http-status-aware-error-handling

Context

ASP.NET Core Web controllers calling downstream APIs via HttpClient should differentiate HTTP error responses based on status code, providing appropriate user feedback:

• 4xx client errors: User-actionable messages (validation, conflict, not found)
• 5xx server errors: Technical error messages with details for support
• 409 Conflict (idempotency): Warning-level feedback, not error-level

This applies when:

• Web layer calls API via IDownstreamApi or HttpClient
• Operations can return 409 Conflict for "already done" scenarios (duplicate resource, idempotent retry)
• User experience should distinguish between "this is fine" vs "something broke"

Patterns

1. Check HttpRequestException.StatusCode

try
{
    var result = await _service.AddResourceAsync(id, data);
    if (result is null)
    {
        TempData["ErrorMessage"] = "Failed to add resource.";
    }
    else
    {
        TempData["SuccessMessage"] = "Resource added successfully.";
    }
}
catch (HttpRequestException ex)
{
    // Differentiate based on status code
    if (ex.StatusCode == System.Net.HttpStatusCode.Conflict)
    {
        TempData["WarningMessage"] = "This resource is already associated.";
    }
    else if (ex.StatusCode == System.Net.HttpStatusCode.NotFound)
    {
        TempData["ErrorMessage"] = "The target resource was not found.";
    }
    else
    {
        TempData["ErrorMessage"] = $"Failed to add resource: {ex.Message}";
    }
}

2. Support TempData Warning Messages in Layout

Ensure _Layout.cshtml (or equivalent) displays warning-level messages:

@if (TempData["SuccessMessage"] != null)
{
    <div class="alert alert-success alert-dismissible fade show" role="alert">
        @TempData["SuccessMessage"]
        <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
    </div>
}
@if (TempData["WarningMessage"] != null)
{
    <div class="alert alert-warning alert-dismissible fade show" role="alert">
        @TempData["WarningMessage"]
        <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
    </div>
}
@if (TempData["ErrorMessage"] != null)
{
    <div class="alert alert-danger alert-dismissible fade show" role="alert">
        @TempData["ErrorMessage"]
        <button type="button" class="btn-close" data-bs-dismiss="alert"></button>
    </div>
}

3. Test Both Happy and Error Paths

[Fact]
public async Task Action_When409Conflict_RedirectsWithWarningMessage()
{
    // Arrange
    var conflictException = new HttpRequestException(
        "Conflict",
        null,
        System.Net.HttpStatusCode.Conflict);
    _service.Setup(s => s.AddAsync(1, 2))
        .ThrowsAsync(conflictException);

    // Act
    var result = await _controller.Add(viewModel);

    // Assert
    var redirectResult = Assert.IsType<RedirectToActionResult>(result);
    Assert.Equal("This resource is already associated.", _controller.TempData["WarningMessage"]);
}

Examples

Real-world case: Issue #708 AddPlatform

Problem: Adding a social media platform to an engagement returns 409 if already associated. Original code showed generic error.

Solution:

• EngagementsController.AddPlatform POST catches HttpRequestException
• 409 → TempData["WarningMessage"] = "This platform is already associated with this engagement."
• Other errors → TempData["ErrorMessage"] with technical details

Files:

• src/JosephGuadagno.Broadcasting.Web/Controllers/EngagementsController.cs (lines 250-280)
• src/JosephGuadagno.Broadcasting.Web/Views/Shared/_Layout.cshtml (lines 148-161)
• src/JosephGuadagno.Broadcasting.Web.Tests/Controllers/EngagementsControllerTests.cs (AddPlatform tests)

Anti-Patterns

❌ Treat all HttpRequestException the same

catch (HttpRequestException ex)
{
    TempData["ErrorMessage"] = $"Failed: {ex.Message}"; // ← No differentiation
}

Problem: User can't tell if retry is safe or if something is broken.

❌ Suppress 409 entirely

if (ex.StatusCode == HttpStatusCode.Conflict)
{
    // Silent ignore ← User has no feedback
}

Problem: User doesn't know the operation was a no-op.

❌ Treat 409 as success

if (ex.StatusCode == HttpStatusCode.Conflict)
{
    TempData["SuccessMessage"] = "Resource added successfully."; // ← Misleading
}

Problem: User thinks resource was just added when it was already there.

Guidelines

1. 409 Conflict: Use warning-level message ("This resource is already..."). Safe to retry, benign outcome.
2. 404 Not Found (on add/update): Error-level message. Parent resource missing or ID invalid.
3. 400 Bad Request: Error-level message with details. Validation or contract issue.
4. 500 Server Error: Error-level message. Log details, show generic user message.

HTTP Status Code Reference for Web Controllers

Status Code	Level	User Message Example	Retry Safe?
200/201/204	Success	"Resource added successfully."	N/A
400	Error	"Invalid request: {details}"	No (fix data)
404	Error	"Resource not found."	No
409	Warning	"This resource is already associated."	Yes (no-op)
422	Error	"Validation failed: {details}"	No (fix data)
500	Error	"An error occurred. Please try again later."	Maybe