ASP.Net Core MVC存储库模式意外处置

当我尝试添加评论时，出现以下错误：

ObjectDisposedException：无法访问已处置的对象。

当代码运行第二行时：

m_context.Comments.Add(comment);
m_context.SaveChanges();

为什么要处理上下文？如果将TryAddComment方法移入控制器，则不会提早调用Dispose。

这是我的Controller和Repository类的样子（简化）。

CommentsController.cs：

public class CommentsController : Controller
{

    private ICommentRepository m_commentRepository;

    public CommentsController(ICommentRepository commentRepository)
    {
        m_commentRepository = commentRepository;
    }

    // POST: api/Comments
    [HttpPost]
    public async Task<IActionResult> PostComment([FromBody] CommentAddViewModel commentVM)
    {
        Comment comment = new Comment
        {
            ApplicationUserId = User.GetUserId(),
            PostId = commentVM.PostId,
            Text = commentVM.Text
        };

        bool didAdd = m_commentRepository.TryAddComment(comment);

        if (!didAdd)
        {
            return new HttpStatusCodeResult(StatusCodes.Status409Conflict);
        }

        return CreatedAtRoute("GetComment", new { id = comment.CommentId }, comment);
    }

}

CommentRepository.cs：

public class CommentRepository : ICommentRepository, IDisposable
{

    public ApplicationDbContext m_context;

    public CommentRepository(ApplicationDbContext context)
    {
        m_context = context;
    }
    public bool TryAddComment(Comment comment)
    {
        m_context.Comments.Add(comment);
        m_context.SaveChanges();

        return true;
    }
    private bool disposed = false;

    protected virtual void Dispose(bool disposing)
    {
        if (!this.disposed)
        {
            if (disposing)
            {
                m_context.Dispose();
            }
        }
        this.disposed = true;
    }

    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }
}

编辑：

如果我使用本地CommentRepository，它将按预期工作。例如：

    CommentRepository localCommentRepo = new CommentRepository(m_context);
    bool didAdd = localCommentRepo.TryAddComment(comment);

编辑2：

在Startup.cs中，我将IcommentRepository注册为Scoped并按预期工作。最初是Singleton。为什么单身人士会导致此问题？

services.AddSingleton<ICommentRepository, CommentRepository>(); //breaks
services.AddScoped<ICommentRepository, CommentRepository>(); //works

编辑3：

ApplicationDbContext.cs：

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>
{
    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        // Customize the ASP.NET Identity model and override the defaults if needed.
        // For example, you can rename the ASP.NET Identity table names and more.
        // Add your customizations after calling base.OnModelCreating(builder);

    }
    public DbSet<Post> Posts { get; set; }
    public DbSet<Comment> Comments { get; set; }
}