Unhandled exception rendering component: '<' is an invalid start of a value

Question

Hi, my app is with blazor wasm

I just create a controller with razor page and I obtain

the message below

Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
System.Text.Json.JsonException: '<' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.
 ---> System.Text.Json.JsonReaderException: '<' is an invalid start of a value. LineNumber: 0 | BytePositionInLine: 0.
   at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan`1 bytes)
   at System.Text.Json.Utf8JsonReader.ConsumeValue(Byte marker)
   at System.Text.Json.Utf8JsonReader.ReadFirstToken(Byte first)
   at System.Text.Json.Utf8JsonReader.ReadSingleSegment()
   at System.Text.Json.Utf8JsonReader.Read()
   at System.Text.Json.Serialization.JsonConverter`1[[FollowUpDash.Shared.Models.TodoListModel[], FollowUpDash.Shared, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   --- End of inner exception stack trace ---
   at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& state, JsonReaderException ex)
   at System.Text.Json.Serialization.JsonConverter`1[[FollowUpDash.Shared.Models.TodoListModel[], FollowUpDash.Shared, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore[TodoListModel[]](JsonConverter jsonConverter, Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state)
   at System.Text.Json.JsonSerializer.ReadCore[TodoListModel[]](JsonReaderState& readerState, Boolean isFinalBlock, ReadOnlySpan`1 buffer, JsonSerializerOptions options, ReadStack& state, JsonConverter converterBase)
   at System.Text.Json.JsonSerializer.ContinueDeserialize[TodoListModel[]](ReadBufferState& bufferState, JsonReaderState& jsonReaderState, ReadStack& readStack, JsonConverter converter, JsonSerializerOptions options)
   at System.Text.Json.JsonSerializer.<ReadAllAsync>d__65`1[[FollowUpDash.Shared.Models.TodoListModel[], FollowUpDash.Shared, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].MoveNext()
   at System.Net.Http.Json.HttpContentJsonExtensions.<ReadFromJsonAsyncCore>d__4`1[[FollowUpDash.Shared.Models.TodoListModel[], FollowUpDash.Shared, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].MoveNext()
   at System.Net.Http.Json.HttpClientJsonExtensions.<GetFromJsonAsyncCore>d__13`1[[FollowUpDash.Shared.Models.TodoListModel[], FollowUpDash.Shared, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]].MoveNext()
   at FollowUpDash.Client.Pages.ToDoList.OnInitializedAsync() in C:\App\sbd\FollowUpDash\FollowUpDash\Client\Pages\ToDoList.razor:line 42
   at Microsoft.AspNetCore.Components.ComponentBase.RunInitAndSetParametersAsync()
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task taskToHandle, ComponentState owningComponentState)

I already have this message and I resolve it to add a route in controller or running server side. I checked it and I don't understand why I the problem.

to resolve my problem I had disabled the cache In devTool, do you know why?

Answer 1

The error is pretty straight forward. The blazor app is doing a json fetch, but the server is not returning json, but probably a html error page.

just use the browsers network debug tools to view the response and get the error.