Fix “Error Constructing Handler for Request of Type MediatR.IRequestHandler”

The error message “Error constructing handler for request of type MediatR.IRequestHandler`2” in your application using MediatR can be frustrating.Sometimes, getting an error message about MediatR might seem confusing because the real problem is hidden. This article will show you easy fixes to get your MediatR stuff working again!.

This error occurs when MediatR can not find appropriate handler class for processing a specific request. Here’s why this happen:

1. Handler Registration Missing: The program doesn’t know how to handle this request yet. It needs a special helper (called a handler) to be plugged in first.
2. Incorrect Assembly Scanning: The AddMediatR method isn’t scanning the assembly containing handler class.
3. Dependency Issues: A dependency required by the handler class isn’t registered correctly.
4. Mismatch Of Version: Different versions of the same assembly during development leading to type mismatches.

How To Fix “Error Constructing Handler for Request of Type MediatR.IRequestHandler”

Here are several ways to fix this error:

1. Register Your Handlers

Make sure your handler class is registered with the dependency injection container. For example, if your handler is in the Application assembly, you can use the following code:

C#

services.AddScoped<IRequestHandler<CreateAccountCommand, GenericResponse<string>>, CreateAccountCommandHandler>();

2. Verify Assembly Scanning

Double-check the assemblies passed to the AddMediatR method. Ensure it includes the assembly containing your handler class. You can use Assembly.GetExecutingAssembly() to reference the current assembly.

3. Fix Dependency Injection Issues

If your handler class depends on other services, ensure those services are registered with the dependency injection container. For instance, if your CreateAccountCommandHandler requires an IUserRepository:

C#

services.AddScoped<IRequestHandler<CreateAccountCommand, GenericResponse<string>>, CreateAccountCommandHandler>();
services.AddScoped<IUserRepository, UserRepository>();

4. Resolve Version Mismatches

Make sure you’re using the same versions of all referenced assemblies in development and deployment environments. Version mismatches can cause type resolution problems and lead to this error.

5. Check For Inner Exceptions

The original error message might not reveal the root cause. Inspect the inner exception for more details. It might point to issues like missing configuration or connection strings.

6. Leverage AddApplication Method

If you have a separate Application layer in your project, consider creating a helper method like this:

C#

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

Call this method from your Program.cs to ensure the Application assembly is scanned for handlers.

By using these solutions and understanding the cause of error, you can easily fix the “Error constructing handler for request of type MediatR.IRequestHandler” and get your MediatR handlers working as expected.

Read: Fix React Local Host Not Working