{"record":{"id":"9d2d081b62bd8af1","repo":"dotnet/AspNetCore.Docs","slug":"no-weather-forecast","errorCode":null,"errorMessage":"No weather forecast!","messagePattern":"No weather forecast!","errorType":"exception","errorClass":"IOException","httpStatus":null,"severity":"error","filePath":"aspnetcore/blazor/call-web-api.md","lineNumber":1072,"sourceCode":"\n> [!NOTE]\n> In non-`Production` environments, the preceding example uses to simplify authentication while developing apps that deploy to Azure by combining credentials used in Azure hosting environments with credentials used in local development. When moving to production, an alternative is a better choice, such as the shown in the preceding example. For more information, see [Authenticate Azure-hosted .NET apps to Azure resources using a system-assigned managed identity](/dotnet/azure/sdk/authentication/system-assigned-managed-identity).\n\nInject and call when calling on behalf of a user:\n\n```csharp\ninternal sealed class ServerWeatherForecaster(IDownstreamApi downstreamApi) : IWeatherForecaster\n{\n public async Task> GetWeatherForecastAsync()\n {\n var response = await downstreamApi.CallApiForUserAsync(\"DownstreamApi\",\n options =>\n {\n options.RelativePath = \"/weather-forecast\";\n });\n\n return await response.Content.ReadFromJsonAsync() ??\n throw new IOException(\"No weather forecast!\");\n }\n}\n```\n\nThis approach is used by the `BlazorWebAppEntra` and `BlazorWebAppEntraBff` sample apps described in the *Sample apps* section of this article.\n\nFor more information, see the following resources:\n\n* \n* \n* [Use the Azure SDK for .NET in ASP.NET Core apps](/dotnet/azure/sdk/aspnetcore-guidance?tabs=api)\n* [Web API documentation | Microsoft identity platform](/entra/identity-platform/index-web-api)\n* [A web API that calls web APIs: Call an API: Option 2: Call a downstream web API with the helper class](/entra/identity-platform/scenario-web-api-call-api-call-api?tabs=aspnetcore#option-2-call-a-downstream-web-api-with-the-helper-class)\n* \n* *Secure an ASP.NET Core Blazor Web App with Microsoft Entra ID*\n * [With YARP and Aspire (Interactive Auto)](xref:blazor/security/blazor-web-app-entra?pivots=with-yarp-and-aspire)\n * [Without YARP and Aspire (Interactive Auto)](xref:blazor/security/blazor-web-app-entra?pivots=without-yarp-and-aspire)\n* [Host ASP.NET Core in a web farm: Data Protection](xref:host-and-deploy/web-farm#data-protection)","sourceCodeStart":1054,"sourceCodeEnd":1090,"githubUrl":"https://github.com/dotnet/AspNetCore.Docs/blob/c67a80103a1a74db20784debd919c7fdda96c510/aspnetcore/blazor/call-web-api.md#L1054-L1090","documentation":"The `ServerWeatherForecaster` sample deserializes the downstream API response as `WeatherForecast[]` and throws `IOException(\"No weather forecast!\")` when the result is null. ReadFromJsonAsync returns null for an empty body or JSON `null`, so this converts a silent null into an explicit failure.","triggerScenarios":"The downstream `/weather-forecast` endpoint returns an empty response body, a JSON `null`, or a payload that deserializes to null. The null-coalescing throw then fires.","commonSituations":"Downstream API returned 204 No Content or an empty 200; route mismatch returning a different shape; auth/token failure resulting in a non-JSON error page that deserializes to null; deserialization type mismatch (private setter, missing parameterless constructor) collapsing the array to null.","solutions":["Check the downstream endpoint actually returns a JSON array — call it directly and inspect the body/status.","Verify the access token was obtained (`GetTokenAsync(\"access_token\")`) and the downstream accepted it.","Ensure `WeatherForecast` has a parameterless constructor and settable properties so deserialization succeeds.","Handle the IOException at the call site and fall back to cached/empty data only if appropriate."],"exampleFix":"// before\nreturn await response.Content.ReadFromJsonAsync()\n ?? throw new IOException(\"No weather forecast!\");\n\n// after (surface the real cause)\nresponse.EnsureSuccessStatusCode();\nvar forecasts = await response.Content.ReadFromJsonAsync()\n ?? throw new IOException($\"No weather forecast! Status {(int)response.StatusCode}, length {response.Content.Headers.ContentLength}\");","handlingStrategy":"validation","validationCode":"public async Task> GetWeatherForecastAsync()\n{\n var response = await downstreamApi.CallApiForUserAsync(\"DownstreamApi\",\n o => o.RelativePath = \"/weather-forecast\");\n response.EnsureSuccessStatusCode();\n var body = await response.Content.ReadAsStringAsync();\n if (string.IsNullOrWhiteSpace(body))\n throw new IOException($\"Empty body, status {(int)response.StatusCode}\");\n return JsonSerializer.Deserialize(body)\n ?? throw new IOException(\"Deserialized to null\");\n}","typeGuard":"static bool IsForecastPayload(WeatherForecast[]? arr) => arr is { Length: > 0 };","tryCatchPattern":"try\n{\n return await forecaster.GetWeatherForecastAsync();\n}\ncatch (IOException ex) when (ex.Message.Contains(\"No weather forecast\"))\n{\n logger.LogWarning(ex, \"Downstream returned no forecast\");\n return Array.Empty();\n}","preventionTips":["Call `EnsureSuccessStatusCode` before deserializing.","Verify WeatherForecast has a parameterless constructor and public setters.","Log status + body length when null is returned to find the root cause."],"tags":["blazor","web-api","http","deserialization","entra"],"backgroundTag":null,"analyzedSha":"c67a80103a1a74db20784debd919c7fdda96c510","analyzedAt":"2026-08-13T17:46:11.763Z","schemaVersion":2},"datasetVersion":"2026-08-13T19:17:28.613Z"}