How can you add a Microsoft Graph client service as a MediatR service in .NET Core 3.1?

Question

So I have a .NET Core web API with it's own local data context, and I'd like to add the ability to call Microsoft Graph as a downstream API.

However, when I try to add the necessary properties to call the Graph API, I get a build error:

Unhandled exception. System.AggregateException: Some services are not able to be constructed (Error while validating the service descriptor 'ServiceType: MediatR.IRequestHandler`2[Application.Users.Me+Query,Microsoft.Graph.User] Lifetime: Transient ImplementationType: Application.Users.Me+Handler': Unable to resolve service for type 'Microsoft.Graph.GraphServiceClient' while attempting to activate 'Application.Users.Me+Handler'.)

Here is my startup class:

using API.Middleware;
using Application.TestEntities;
using FluentValidation.AspNetCore;
using MediatR;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc.Authorization;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Persistence;
using Microsoft.Identity.Web;

namespace API
{
    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<DataContext>(opt =>
            {
                opt.UseSqlite(Configuration.GetConnectionString("DefaultConnection"));
            });
            services.AddCors(opt =>
            {
                opt.AddPolicy("CorsPolicy", policy =>
                {
                    policy.AllowAnyHeader().AllowAnyMethod().WithOrigins("http://localhost:3000");
                });
            });

            services.AddMicrosoftIdentityWebApiAuthentication(Configuration)
                .EnableTokenAcquisitionToCallDownstreamApi()
                .AddInMemoryTokenCaches();

            services.AddMediatR(typeof(List.Handler).Assembly);
            services.AddControllers(opt =>
            {
                var policy = new AuthorizationPolicyBuilder().RequireAuthenticatedUser().Build();
                opt.Filters.Add(new AuthorizeFilter(policy));
            })
            .AddFluentValidation(cfg => cfg.RegisterValidatorsFromAssemblyContaining<Create>());
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            app.UseMiddleware<ErrorHandlingMiddleware>();
            if (env.IsDevelopment())
            {
                // app.UseDeveloperExceptionPage();
            }

            app.UseCors("CorsPolicy");

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}

And my application handler for calling downstream:

using System.Threading;
using System.Threading.Tasks;
using MediatR;
using Microsoft.Graph;
using Microsoft.Identity.Web;

namespace Application.Users
{
    public class Me
    {
        public class Query : IRequest<User> { }

        public class Handler : IRequestHandler<Query, User>
        {
            private readonly ITokenAcquisition _tokenAcquisition;
            private readonly GraphServiceClient _graphServiceClient;
            public Handler(ITokenAcquisition tokenAcquisition, GraphServiceClient graphServiceClient)
            {
                _tokenAcquisition = tokenAcquisition;
                _graphServiceClient = graphServiceClient;
            }

            public async Task<User> Handle(Query request, CancellationToken cancellationToken)
            {
                var user = await _graphServiceClient.Me.Request().GetAsync();
                return user;
            }
        }
    }
}

Hopefully I'm on the right track here, but please let me know if I'm not.

Answer 1

Right so this was a simple oversight on my part.

As per @franklores, you need to register Microsoft Graph in your startup class services:

services.AddMicrosoftIdentityWebApiAuthentication(Configuration)
.EnableTokenAcquisitionToCallDownstreamApi()
.AddMicrosoftGraph(Configuration.GetSection("DownstreamApi"))
.AddInMemoryTokenCaches();

Adding the following to appsettings (scopes may differ):

  "DownstreamAPI": {
    "BaseUrl": "https://graph.microsoft.com/v1.0",
    "Scopes": "user.read"
  },

And be sure to install Microsoft.Identity.Web.MicrosoftGraph to enable the AddMicrosoftGraph() function.