Issue description:

While using .NET API project and attempting to run application, it gives below error,

InvalidOperationException: Unable to resolve service for type 'IEmployeeRepository ' while attempting to activate 'EmployeeController'.

Sampel code which produces this error

Please refer this article for complete code

    [Route("api/[controller]")]
    [ApiController]
    public class EmployeeController : ControllerBase
    {
        private readonly IEmployeeRepository _employeeRepository;
        public EmployeeController(IEmployeeRepository employeeRepository)
        {
            _employeeRepository = employeeRepository;
        }
        // GET: api/Employee
        [HttpGet]
        public ActionResult<IEnumerable<Employee>> Get()
        {
            return Ok(_employeeRepository.GetEmployees());
        }
        // GET: api/Employee/5
        [HttpGet("{id}")]
        public ActionResult Get(int id)
        {
            return Ok(_employeeRepository.GetEmployeeByID(id));
        }

Repository Interfaces was be defined as below,

public interface IEmployeeRepository
{
    IEnumerable<Employee> GetEmployees();
    Employee GetEmployeeByID(int employeeID);
}

Root cause and Issue resolution

The main reason for issue was found to be not registering the instance type correctly in the IOC contaier of API pipeline or middleware..

After adding the type and interface registration in the Container as below, the issue got resolved,

services.AddScoped<IEmployeeRepository, EmployeeRepository>();

Below is the complete sample code,

public void ConfigureServices(IServiceCollection services)
       {
           services.AddControllers();
           services.AddScoped<IEmployeeRepository, EmployeeRepository>();
           services.AddDbContext<EmployeeContext>(options =>
           {
               options.UseSqlServer(Configuration.GetConnectionString("EmployeeDB"),
                sqlServerOptionsAction: sqlOptions =>
                {
                    sqlOptions.EnableRetryOnFailure();
                });
           });
       }

If you are using .NET 6 and above framework please define the type as below,

  builder.services.AddScoped<IEmployeeRepository, EmployeeRepository>();

Service lifetime can be defined using any the below approaches as below,

• Transient
• Scoped
• Singleton

Other possible root cause

Please ensure that you are injecting Interface in Controller and not the class type from Controller or other module where you want to use that type or service.

Example: Failed

        private readonly IEmployeeRepository _employeeRepository;
        public EmployeeController(EmployeeRepository employeeRepository)
        {
            _employeeRepository = employeeRepository;
        }

Example: Working

        private readonly IEmployeeRepository _employeeRepository;
        public EmployeeController(IEmployeeRepository employeeRepository)
        {
            _employeeRepository = employeeRepository;
        }

That’s all! Happy coding!

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Today in this article, we will learn a simple and easy approach In C# .NET – Compare Two Different Files line-by-line. You might find need to compare two files line by line.

The file can be compared in a variety of methods to identify differences or common lines.

This piece walks readers through the process of comparing two files to determine whether their contents differ and how to create a file of differences if they do.

This comparison examines the contents of the two files line by line, checking each line for any order that may or may not correspond to other files’ names, locations, times, or other characteristics.

Multiple modules can be used in C# to identify variations or commonalities.

File A – We have the below file with the content.

File B – We have the below file with the content different from the previous file as highlighted.

Approach 1 – File Line compare using Linq Contains method

Using LInq Contains method determines whether a sequence contains a specified element by using the default equality comparer.

var listA = File.ReadAllLines("DataA");
 
var listExceptAB = File.ReadAllLines("DataB")
.Where(line =>!listA.Contains(line));

Approach 2 – File Line compare using Linq Except method

var listA = File.ReadAllLines("DataA");
 
var listExceptAB = File.ReadAllLines("DataB")
.Except(listA );

We can use a linq Except for method which helps produce a set difference of two file line by line by using the default equality comparer to compare values.

Except works fine on any IEnumerable properties as explained below

Output

Once the code is executed, you shall see the file is generated with only delta changes having the differences i.e return all the elements which exist in File A but don’t in File B.

27 APOSTROPHE A7 SECTION SIGN
23 NUMBER SIGN A3 POUND SIGN

Do you have any comments or ideas or any better suggestions to share?

Please sound off your comments below.

Happy Coding !!

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In today’s post, we will write client-side code to Send HTTP POST requests in .NET.

We will see the technique of regular HTTPClient and also use HttpClientFactory (recommended) examples

Let’s look at a few examples to invoke HTTP POST methods using HttpClientfactory.

As discussed in the previous article I have already setup the named HttpClientclientFactory to create the required HttpClient instance as below,

Send HTTP POST request in .NET

Below is a sample POST method example.

Below we are creating an instance of HttpClient using HttpClientFactory and invoking the POST method asynchronously.

One can create a HttpClient instance using a static instance or named or typed instances as needed.

Note: Below samples are shown using direct usage of Post call in Controller.

Ideally please structure your code considering the separation of concerns using Repository pattern etc.

Ideally, the POST method should also return the location of newly created resources as per REST guidelines.

        [HttpPost]
        public async Task<IActionResult> Post([FromBody]Customer customer)
        {

            HttpContent httpContent = new StringContent(JsonConvert.SerializeObject(customer)
                , Encoding.UTF8, "application/json");

            HttpResponseMessage response = await client.PostAsync("api/acounts", httpContent);

          

            if (response.IsSuccessStatusCode)
            {
                return Ok(response.Content.ReadAsStreamAsync().Result);
            }

            else
            {
                return StatusCode(500, "Something Went Wrong! Error Occured");
            }

        }

The above-discussed HttpClient object can be created using any of the below methods mentioned and can be initialized in the Controller constructor,

HttpClientfactory alternative ?

If you are not using the HttpClientfactory instance then please see below the way to initialize the HttpClient instance,

References:

• Using HTTPClient Best Practices and Anti-Patterns

That’s all! Happy coding!

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Today in this article, we will see how to Inject service in AutoMapper profile class. While setting up the Automapper profile during Startup services bootstrap you might want to use services within in Automapper profile classes.

Today in this article, we will cover below aspects,

It’s easy to configure services using the explicit dependency injection principle.

Service injection into the AutoMapper profile can be done and it’s done very rarely. But if you have valid use cases and want to perform some conditional mapping or want to consumer values of injected services from other modules or middleware then the below approach is very simple.

As we already discussed in our last article,

AutoMapper is an object mapper that helps you transform one object of one type into an output object of a different type. This type of transformation is often needed when you work on business logic.

Automapper helps to centralize your repeated boring mapping logic, address the separation of concerns, provide better control on mapping making your code clean and easy to maintain.

Here the idea with Automapper usage is simple, You have source type and destination type. You just transform object values from one source to the destination object. All you need is two types and profile registration. We will see what is profile registration in detail below.

For more details on dependency Injection of the IMapper interface is explained here already with more details,

• Configure Automapper in ASP.NET Core – Getting Started

Define the Automapper Profile with service Dependency Injection

Automapper Profile with service Dependency Injection can be defined as below.

Below example “SourceMappingProfile” is a profile class with constructor injection of service instance type i.e IEmployeeType.

Below I am defining the conditional profile for Automapper depending on the IEmployeeType.

Example

    public class SourceMappingProfile : Profile
    {
        private IEmployeeType _employeeType;

        public SourceMappingProfile()
        {
        }

        public SourceMappingProfile(IEmployeeType employeeType)
        {
            _employeeType = employeeType;

            if (_employeeType.empType == EmpType.Contract)
            {
                CreateMap<Source, Destination>()
                    .ForMember(dest => dest.Name, o => o.MapFrom(src => src.FirstName))
                    .ForMember(dest => dest.Location, o => o.MapFrom(src => src.Address))
                    .ForMember(dest => dest.Employee, o => o.MapFrom(src => src.Employee))
                    .ForMember(dest => dest.Identity, opt =>
                    {
                        opt.PreCondition(src => (src.Id != null));
                        opt.MapFrom(src => src.Id);
                    });
            }

            if (_employeeType.empType == EmpType.Permanent)
            {
                CreateMap<Source, Destination>()
                    .ForMember(dest => dest.Name, o => o.MapFrom(src => src.FirstName));
            }
        }
    }

Define ConfigServices in Startup

I have defined the Define ConfigServices in Startup as below,

Example

            services.AddScoped<ValidateInputAttribute<string>>();
            services.AddScoped<IUniqueIdService, UniqueIdService>();
            services.AddScoped<IEmployeeType,EmployeeType>();

            services.AddSingleton(provider => new MapperConfiguration(cfg =>
            {
                cfg.AddProfile(new SourceMappingProfile(provider.CreateScope().ServiceProvider.GetService<IEmployeeType>()));

            }).CreateMapper());

        }

Above we are using scoped service instance. While scope service instances are created once per request it is often recommended they are resolved using a scoped instance of Application Builder. Then followed by using a service provider one can resolve the dependencies.

Do you have any comments or ideas or any better suggestions to share?

Please sound off your comments below.

Happy Coding !!

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Today in this article, we will see more details about MongoDB ObjectId Data type.

Updated October 2026: a section on string-versus-ObjectId query bugs, a C# id modelling section, corrected ObjectId layout and an FAQ.

ObjectId is the 12-byte element consisting of a timestamp value, a random value, and an incremental counter.

Today in this article, we will cover below aspects,

MongoDB _id field as ObjectId

MongoDB uses the _id field as ObjectId this Id has few behavioral characteristics and it is explained below, A 4-byte timestamp is a UNIX timestamp.

• MongoDB database automatically generates _id the field for insertion.

• This field is unique for every document in the collection, except in time series collections, which don’t require a unique _id.

• _id field is automatically indexed and the index is unique.

It represents the ObjectId’s creation, measured in seconds since the Unix epoch. for more details, please visit here.

MongoDB ObjectId fields details

ObjectId is the 12-byte element consisting of a timestamp value, a random value, and an incremental counter.

• A 4-byte timestamp is a UNIX timestamp. It represents the ObjectId’s creation timestamp, measured in seconds since the Unix epoch.

If interested to know how to get timestamp using these 4 bytes, please visit the below article,

• Query MongoDB using ObjectId by Date -MongoShell

• A 5-byte random value is generated once per process. This random value is unique to the machine and process.

• A 3-byte incrementing counter, initialized to a random value.

References:

• MongoDB Naming Standards and Guidelines

Do you have any comments or ideas or any better suggestions to share?

Please sound off your comments below.

Happy Coding !!

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Why a query by _id returns nothing: ObjectId vs string

A query by _id returns nothing when you filter with a string, because the stored value is an ObjectId and BSON treats the two as different types. The 24-character text looks identical, but it never matches.

Why doesn’t a string match an ObjectId?

BSON compares type as well as value. The string "507f1f77bcf86cd799439011" and the ObjectId with the same hex digits are two different values, so an equality filter on one never finds the other. MongoDB raises no error. You just get an empty result.

In plain English: It’s like a shoe size of 42 and bus route 42. The digits match, but they’re different kinds of thing, so you can’t swap one for the other.

How do you query by _id correctly in Node.js?

Validate the input, then convert it before it reaches the filter:

1. Validate: confirm the string is exactly 24 hex characters. new ObjectId() throws on malformed input, so reject bad values first and return a clean 400 instead of a server error.
2. Wrap: pass new ObjectId(idString) in the _id filter, not the raw string.
3. Compare with .equals(): === checks object identity in JavaScript, so two ObjectIds with the same bytes still compare as different. Use a.equals(b).

How should you store references to another document?

Store a reference to another document’s _id as an ObjectId, not as its string form. Otherwise every join or lookup has to convert types, and one missed conversion gives you the empty-result problem again.

If you only have the string, convert it inside the query or aggregation with $convert and to: "objectId". Treat that as a workaround for existing data. Fixing the stored type is the better long-term answer.

Can you trust _id order and the embedded timestamp?

Not completely. ObjectIds are only approximately ordered by creation time, so don’t treat _id order as a strict insertion sequence. Several clients can generate ids within the same second, and the counter and random bytes decide the order among them.

The embedded timestamp records when the id was generated, which isn’t always when the record was really created. If you may migrate, re-insert or bulk-load documents, store an explicit createdAt field and use that for reporting and sorting.

Frequently asked questions

Why does my MongoDB query by _id return nothing when the string looks right?

The stored _id is an ObjectId and you are filtering with a string, so the types differ and nothing matches. Convert the string to an ObjectId first (after checking it is 24 hex characters), or map the property with BsonRepresentation(BsonType.ObjectId) in C#. With the attribute on a string property, the C# driver converts between the string and the stored ObjectId for you, so your filters can keep using the string.

Is a MongoDB ObjectId strictly ordered and guaranteed unique?

ObjectIds are designed to be unique by combining a timestamp, a per-process random value and a counter. They are only approximately ordered by creation time, because clients generate them independently, so sort by an explicit field if strict order matters. Two clients creating documents in the same second can produce ids that sort differently from the real insertion sequence.

How do I compare two ObjectIds correctly in JavaScript?

Use .equals(), because === compares object identity and returns false for two separate objects holding the same value. Convert both to strings with toString() if you need a plain comparison. For example, a.equals(b) and a.toString() === b.toString() both work for ids that hold the same 12 bytes.

How do I get the creation time from an ObjectId?

Call getTimestamp() in mongosh or the driver; it reads the 4-byte seconds-since-epoch value. It reflects when the id was generated, so keep a separate createdAt field if documents may be migrated or re-inserted. The value has one-second resolution, so it can’t tell you the order of documents created within the same second.

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