Implementing an Interface on a class with Dependency Injection

Question

I'm developing a Blazor app and somewhere I'm implementing an interface:

public class UserData : IUserData
{
    private readonly ISqlDataAccess _db;

    public UserData(ISqlDataAccess db)
    {
        _db = db;
    }

    public void SomeFunction ()
    {
        ...
    }
}
public interface IUserData
{
    void SomeFunction();
}

While on .razor I can do: @inject IUserData UserData; and ((UserData)UserData).SomeFunction();; I'm failing to discover how to do it on .cs file.

//There is no such constructor and If I create a new one, then I won't get the _db dependecy injection
IUserData userDate = new UserData();
userDate.SomeFunction();

Edit

So now, when I'm calling the method from the .cs file, the app freezes; it doesn't throw an error and I am able to refresh the page, so it seems it's stuck on the call to the db; but if I call it from the .razor it works flawlessy.

.cs

public AccountService(IUserData userData)
{
    _userData = userData;
}
...
public async Task<bool> Validate(string userId, string password)
{
    ...
    try
    {
        List<UserModel> users = new List<UserModel<();
        users = await _userData.GetUsers();
        //NEVER GETS HERE
        return true;
    }
    catch (Exception ex)
    {
         return false;
    }
    ...
}

.razor

@inject IUserData _db;

@code {
    private List<UserModel> users;
    ...
    protected override async Task OnInitializedAsync()
    {
        users = await _db.GetUsers();
    }
    ...

UserData

    public class UserData : IUserData
    {
        private readonly ISqlDataAccess _db;

        public UserData(ISqlDataAccess db)
        {
            _db = db;
        }

        public Task<List<UserModel>> GetUsers()
        {
            string sql = "Select *from dbo.Users";
            return _db.LoadData<UserModel, dynamic>(sql, new { });
    }
    ...
}

IUserData

    public interface IUserData
    {
        Task<List<UserModel>> GetUsers();
        ...
    }

Edit2 It turns out I was missing an await when calling Validate() service, and thus not running it asynchronous.

Answer 1

In Startup.cs you can register your interface and implementation;

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddScoped<IUserData, Userdata>();
    }

Then you can use the interface in a class:

public class TestClass{
  private IUserData _userData;
  public TestClass(IUserData userdata){
    _userData = userdata;
  }
}

Answer 2

At some point in the program, you need to setup dependency injection. This is most common to do in the ConfigureServices method in Startup.cs by convention.

public class Startup
{
    public IConfiguration Configuration { get; }

    public Startup(IHostingEnvironment env)
    {
        var builder = new ConfigurationBuilder()
           .SetBasePath(env.ContentRootPath)
           .AddJsonConfigurationProvider("appsettings.json", optional: false, reloadOnChange: true)
           .AddEnvironmentVariables();
            Configuration = builder.Build();
    }
    public void ConfigureServices(IServiceCollection services)
    {
         services.AddTransient <ISqlDataAccess, SqlDataAccess>(); //Second argument is the implementation of the interface
         services.AddTransient <IUserData, UserData>();
    }
}

You need to pass the ISqlDataAccess to the constructor of the UserData but you had it covered already.

public class UserData : IUserData
{
    private readonly ISqlDataAccess _db;

    public UserData(ISqlDataAccess db)
    {
        _db = db;
    }
    //...
}

Then you need to pass your IUserData to your objects via constructors:

public class ClassWithIUserDataDependency {
    private IUserData _userData;
    public ClassWithIUserDataDependency (IUserData userData) {
        _userData = userData;
    }

    //rest of the class
}

One note: You would need to pass IUserData to all dependency classes. Based on the name, this looks like a POCO object (If it is not, don't mind this comment) If this is a POCO class, or anything representing a DTO or Data, then it is better to separate db from it and allow users to just new it. If it is not, you may want to change its name.