Refactor API client for richer error handling

Refactored CatalogApiClient methods to return ApiResult<T> for create, update, and delete operations, enabling more detailed error reporting. Introduced ApiResult<T> and ProblemDetailsDto types, and added logic to parse and display informative error messages. Updated Catalogs.razor to use the new pattern and show user-friendly error feedback. Added necessary using directives.

DbFirst.BlazorWasm/Services/CatalogApiClient.cs

@@ -1,4 +1,6 @@
+using System.Net;
 using System.Net.Http.Json;
+using System.Text.Json;
 using DbFirst.BlazorWasm.Models;
 
 namespace DbFirst.BlazorWasm.Services;
@@ -24,26 +26,114 @@ public class CatalogApiClient
         return await _httpClient.GetFromJsonAsync<CatalogReadDto>($"{Endpoint}/{id}");
     }
 
-    public async Task<CatalogReadDto?> CreateAsync(CatalogWriteDto dto)
+    public async Task<ApiResult<CatalogReadDto?>> CreateAsync(CatalogWriteDto dto)
     {
         var response = await _httpClient.PostAsJsonAsync(Endpoint, dto);
-        if (!response.IsSuccessStatusCode)
+        if (response.IsSuccessStatusCode)
         {
-            return null;
+            var payload = await response.Content.ReadFromJsonAsync<CatalogReadDto>();
+            return ApiResult<CatalogReadDto?>.Ok(payload);
         }
 
-        return await response.Content.ReadFromJsonAsync<CatalogReadDto>();
+        var error = await ReadErrorAsync(response);
+        return ApiResult<CatalogReadDto?>.Fail(error);
     }
 
-    public async Task<bool> UpdateAsync(int id, CatalogWriteDto dto)
+    public async Task<ApiResult<bool>> UpdateAsync(int id, CatalogWriteDto dto)
     {
         var response = await _httpClient.PutAsJsonAsync($"{Endpoint}/{id}", dto);
-        return response.IsSuccessStatusCode;
+        if (response.IsSuccessStatusCode)
+        {
+            return ApiResult<bool>.Ok(true);
+        }
+
+        var error = await ReadErrorAsync(response);
+        return ApiResult<bool>.Fail(error);
     }
 
-    public async Task<bool> DeleteAsync(int id)
+    public async Task<ApiResult<bool>> DeleteAsync(int id)
     {
         var response = await _httpClient.DeleteAsync($"{Endpoint}/{id}");
-        return response.IsSuccessStatusCode;
+        if (response.IsSuccessStatusCode)
+        {
+            return ApiResult<bool>.Ok(true);
+        }
+
+        var error = await ReadErrorAsync(response);
+        return ApiResult<bool>.Fail(error);
+    }
+
+    private static async Task<string> ReadErrorAsync(HttpResponseMessage response)
+    {
+        string? problemTitle = null;
+        string? problemDetail = null;
+
+        try
+        {
+            var problem = await response.Content.ReadFromJsonAsync<ProblemDetailsDto>();
+            if (problem != null)
+            {
+                problemTitle = problem.Title;
+                problemDetail = problem.Detail ?? problem.Type;
+            }
+        }
+        catch
+        {
+            // ignore parse errors
+        }
+
+        var status = response.StatusCode;
+        var reason = response.ReasonPhrase;
+        var body = await response.Content.ReadAsStringAsync();
+
+        string detail = problemDetail;
+        if (string.IsNullOrWhiteSpace(detail) && !string.IsNullOrWhiteSpace(body))
+        {
+            detail = body;
+        }
+
+        // Friendly overrides
+        if (status == HttpStatusCode.Conflict)
+        {
+            return "Datensatz existiert bereits. Bitte wählen Sie einen anderen Titel.";
+        }
+        if (status == HttpStatusCode.BadRequest && (detail?.Contains("CatTitle cannot be changed", StringComparison.OrdinalIgnoreCase) ?? false))
+        {
+            return "Titel kann nicht geändert werden.";
+        }
+
+        return status switch
+        {
+            HttpStatusCode.BadRequest => $"Eingabe ungültig{FormatSuffix(problemTitle, detail, reason)}",
+            HttpStatusCode.NotFound => $"Nicht gefunden{FormatSuffix(problemTitle, detail, reason)}",
+            HttpStatusCode.Conflict => $"Konflikt{FormatSuffix(problemTitle, detail, reason)}",
+            HttpStatusCode.Unauthorized => $"Nicht autorisiert{FormatSuffix(problemTitle, detail, reason)}",
+            HttpStatusCode.Forbidden => $"Nicht erlaubt{FormatSuffix(problemTitle, detail, reason)}",
+            HttpStatusCode.InternalServerError => $"Serverfehler{FormatSuffix(problemTitle, detail, reason)}",
+            _ => $"Fehler {(int)status} {reason ?? string.Empty}{FormatSuffix(problemTitle, detail, reason)}"
+        };
+    }
+
+    private static string FormatSuffix(string? title, string? detail, string? reason)
+    {
+        var parts = new List<string>();
+        if (!string.IsNullOrWhiteSpace(title)) parts.Add(title);
+        if (!string.IsNullOrWhiteSpace(detail)) parts.Add(detail);
+        if (parts.Count == 0 && !string.IsNullOrWhiteSpace(reason)) parts.Add(reason);
+        if (parts.Count == 0) return string.Empty;
+        return ": " + string.Join(" | ", parts);
     }
 }
+
+public record ApiResult<T>(bool Success, T? Value, string? Error)
+{
+    public static ApiResult<T> Ok(T? value) => new(true, value, null);
+    public static ApiResult<T> Fail(string? error) => new(false, default, error);
+}
+
+internal sealed class ProblemDetailsDto
+{
+    public string? Type { get; set; }
+    public string? Title { get; set; }
+    public string? Detail { get; set; }
+}