _next;
+                }
+            }
         }
 
         public async Task Invoke(HttpContext context)

src/Microsoft.AspNetCore.Diagnostics/Properties/Resources.Designer.cs

@@ -584,6 +584,20 @@ namespace Microsoft.AspNetCore.Diagnostics
         internal static string FormatRuntimeInfoPage_Environment()
             => GetString("RuntimeInfoPage_Environment");
 
+        /// <summary>
+        /// An error occurred when configuring the exception handler middleware. Either the 'ExceptionHandlingPath' or the 'ExceptionHandler' option must be set in 'UseExceptionHandler()'.
+        /// </summary>
+        internal static string ExceptionHandlerOptions_NotConfiguredCorrectly
+        {
+            get => GetString("ExceptionHandlerOptions_NotConfiguredCorrectly");
+        }
+
+        /// <summary>
+        /// An error occurred when configuring the exception handler middleware. Either the 'ExceptionHandlingPath' or the 'ExceptionHandler' option must be set in 'UseExceptionHandler()'.
+        /// </summary>
+        internal static string FormatExceptionHandlerOptions_NotConfiguredCorrectly()
+            => GetString("ExceptionHandlerOptions_NotConfiguredCorrectly");
+
         private static string GetString(string name, params string[] formatterNames)
         {
             var value = _resourceManager.GetString(name);

src/Microsoft.AspNetCore.Diagnostics/Resources.resx

@@ -247,4 +247,7 @@
   <data name="RuntimeInfoPage_Environment" xml:space="preserve">
     <value>Environment:</value>
   </data>
+  <data name="ExceptionHandlerOptions_NotConfiguredCorrectly" xml:space="preserve">
+    <value>An error occurred when configuring the exception handler middleware. Either the 'ExceptionHandlingPath' or the 'ExceptionHandler' option must be set in 'UseExceptionHandler()'.</value>
+  </data>
 </root>

test/Microsoft.AspNetCore.Diagnostics.Tests/ExceptionHandlerTest.cs

@@ -494,5 +494,27 @@ namespace Microsoft.AspNetCore.Diagnostics
             Assert.NotNull(listener.DiagnosticHandledException?.HttpContext);
             Assert.NotNull(listener.DiagnosticHandledException?.Exception);
         }
+
+        [Fact]
+        public void UsingExceptionHandler_ThrowsAnException_WhenExceptionHandlingPathNotSet()
+        {
+            // Arrange
+            DiagnosticListener diagnosticListener = null;
+
+            var builder = new WebHostBuilder()
+                .Configure(app =>
+                {
+                    diagnosticListener = app.ApplicationServices.GetRequiredService<DiagnosticListener>();
+                    app.UseExceptionHandler();
+                });
+
+            // Act
+            var exception = Assert.Throws<InvalidOperationException>(() => new TestServer(builder));
+
+            // Assert
+            Assert.Equal($"An error occurred when configuring the exception handler middleware. " +
+                $"Either the 'ExceptionHandlingPath' or the 'ExceptionHandler' option must be set in 'UseExceptionHandler()'.",
+                exception.Message);
+        }
     }
 }