无法将“AttachmentDetailsDto”类型的对象转换为“System.String”类型

Question

Have method

public async Task<IEnumerable<IGrouping<string, AttachmentDetailsDto>>> GetAttachmentsAsync(long assetInfoId)

within it I do...

var results = new List<AttachmentDetailsDto>();
var attachmentEntities = await _attachmentRepository.GetAsync(x => x.ParentAssetInfoId == assetInfoId && !x.IsDeleted).ConfigureAwait(false);

foreach (var attachmentEntity in attachmentEntities)
{
    var attachmentTypeEntity = await _attachmentTypeRepository.GetByIdAsync(attachmentEntity.MetattachTypeId).ConfigureAwait(false);

    var metattachDetail = new AttachmentDetailsDto
    {
        Id = attachmentEntity.Id,
        TypeId = attachmentEntity.MetattachTypeId,
        Type = attachmentTypeEntity.Name,
        Name = attachmentEntity.Name,
        Notes = attachmentEntity.Notes,
    };

    attachmentType = attachmentEntity.MetaType;
    var assetInfoEntity = default(AssetInfo);

    switch (attachmentType)
    {
        case "DigiDoc":
            var digiDocEntity = await _digiDocRepository.GetByIdAsync(attachmentEntity.TypeId).ConfigureAwait(false);
            metattachDetail.DigiDoc = new DigiDocDto
            {
                Id = digiDocEntity.Id,
                ContentType = digiDocEntity.ContentType,
                Url = digiDocEntity.Url
            };
            break;
... other cases ...
    }
    results.Add(metattachDetail);
}

return results.GroupBy(x => x.Type);

No errors here, but in my controller...

        public async Task<ActionResult> GetAttachmentsAsync(long assetInfoId)
        {
            var result = await _attachmentManager.GetAttachmentsAsync(assetInfoId).ConfigureAwait(false);
            return Ok(result);
        }

An error is thrown on line " return Ok(result); "

... I don't understand it, same code in .Net Framework works perfectly, now in .Net Core 3.1 it errors...

Error:

Unable to cast object of type 'AttachmentDetailsDto' to type 'System.String'.

Stack Trace:

at System.Text.Json.JsonPropertyInfoNotNullable'4.OnWriteEnumerable(WriteStackFrame& current, Utf8JsonWriter writer) at System.Text.Json.JsonPropertyInfo.WriteEnumerable(WriteStack& state, Utf8JsonWriter writer) at System.Text.Json.JsonSerializer.HandleEnumerable(JsonClassInfo elementClassInfo, JsonSerializerOptions options, Utf8JsonWriter writer, WriteStack& state) at System.Text.Json.JsonSerializer.Write(Utf8JsonWriter writer, Int32 originalWriterDepth, Int32 flushThreshold, JsonSerializerOptions options, WriteStack& state) at System.Text.Json.JsonSerializer.d__50.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.d__5.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at Microsoft.AspNetCore. Mvc.Formatters.SystemTextJsonOutputFormatter.d__5.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<g__Logged|21_0>d.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<g__Awaited|29_0>d`2.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Sco pe& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeResultFilters() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<g__Awaited|24_0>d.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResourceExecutedContextSealed context) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.InvokeFilterPipelineAsync() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.R untime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<g__Logged|17_1>d.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Routing.EndpointMiddleware.<g__AwaitRequestTask|6_0>d.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.d__5.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerService s.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.d__6.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.d__5.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.d__4.MoveNext() at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Tas k task) at HomeaZZon.Api.Middlewares.ExceptionMiddleware.d__3.MoveNext() in C:\\repo\\homeazzon\\api\\api-.core-monolothic\\HomeaZZon.Api\\Middlewares\\ExceptionMiddleware.cs:line 28

...

Startup.cs

...

            services.AddMvc().AddJsonOptions(o =>
            {
                o.JsonSerializerOptions.IgnoreNullValues = true;
                o.JsonSerializerOptions.PropertyNamingPolicy = null;
            });

Answer 1

As someone else mentioned, it's hard to really nail down your error but if you simply want the same functionality you had in .NET Framework in terms of serialization, then here's how you do it.

Install the following nuget package :

Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson 

Then in your services method you'll have something like "AddMVC". Modify that like so :

services.AddMvc().AddNewtonsoftJson();

More info : https://dotnetcoretutorials.com/2019/12/19/using-newtonsoft-json-in-net-core-3-projects/

.NET Core projects 3+ use a new JSON serializer that honestly isn't bad, but has a couple of issues here and there. The performance benefits in later versions are a pretty big boon, but if you just need things to be exactly like they were before, then you need to use JSON.NET (With the above instructions), and you'll be back to old times!