运行 .net 核心 web api 项目时出错

Question

I am learning .net core web api. trying to connect with mysql. I am getting error following error

System.InvalidOperationException: Unable to resolve service for type 'WebApplication4.Models.ConnectionStrings' while attempting to activate 'WebApplication4.Controllers.UserController'.
  at Microsoft.Extensions.DependencyInjection.ActivatorUtilities.GetService(IServiceProvider sp, Type type, Type requiredBy, Boolean isDefaultParameterRequired)
  at lambda_method9(Closure , IServiceProvider , Object[] )
  at Microsoft.AspNetCore.Mvc.Controllers.ControllerActivatorProvider.<>c__DisplayClass7_0.<CreateActivator>b__0(ControllerContext controllerContext)
  at Microsoft.AspNetCore.Mvc.Controllers.ControllerFactoryProvider.<>c__DisplayClass6_0.<CreateControllerFactory>g__CreateController|0(ControllerContext controllerContext)
  at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
  at Microsoft.AspNetCore.Mvc.Infrastructure.ControllerActionInvoker.InvokeInnerFilterAsync()

Here's my Startup.cs looks like this

using Microsoft.AspNetCore.Builder;
using Microsoft.OpenApi.Models;
using WebApplication4.Models;

namespace WebApplication4
{
    public class Startup
    {
        public IConfigurationRoot Configuration { get; }
        public Startup(Microsoft.Extensions.Hosting.IHostingEnvironment env)
        {
            var appsettings = new ConfigurationBuilder()
                .SetBasePath(env.ContentRootPath)
                .AddJsonFile("appsettings.json")
                .Build();
            Configuration = appsettings;
        }

        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            ConnectionStrings con = new ConnectionStrings();
            Configuration.Bind("ConnectionStrings", con);
            services.AddSingleton(con);

            services.AddControllers();
            services.AddMvc();

            services.AddSwaggerGen(c =>
            {
                c.SwaggerDoc("v1", new OpenApiInfo { Title = ".Net Core 3 Web API", Version = "v1" });
            var filePath = Path.Combine(AppContext.BaseDirectory, "NetCore3WebAPI.xml");
            c.IncludeXmlComments(filePath);
            });
        }

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

            app.UseHttpsRedirection();
            app.UseRouting();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
            app.UseSwagger();
            app.UseSwaggerUI(c =>
            {
                c.SwaggerEndpoint("/swagger/v1/swagger.json", ".Net Core 3 Web API V1");
            });
        }
    }
}

Usercontroller.cs

using Dapper;
using Microsoft.AspNetCore.Mvc;
using MySqlConnector;
using WebApplication4.Models;
using System.Threading.Tasks;

namespace WebApplication4.Controllers
{
    [Route("api/User")]
    [ApiController]
    public class UserController : ControllerBase
    {
        private readonly Models.ConnectionStrings con;
        public UserController(Models.ConnectionStrings c)
        {
            con = c;
        }

        /// <summary>
        /// List users
        /// </summary>
        /// <returns></returns>
        [HttpGet]
        public async Task<IActionResult> Get([FromQuery] Models.User vm)
        {
            return await Task.Run(() =>
            {
                using (var c = new MySqlConnection(con.MySQL))
                {
                    var sql = @"SELECT * FROM user 
                                WHERE (@id = 0 OR id = @id) 
                                AND (@name IS NULL OR UPPER(name) = UPPER(@name))";
                    var query = c.Query<Models.User>(sql, vm, commandTimeout: 30);
                    return Ok(query);
                }
            });
        }

        /// <summary>
        /// Create user
        /// </summary>
        /// <returns></returns>
        [HttpPost]
        public async Task<IActionResult> Post([FromBody] Models.User vm)
        {
            return await Task.Run(() =>
            {
                using (var c = new MySqlConnection(con.MySQL))
                {
                    var sql = @"INSERT INTO user (name) VALUES (@name)";
                    c.Execute(sql, vm, commandTimeout: 30);
                    return Ok();
                }
            });
        }
    }
}

ConnectionStrings.cs

namespace WebApplication4.Models
{
    public class ConnectionStrings
    {
        public string MySQL { get; set; }
    }
}

Since the error mentions about UserController and ConnectionString classes, I have added only 2 of them.

Let me know how to solve this. Thanks

Answer 1

You are creating a Singleton of a connection string. But then again not injecting it anywhere. As a result the controller cant find what it is looking for.

The better approach to do would be to use the configuration object itself. Which can be reused anywhere on the application. Here is how you can build and inject the configuration in controllers:

Startup class:

public IConfiguration Configuration { get; }

public Startup(IConfiguration configuration, Microsoft.Extensions.Hosting.IHostingEnvironment env)
{
    Configuration = configuration;
}

Controller:

using Microsoft.Extensions.Configuration;

private IConfiguration _configuration;

public UserController(IConfiguration configuration)
{
    _configuration = configuration;
}

public async Task<IActionResult> Get([FromQuery] Models.User vm)
{
    var connstring= _configuration.GetValue<string>("ConnectionStrings:DefaultConnection");
}