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A .Net5 Sample Using CQRS(Command Query Responsibility Segregation) And MediatR Patterns

CQRS stands for Command Query Responsibility Segregation. CQRS guides us to separate our logical implementations into 2 categories like 'Commands', 'Query'. The 'Commands' specifies the operations like creation or updating of data into the data source(database). The 'Query' specifies the operations to fetch the data.

In CQRS models(Request/Response classes) are independent or owned by a single operation, which means model classes can not be shared between the different 'Commands' or different 'Queries' or between a 'Command' and 'Query'.
From the diagram one thing to observe Request/Response model(optional), that's because some times we will use query parameters or return a simple scalar type in those cases we won't create models.

Create .Net5 Web API:

To implement the CQRS pattern let's create a sample .Net5 Web API application.

Configure Entity Framework Core Database Context:

For this demo, I had created a table like 'Products'. So to establish communication between our API project and database, here I will implement Code First Technique with an existing database.

Let's install entity framework core NuGet.
Package Manager Command:
Install-Package Microsoft.EntityFrameworkCore -Version 5.0.4

.Net CLI Command:
dotnet add package Microsoft.EntityFrameworkCore --version 5.0.4
Let's install the SQL server extension library for the entity framework core. 
Package Manager Command:
Install-Package Microsoft.EntityFrameworkCore.SqlServer -Version 5.0.4

.Net CLI Command:
dotnet add package Microsoft.EntityFrameworkCore.SqlServer --version 5.0.4
Let's add the 'Products' table entity. Let's create a folder like 'Data' and inside create a subfolder like 'Entities' and add a file like 'Products.cs'
Data/Entities/Products.cs:
namespace API.CQRS.Sample.Data.Entites
{
    public class Products
    {
        public int ProductId { get; set; }
        public string Name { get; set; }
        public string Manufacturer { get; set; }
        public string Description { get; set; }
        public decimal Price{get;set;}
    }
}
Inside the 'Data' folder create a database context file like 'MyWorldDbContext.cs'
Data/MyWorldDbContext.cs:
public class MyWorldDbContext : DbContext
{
	public MyWorldDbContext(DbContextOptions<MyWorldDbContext> options) : base(options)
	{

	}
	public DbSet<Products> Products { get; set; }
	

	protected override void OnModelCreating(ModelBuilder modelBuilder)
	{
		modelBuilder.Entity<Products>(build => {
			build.HasKey(_ => _.ProductId);
		});
	}
}
In the JSON config file add the connection string
appsettings.Development.json:
"ConnectionStrings":{
"MyWorldDbConnection":"Your_connection_string"
}
Now register database context in the 'Startup.cs' file.
Startup.cs:
services.AddDbContext<MyWorldDbContext>(options =>
{
	options.UseSqlServer(Configuration.GetConnectionString("MyWorldDbConnection"));
});

CQRS Folder Structure:

Let's have a look at the CQRS pattern folder structure.


Create Command Handler To Save A New Product:

So let's begin our sample by creating a command handler to save a new 'Product' into the table. So we have to create a 'RequestModel'(payload class). So in folder hierarchy like 'RequestModels/CommandRequestModels' add a file 'SaveProductRequestModel.cs'.
RequestModels/CommandRequestModels/SaveProductRequestModel.cs:
namespace API.CQRS.Sample.RequestModels.CommandRequestModels
{
    public class SaveProductRequestModel
    {
        public string Name { get; set; }
        public string Manufacturer { get; set; }
        public string Description { get; set; }
        public decimal Price{get;set;}
    }
}
Now let's create an interface with an abstract method to save new products. So folder hierarchy like 'Contracts/CommandHandlers' and add a file like 'ISaveProductCommandHandler.cs'
Contracts/CommandHandler/ISaveProductCommandHandler.cs:
using System.Threading.Tasks;
using API.CQRS.Sample.RequestModels.CommandRequestModels;

namespace API.CQRS.Sample.Contracts.CommandHandlers
{
    public interface ISaveProductCommandHandler
    {
        Task<int> SaveAsync(SaveProductRequestModel requestModel);
    }
}
  • From this method definition, we conclude that this command had only 'RequestModel'(SaveProductRequestMode) but no 'ResponseModel' because its return type is scalar type Integer.
Let's implement the 'SaveAsync' method. In folder hierarchy 'Handlers/CommandHandlers' add a file like 'SaveProductCommandHandler.cs'.
Handlers/CommandHandlers/SaveProductCommandHandler.cs:
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.CommandHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.Data.Entites;
using API.CQRS.Sample.RequestModels.CommandRequestModels;

namespace API.CQRS.Sample.Handlers.CommandsHandlers
{
    public class SaveProductCommandHandler: ISaveProductCommandHandler
    {
        public readonly MyWorldDbContext _myWorldDbContext;
        public SaveProductCommandHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }

        public async Task<int> SaveAsync(SaveProductRequestModel requestModel)
        {
            var newProducts = new Products
            {
                Description = requestModel.Description,
                Manufacturer = requestModel.Manufacturer,
                Name = requestModel.Name,
                Price = requestModel.Price
            };

            _myWorldDbContext.Products.Add(newProducts);
            await _myWorldDbContext.SaveChangesAsync();
            return newProducts.ProductId;
        }
    }
}
Register CommandHandler in 'Startup.cs' file.
Startup.cs:
services.AddScoped<ISaveProductCommandHandler, SaveProductCommandHandler>();
Let's create an endpoint to save our 'Product'. Let's create a new controller like 'ProductController.cs'.
Controller/ProductController.cs:
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.CommandHandlers;
using API.CQRS.Sample.RequestModels.CommandRequestModels;
using Microsoft.AspNetCore.Mvc;

namespace API.CQRS.Sample.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class ProductController : ControllerBase
    {
        private readonly ISaveProductCommandHandler _saveProductCommandHandler;
        public ProductController(ISaveProductCommandHandler saveProductCommandHandler)
        {
            _saveProductCommandHandler = saveProductCommandHandler;
        }

        [HttpPost]
        [Route("create")]
        public async Task<IActionResult> SaveProductAsync(SaveProductRequestModel requestModel)
        {
            var result = await _saveProductCommandHandler.SaveAsync(requestModel);
            return Ok(result);
        }
    }
}

Create QueryHandler To Fetch All Products:

Let's begin creating a QueryHandler to fetch all 'Products'. Let's begin by creating a 'ResponseModel'. In the folder hierarchy 'ResponseModels/QueryResponseModels' add a file like 'AllProductsResponseModel'
ResponseModels/QueryResponseModels/AllProductsResponseModel.cs:
namespace API.CQRS.Sample.ResponseModels.QueryResponseModels
{
    public class AllProductsResponseModel
    {
        public int ProductId { get; set; }
        public string Name { get; set; }
        public string Manufacturer { get; set; }
        public string Description { get; set; }
        public decimal Price { get; set; }
    }
}
Now let's declare a abstract method definition in query handler interface to fetch the all 'Products'. In folder hierarchy like 'Contracts/QueryHandlers' add a file 'IAllProductsQueryHandler.cs'.
Contracts/QueryHandlers/IAllProductQueryHandler.cs:
using System.Collections.Generic;
using System.Threading.Tasks;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;

namespace API.CQRS.Sample.Contracts.QueryHandlers
{
    public interface IAllProductsQueryHandler
    {
        Task<List<AllProductsResponseModel>> GetListAsync();
    }
}
  • Here we can observe that our query handler don't have a 'RequestModel', it had 'ResponseModel'(AllProductResponseModel).
Now let's implement logic for abstract method 'GetListAsync'. In folder hierarchy like 'Handlers/QueryHandlers' add a file 'AllProductsQueryHandler.cs'
Handlers/QueryHandlers/AllProductsQueryHandler.cs:
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.QueryHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using Microsoft.EntityFrameworkCore;

namespace API.CQRS.Sample.Handlers.QueryHandlers
{
    public class AllProductsQueryHandler:IAllProductsQueryHandler
    {
        private readonly MyWorldDbContext _myWorldDbContext;
        public AllProductsQueryHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }

        public async Task<List<AllProductsResponseModel>> GetListAsync()
        {
            return await _myWorldDbContext.Products
            .Select(_ => new AllProductsResponseModel{
                Description = _.Description,
                ProductId = _.ProductId,
                Manufacturer = _.Manufacturer,
                Name = _.Name,
                Price = _.Price
            }).ToListAsync();
        }
    }
}
Now register our query handler in the 'Startup.cs' file
Startup.cs:
services.AddScoped<IAllProductsQueryHandler,AllProductsQueryHandler>();
Now inject our query handler into the 'ProductController.cs'.
Controllers/ProductController.cs:
private readonly ISaveProductCommandHandler _saveProductCommandHandler;
private readonly IAllProductsQueryHandler _allProductsQueryHandler;
public ProductController(
	ISaveProductCommandHandler saveProductCommandHandler,
	IAllProductsQueryHandler allProductsQueryHandler)
{
	_saveProductCommandHandler = saveProductCommandHandler;
	_allProductsQueryHandler = allProductsQueryHandler;
}
Let's add our new endpoint that fetches all 'Products'.
Controllers/ProductController.cs:
[HttpGet]
[Route("all")]
public async Task<IActionResult> AllProducts()
{
	var result = await _allProductsQueryHandler.GetListAsync();
	return Ok(result);
}

Create Query Handler To Filter Products By Prices:

For better understanding let's create one more query handler that fetches the 'Products' by price range. Let's create 'ResponseModel' in the folder hierarchy like 'ResponeModels/QueryResponeModels' add a file like 'PriceRangeProductsResponseModel.cs'.
ResponseModels/QueryResponseModels/PriceRangeProductsResponseModel.cs:
namespace API.CQRS.Sample.ResponseModels.QueryResponseModels
{
    public class PriceRangeProductsResponseModel
    {
        public int ProductId { get; set; }
        public string Name { get; set; }
        public decimal Price { get; set; }
    }
}
Now let's define abstract method in our query handler interface. In the folder hierarchy like 'Contracts/QueryHandlers' add a file like 'IPriceRangeProductsQueryHandler.cs'
Contracts/QueryHandlers/IPriceRangeProductsQueryHandler.cs:
using System.Collections.Generic;
using System.Threading.Tasks;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;

namespace API.CQRS.Sample.Contracts.QueryHandlers
{
    public interface IPriceRangeProductsQueryHandler
    {
        Task<List<PriceRangeProductsResponseModel>> PriceRangeProductsAsync(int minPrice, int maxPrice);
    }
}
Now let's implement the 'PriceRangeProductsAsync' method. In the folder hierarchy like 'Handlers/QueryHandlers' add a file like 'PriceRangeProductsQueryHandler.cs'.
Handlers/QueryHandlers/PriceRangeProductsQueryHandler.cs:
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.QueryHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using Microsoft.EntityFrameworkCore;

namespace API.CQRS.Sample.Handlers.QueryHandlers
{
    public class PriceRangeProductsQueryHandler : IPriceRangeProductsQueryHandler
    {
        private readonly MyWorldDbContext _myWorldDbContext;
        public PriceRangeProductsQueryHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }
        public async Task<List<PriceRangeProductsResponseModel>> PriceRangeProductsAsync(int minPrice, int maxPrice)
        {
            return await _myWorldDbContext.Products
            .Where(_ => _.Price >= minPrice && _.Price <= maxPrice)
            .Select(_ => new PriceRangeProductsResponseModel
            {
                Name = _.Name,
                ProductId = _.ProductId,
                Price = _.Price
            }).ToListAsync();
        }
    }
}
Register the query handlers in the 'Startup.cs'.
Startup.cs:
services.AddScoped<IPriceRangeProductsQueryHandler, PriceRangeProductsQueryHandler>();
Now inject the query handler into the 'ProductController.cs'
Controllers/ProductController.cs:
[HttpGet]
[Route("price-range")]
public async Task<IActionResult> PriceRangeProducts(int minPrice, int maxPrice)
{
	var result = await _priceRangeProductsQueryHandler.PriceRangeProductsAsync(minPrice, maxPrice);
	return Ok(result);
}
Finally we have completed our sample by implementing few Query/Command handlers. So our project folder structure will looks like:


MediatR:

MediatR is another design pattern. MediatR builds with handlers that have the capability to work on Commands and Queries.

MediatR reduces a lot of boilerplate code like injecting multiple services into the controller. MediatR provides a single entry point that expects a RequestModel, so based on the RequestModel corresponding Handler gets invoked. It works as a centralized communication hub because from the controller any handler gets invoked only through it. So it's the most common recommendation to use MediatR with CQRS.
From the diagram, we have to understand MediatR should have both RequestModel and ResponseMode. In MediatR 'RequestModel' always be a user-defined class whereas 'ResponseModel' either a user-defined class or scalar type.

Install MediatR NuGet:

Let's install the MediatR package for dotnet.
Package Manager Command:
Install-Package MediatR -Version 9.0.0

.Net CLI Command:
dotnet add package MediatR --version 9.0.0
Let's install the Dependency Injection extension library for MediatR. 
Package Manager Command:
Install-Package MediatR.Extensions.Microsoft.DependencyInjection -Version 9.0.0

.Net CLI Command:
dotnet add package MediatR.Extensions.Microsoft.DependencyInjection --version 9.0.0

Register And Inject MediatR:

Let's register MediatR service in 'Startup.cs'
Startup.cs:
services.AddMediatR(Assembly.GetExecutingAssembly());
Now inject the 'IMediator' into the 'ProductController' constructor.
Controllers/ProductController.cs:
private readonly ISaveProductCommandHandler _saveProductCommandHandler;
private readonly IAllProductsQueryHandler _allProductsQueryHandler;
private readonly IPriceRangeProductsQueryHandler _priceRangeProductsQueryHandler;
private readonly IMediator _mediator;
public ProductController(
	ISaveProductCommandHandler saveProductCommandHandler,
	IAllProductsQueryHandler allProductsQueryHandler,
	IPriceRangeProductsQueryHandler priceRangeProductsQueryHandler,
	IMediator mediator)
{
	_saveProductCommandHandler = saveProductCommandHandler;
	_allProductsQueryHandler = allProductsQueryHandler;
	_priceRangeProductsQueryHandler = priceRangeProductsQueryHandler;
	_mediator = mediator;
}

Update Logic In Command Handler To Save A New Product Using MediatR:

In MediatR pattern 'RequestModel' required, based on the 'RequestModel' corresponding handler will be automatically invoked by the MedaitR. The 'RequesModel' should always be a user-define class. In folder hierarchy 'RequestModels/CommandRequestModels' update the 'SaveProductRequestModel.cs' like below:
RequestModels/CommandRequestModels/SaveProductRequestModel.cs:
using MediatR;

namespace API.CQRS.Sample.RequestModels.CommandRequestModels
{
    public class SaveProductRequestModel:IRequest<int>
    {
        public string Name { get; set; }
        public string Manufacturer { get; set; }
        public string Description { get; set; }
        public decimal Price{get;set;}
    }
}
  • In mediatr pattern our RequestModels(eg: SaveProductRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel' our handler.
 Now we have to update the logic of our command handler 'SaveProductCommandHandler.cs'.
 Handlers/CommandHandlers/SaveProductCommandHandler.cs:
using System.Threading;
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.CommandHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.Data.Entites;
using API.CQRS.Sample.RequestModels.CommandRequestModels;
using MediatR;

namespace API.CQRS.Sample.Handlers.CommandsHandlers
{
    public class SaveProductCommandHandler : IRequestHandler<SaveProductRequestModel, int>
    {
        public readonly MyWorldDbContext _myWorldDbContext;
        public SaveProductCommandHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }

        public async Task<int> Handle(SaveProductRequestModel request, CancellationToken cancellationToken)
        {
            var newProducts = new Products
            {
                Description = request.Description,
                Manufacturer = request.Manufacturer,
                Name = request.Name,
                Price = request.Price
            };

            _myWorldDbContext.Products.Add(newProducts);
            await _myWorldDbContext.SaveChangesAsync();
            return newProducts.ProductId;
        }
    }
}
  • By using MediatR, we no need to create interfaces to our Command/Query handlers. So we can delete all our interface classes inside of the 'Contracts' folder in our project.
  • Our Command Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
  • Our Command Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'. The 'Handle' method return type is our 'ResponseModel'.
Now we need to comment out our 'ISaveProductCommandHandler' references in 'Startup.cs' and then in 'ProductControll.cs' as well.

Let's update our endpoint to use MediatR to trigger our Command Handler.
Controllers/ProductController.cs:
[HttpPost]
[Route("create")]
public async Task<IActionResult> SaveProductAsync(SaveProductRequestModel requestModel)
{
	//var result = await _saveProductCommandHandler.SaveAsync(requestModel);
	var result = await _mediator.Send(requestModel);
	return Ok(result);
}
  • So here 'Send' method of 'IMediator' that takes our RequestModel(eg: SaveProductRequestModel) as input value. So 'Send' method will invoke the appropriate Command Handler(eg: SaveProductCommandHandler) with the help of 'RequestModel'(eg: SaveProductRequestModel).

Update Logic In QueryHandler To Fetch All Products Using MediatR:

Currently 'AllProductsQueryHandler' class has method 'GetListAsync' that don't have 'RequestModel'. But we know MediatR fully depends on the 'RequestModel'. So we have to create an empty 'RequestModel' to make it work with 'MediatR'. In folder hierarchy like 'RequestModel/QueryRequestModels' add a new file like 'AllProductsRequestModel.cs'
RequestModel/QueryRequestModels/AllProductsRequestModel.cs:
using System.Collections.Generic;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using MediatR;

namespace API.CQRS.Sample.RequestModels.QueryRequestModels
{
    public class AllProductsRequestModel:IRequest<List<AllProductsResponseModel>>
    {

    }
}
  • In mediatr pattern our RequestModels(eg: AllProductsRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel'(eg: List<AllProductsResponseModel>) of our handler.
Update the logic of our query handler 'AllProductsQueryHandler'.
Handlers/QueryHandlers/AllProductsQueryHandler.cs:
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.QueryHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.RequestModels.QueryRequestModels;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using MediatR;
using Microsoft.EntityFrameworkCore;

namespace API.CQRS.Sample.Handlers.QueryHandlers
{
    public class AllProductsQueryHandler : IRequestHandler<AllProductsRequestModel, List<AllProductsResponseModel>>
    {
        private readonly MyWorldDbContext _myWorldDbContext;
        public AllProductsQueryHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }
        public async Task<List<AllProductsResponseModel>> Handle(AllProductsRequestModel request, CancellationToken cancellationToken)
        {
            return await _myWorldDbContext.Products
            .Select(_ => new AllProductsResponseModel
            {
                Description = _.Description,
                ProductId = _.ProductId,
                Manufacturer = _.Manufacturer,
                Name = _.Name,
                Price = _.Price
            }).ToListAsync();
        }
    }
}
  • Our Query Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
  • Our Query Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'(eg: AllProductsRequestModel). The 'Handle' method return type is our 'ResponseModel'(eg: List<AllProductsResponseModel>).
Now we can remove the reference of our interface 'IAllProductsQueryHandler', in 'Startup.cs' and in the constructor of the 'ProductController' as well.

Update the endpoint logic to use Mediator to invoke the query handler.
Controllers/ProductController.cs:
[HttpGet]
[Route("all")]
public async Task<IActionResult> AllProducts()
{
	//var result = await _allProductsQueryHandler.GetListAsync();
	var result = await _mediator.Send(new AllProductsRequestModel());
	return Ok(result);
}
  • So here 'Send' method of 'IMediator' that takes our RequestModel(eg: AllProductsRequestModel) as input value. So 'Send' method will invoke the appropriate Query Handler(eg: AllProductsQueryHandler) with the help of 'RequestModel'(eg: AllProductsRequestModel).

Update Logic In Query Handler To Filter Products By Prices Using MediatR:

Currently 'PriceRangeProductsQueryHandler' has method 'PriceRangeProductsAsync' that contains 2 input parameters like 'minPrice' and 'maxPrice'. Now we have to create 'RequestModel' with those input properties to support MediatR. In the folder hierarchy like 'RequestModel/QueryRequestModels' add a new file like 'PriceRangeProductsRequestModel.cs'
RequestModel/QueryRequestModels/PriceRangeProductsRequestModel.cs:
using System.Collections.Generic;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using MediatR;

namespace API.CQRS.Sample.RequestModels.QueryRequestModels
{
    public class PriceRangeProductsRequestModel:IRequest<List<PriceRangeProductsResponseModel>>
    {
        public int MinPrice { get; set; }
        public int MaxPrice { get; set; }
    }
}
  • In mediatr pattern our RequestModels(eg: PriceRangeProductsRequestModel) should inherit 'IRequest<out ResponseModel>'. The 'IRequest<out T>' 'T' model should be the 'ResponseModel'(eg: List<PriceRangeProductsResponseModel>) of our handler.
Let's update the logic of the query handler 'PriceRangeProductsQueryHandler'.
Handlers/QueryHandlers/PriceRangeProductsQueryHandler.cs:
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using API.CQRS.Sample.Contracts.QueryHandlers;
using API.CQRS.Sample.Data;
using API.CQRS.Sample.RequestModels.QueryRequestModels;
using API.CQRS.Sample.ResponseModels.QueryResponseModels;
using MediatR;
using Microsoft.EntityFrameworkCore;

namespace API.CQRS.Sample.Handlers.QueryHandlers
{

    public class PriceRangeProductsQueryHandler : IRequestHandler<PriceRangeProductsRequestModel, List<PriceRangeProductsResponseModel>>
    {
        private readonly MyWorldDbContext _myWorldDbContext;
        public PriceRangeProductsQueryHandler(MyWorldDbContext myWorldDbContext)
        {
            _myWorldDbContext = myWorldDbContext;
        }
        public async Task<List<PriceRangeProductsResponseModel>> Handle(PriceRangeProductsRequestModel request, CancellationToken cancellationToken)
        {
            
            return await _myWorldDbContext.Products
            .Where(_ => _.Price >= request.MinPrice && _.Price <= request.MaxPrice)
            .Select(_ => new PriceRangeProductsResponseModel
            {
                Name = _.Name,
                ProductId = _.ProductId,
                Price = _.Price
            }).ToListAsync();
        }
    }
}
  • Our Query Handler inherits 'MediatR.IRequestHandler<in TRequest, TResponse>'. The 'TRequest' means our 'RequestModel'(only user defined class), 'TResponse' means our 'ResponseModel'(either user defined class or scalar type).
  • Our Query Handler should implement 'Handle' as a public method. So first input parameter of the 'Handle' is 'RequestModel'(eg: PriceRangeProductsRequestModel). The 'Handle' method return type is our 'ResponseModel'(eg: List<PriceRangeProductsResponseModel>).
Now we remove references of our interface 'IPriceRangeProductsQueryHandler' in 'Startup.cs' and in 'ProductController' as well.

Now update the endpoint logic to use the MediatR.
Controllers/ProductController.cs:
[HttpGet]
[Route("price-range")]
public async Task<IActionResult> PriceRangeProducts([FromQuery]PriceRangeProductsRequestModel requestModel)
{
	//var result = await _priceRangeProductsQueryHandler.PriceRangeProductsAsync(minPrice, maxPrice);
	var result = await _mediator.Send(requestModel);
	return Ok(result);
}
  • So here 'Send' method of 'IMediator' that takes our RequestModel(eg: PriceRangeProductsRequestModel) as input value. So 'Send' method will invoke the appropriate Query Handler(eg: PriceRangeProductsQueryHandler) with the help of 'RequestModel'(eg: PriceRangeProductsRequestModel).
That's all we have implemented .Net5 Web API sample using CQRS And MediatR Patterns.

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In this article, we will implement the ReactJS application authentication using the HTTP-only cookie. HTTP Only Cookie: In a SPA(Single Page Application) Authentication JWT token either can be stored in browser 'LocalStorage' or in 'Cookie'. Storing the JWT token inside of the cookie then the cookie should be HTTP Only. The HTTP-ONly cookie nature is that it will be only accessible by the server application. Client apps like javascript-based apps can't access the HTTP-Only cookie. So if we use the authentication with HTTP-only JWT cookie then we no need to implement the custom logic like adding authorization header or storing token data, etc at our client application. Because once the user authenticated cookie will be automatically sent to the server by the browser on every API call. Authentication API: To authenticate our client application with JWT HTTP-only cookie, I developed a NetJS(which is a node) Mock API. Check the GitHub link and read the document on G
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Angular 14 State Management CRUD Example With NgRx(14)

In this article, we are going to implement the Angular(14) state management CRUD example with NgRx(14) NgRx Store For State Management: In an angular application to share consistent data between multiple components, we use NgRx state management. Using NgRx state helps to avoid unwanted API calls, easy to maintain consistent data, etc. The main building blocks for the NgRx store are: Actions - NgRx actions represents event to trigger the reducers to save the data into the stores. Reducer - Reducer's pure function, which is used to create a new state on data change. Store - The store is the model or entity that holds the data. Selector - Selector to fetch the slices of data from the store to angular components. Effects - Effects deals with external network calls like API. The effect gets executed based the action performed Ngrx State Management flow: The angular component needs data for binding.  So angular component calls an action that is responsible for invoking the API call.  Aft
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Unit Testing Asp.NetCore Web API Using xUnit[.NET6]

In this article, we are going to write test cases to an Asp.NetCore Web API(.NET6) application using the xUnit. xUnit For .NET: The xUnit for .Net is a free, open-source, community-focused unit testing tool for .NET applications. By default .Net also provides a xUnit project template to implement test cases. Unit test cases build upon the 'AAA' formula that means 'Arrange', 'Act' and 'Assert' Arrange - Declaring variables, objects, instantiating mocks, etc. Act - Calling or invoking the method that needs to be tested. Assert - The assert ensures that code behaves as expected means yielding expected output. Create An API And Unit Test Projects: Let's create a .Net6 Web API and xUnit sample applications to accomplish our demo. We can use either Visual Studio 2022 or Visual Studio Code(using .NET CLI commands) to create any.Net6 application. For this demo, I'm using the 'Visual Studio Code'(using the .NET CLI command) editor. Create a fo
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Part-1 Angular JWT Authentication Using HTTP Only Cookie[Angular V13]

In this article, we are going to implement a sample angular application authentication using HTTP only cookie that contains a JWT token. HTTP Only JWT Cookie: In a SPA(Single Page Application) Authentication JWT token either can be stored in browser 'LocalStorage' or in 'Cookie'. Storing JWT token inside of the cookie then the cookie should be HTTP Only. The HTTP-Only cookie nature is that it will be only accessible by the server application. Client apps like javascript-based apps can't access the HTTP-Only cookie. So if we use authentication with HTTP only JWT cookie then we no need to implement custom logic like adding authorization header or storing token data, etc at our client application. Because once the user authenticated cookie will be automatically sent to the server by the browser on every API call. Authentication API: To implement JWT cookie authentication we need to set up an API. For that, I had created a mock authentication API(Using the NestJS Se
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Angular 14 Crud Example

In this article, we will implement CRUD operation in the Angular 14 application. Angular: Angular is a framework that can be used to build a single-page application. Angular applications are built with components that make our code simple and clean. Angular components compose of 3 files like TypeScript File(*.ts), Html File(*.html), CSS File(*.cs) Components typescript file and HTML file support 2-way binding which means data flow is bi-directional Component typescript file listens for all HTML events from the HTML file. Create Angular(14) Application: Let's create an Angular(14) application to begin our sample. Make sure to install the Angular CLI tool into our local machine because it provides easy CLI commands to play with the angular application. Command To Install Angular CLI npm install -g @angular/cli Run the below command to create the angular application. Command To Create Angular Application ng new name_of_your_app Note: While creating the app, you will see a noti
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ReactJS(v18) Authentication With JWT AccessToken And Refresh Token

In this article, we are going to do ReactJS(v18) application authentication using the JWT Access Token and Refresh Token. JSON Web Token(JWT): JSON Web Token is a digitally signed and secured token for user validation. The JWT is constructed with 3 important parts: Header Payload Signature Create ReactJS Application: Let's create a ReactJS application to accomplish our demo. npx create-react-app name-of-your-app Configure React Bootstrap Library: Let's install the React Bootstrap library npm install react-bootstrap bootstrap Now add the bootstrap CSS reference in 'index.js'. src/index.js: import 'bootstrap/dist/css/bootstrap.min.css' Create A React Component 'Layout': Let's add a React component like 'Layout' in 'components/shared' folders(new folders). src/components/shared/Layout.js: import Navbar from "react-bootstrap/Navbar"; import { Container } from "react-bootstrap"; import Nav from "react-boot
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A Small Guide On NestJS Queues

NestJS Application Queues helps to deal with application scaling and performance challenges. When To Use Queues?: API request that mostly involves in time taking operations like CPU bound operation, doing them synchronously which will result in thread blocking. So to avoid these issues, it is an appropriate way to make the CPU-bound operation separate background job.  In nestjs one of the best solutions for these kinds of tasks is to implement the Queues. For queueing mechanism in the nestjs application most recommended library is '@nestjs/bull'(Bull is nodejs queue library). The 'Bull' depends on Redis cache for data storage like a job. So in this queueing technique, we will create services like 'Producer' and 'Consumer'. The 'Producer' is used to push our jobs into the Redis stores. The consumer will read those jobs(eg: CPU Bound Operations) and process them. So by using this queues technique user requests processed very fastly because actually
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