Every time I attempt to call `Send` from MediatR for any Query/Command, I encounter an error: "Error constructing handler request of type MediatR.IRequestHandler`"

We encountered an error related to MediatR in our ASP.NET Core application, this error typically occurs when MediatR handlers are not properly registered with the dependency injection container.

Here's a detailed explanation of the issue:

  • As we know that MediatR is a library used for implementing the Mediator pattern in .NET applications. It helps in decoupling the sender and receiver of requests by introducing a mediator, which handles the communication between components.the error occurs because we forgot to register our MediatR handlers with the dependency injection container during application startup.
  • Let's say we have a MediatR handler class named ProductHandler that handles a specific type of request. If we forget to register this handler with the container, MediatR won't be able to resolve it when a corresponding request is made, leading to the error.

To resolve this error, we need to ensure that all our MediatR handlers are registered with the dependency injection container during application startup. 

// In the ConfigureServices method of Startup.cs

After adding this registration, the error should be resolved, and MediatR handlers will be properly resolved by the container when handling requests.

Error constructing handler for request of type MediatR.IRequestHandler`[EducationService.Application.Commands.UpdatedAddEducation System.Boolean]. Register your handlers withthe container. 

I'm getting this error when tryting to use MediatR "InvalidOperationException: Cannot resolve 'MediatR.IRequestHandler2[PetsHost.Recursos.Queries.Product,System.Collections.Generic.IEnumerable1[PetsHost.Models.Product]]' from root provider because it requires scoped service 'Product.Context.AppDbContext'.",when We encountered an exception with the message "Register your handlers with the container." While this message might seem misleading, the actual error lies within the inner exception:

Cannot resolve 'MediatR.IRequestHandler<GetProduct, IEnumerable>' from the root provider because it requires a scoped service 'Product.Context.AppDbContext'.

Error occurs because we inject IMediator into a singleton consumer, such as ProductService. The Mediator implementation relies on an IServiceProvider, but because ProductService is a singleton, it's resolved from the root container along with all its dependencies. Consequently, when Mediator starts resolving from its IServiceProvider, it also resolves from the root container.

problem occurs because scoped services cannot be resolved from the root container. If they were, it could lead to bugs, as the scoped service would be cached for the lifetime of the root container, potentially leading to indefinite reuse.

Above error show the importance of understanding the lifetime of services and their dependencies within the dependency injection system of our application.

To solve  this issue, we need to ensure that services with dependencies on scoped services are not singletons. We can refactor our code to avoid injecting IMediator into singleton consumers or reconsider the lifetimes of our services.

3 I also get same error when I run the  endpoint that executes a simple `await _mediator.Send(command);` I encounter the following error in my log: Error constructing handler for request of type MediatR.IRequestHandler`2." Below is my code 
// UpdatedAddEducationRequest.cs

public class UpdatedAddEducationRequest : IRequest<bool>
    public int UpdatedUniversityId { get; set; }

    public int UpdatedFacultyId { get; set; }

    public string UpdatedEducationName { get; set; }

// UpdatedAddEducationHandler.cs

public class UpdatedAddEducationHandler : IRequestHandler<UpdatedAddEducationRequest, bool>
    private readonly IManageUniversityRepository _updatedRepository;
    public UpdatedAddEducationHandler(IUniversityRepository repository)
        _updatedRepository = repository;

    public async Task<bool> Handle(UpdatedAddEducationRequest command, CancellationToken cancellationToken)
        var updatedUniversity = await _updatedRepository.GetAsync(command.UpdatedUniversityId);

            .FirstOrDefault(f => f.Id == command.UpdatedFacultyId)

        return await _updatedRepository.UnitOfWork.SaveEntitiesAsync();

Upon investigating the problem, we discovered that the line of code services.AddMediatR(typeof(UpdatedAddEducationRequest).GetTypeInfo().Assembly); handles all MediatR IRequest and IRequestHandlers. However, we have introduced an IRepository interface and its implementation class, which are not automatically handled by MediatR.Extensions.Microsoft.DependencyInjection, to fixed this issue, we need to manually register these interfaces and their implementation classes with the dependency injection container.


services.AddScoped(typeof(IManageUniversityRepository), typeof(IManageUniversityRepositoryDbRepository));

By manually registering the IRepository interface and its implementation class ensures that they are properly handled by the dependency injection container and can be resolved when needed, While MediatR.Extensions.Microsoft.DependencyInjection provides automatic registration for IRequest and IRequestHandler types, it's essential to remember that additional interfaces and their implementations may need to be registered manually to ensure proper dependency resolution. By incorporating this manual registration step into our code, we can address the issue and ensure that all dependencies are correctly resolved within our application.

4 services.AddMediatR(Assembly.GetExecutingAssembly()); 
This ensures that all our handlers and commands are in the assembly we passed. 
If you are using .NET 6, the following line is useful: builder.Services.AddMediatR(AppDomain.CurrentDomain.GetAssemblies()); 
This registers MediatR services in .NET 6, considering all assemblies loaded in the program files.
As we checked, we found that the problem within the Startup.cs file, specifically in the way MediatR service was added. Since in our case Handlers were stored in a separate assembly, it was necessary to explicitly mention the name of that assembly during service registration.

To address this issue, we made the following adjustment in the ConfigureServices method of the Startup.cs:

public void ConfigureServices(IServiceCollection services){
    var assembly = AppDomain.CurrentDomain.Load("ApplicationData");

we replaced the use of typeof(Startup) with the name of our assembly where all the Handlers are stored, which in this case is "ApplicationData".

In this way we ensure that MediatR properly registers all Handlers located in the specified assembly. By explicitly providing the assembly name, we avoid any ambiguity and ensure that the correct assembly is included during service registration.


we addressed it by adding the following code to our application layer and then calling it from our Program:

    public static IServiceCollection AddApplication(this IServiceCollection services)
        var assembly = Assembly.GetExecutingAssembly();
        return services.AddMediatR(assembly);

Our program code calls the methods in the following order:


It involves extending the IServiceCollection interface with a method named AddApplication. Inside this method, we obtain the executing assembly and use it to add MediatR to the services collection.

By encapsulating the logic to add MediatR within the AddApplication method, we ensure that it can be easily reused and maintained across our application.