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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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Introduction: Form validations in any applications are like assures that a valid data is storing on servers. All programing frameworks have their own individual implementations for form validations. In Dotnet Core MVC application server-side validations carried on by the models with the help of Data Annotations and the client-side validations carried by the plugin jQuery Unobtrusive Validation. jQuery Unobtrusive Validation is a custom library developed by Microsoft based on the popular library  jQuery Validate . In this article, we are going to learn how the model validation and client-side validation works in Asp.Net Core MVC Application with sample examples. Getting Started: Let's create an Asp.Net Core MVC application project using preferred editors like Microsoft Visual Studio or Microsoft Visual Studio Code. Here I'm using Visual Studio. Let's create an MVC controller and name it as 'PersonController.cs' and add an action method as bel

.NET Core MVC Application File Upload To Physical Location With Buffered Technique

Buffering Technique In File Upload: The server will use its Memory(RAM) or Disk Storage to save the files on receiving a file upload request from the client.  Usage of Memory(RAM) or Disk depends on the number of file requests and the size of the file.  Any single buffered file exceeding 64KB is moved from Memory to a temp file on disk.  If an application receives heavy traffic of uploading files there might be a chance of out of Disk or RAM memory which leads to crash application. So this Buffer technique used for small files uploading. In the following article, we create a sample for the file uploading using .NET Core MVC application. Create The .NET Core MVC Project: Let's create a .NET Core MVC project, here for this sample I'm using Visual Studio Code as below.   Check the link to use the Visual Studio Code for .NET Core Application . IFormFile: Microsoft.AspNetCore.Http.IFormFile used for file upload with buffered technique. On uploading files f

Ionic Picker Sample Code In Angular

Introduction: Ionic Picker(ion-picker) is a popup slides up from the bottom of the device screen, which contains rows with selectable column separated items. The main building block of ion-picker as follows: PickerController PickerOptions PickerController: PickerController object helps in creating an ion-picker overlay. create(opts?: Opts): Promise<Overlay> PickerController create method helps in create the picker overlay with the picker options PickerOptions: PickerOptions is a configuration object used by PickerController to display ion-picker. Single Column Ionic Picker: single.item.picker.ts: import { Component } from "@angular/core"; import { PickerController } from "@ionic/angular"; import { PickerOptions } from "@ionic/core"; @Component({ selector: "single-column-picker", templateUrl:"single.item.picker.html" }) export class SingleItemPicker { animals: string[] = ["Tiger&quo

.Net Core HttpClient JSON Extension Methods Using System.Net.Http.Json Package

.Net Core 3.0 onwards Microsoft brought up a new package called System.Net.Http.Json. This new package provides JSON extension methods for HttpClient. These JSON extension methods will have a prebuild mechanism for serializing or deserializing response data or payload of HttpClient call. System.Net.Http.Json extension methods that are provided to HttpClient, few of them are mentioned below. GetFromJsonAsync PostAsJsonAsync PutAsJsonAsync ReadFromJsonAsync In this article, we understand System.Net.Http.Json package by implementing the HttpClient samples by with and without JSON extension methods and compare them. Create A .Net Core Web API Sample Application: Let's create a .Net Core sample Web API application, from this application we will consume another Web API by implementing HttpClient calls. We can create a Web API sample application using IDE like Visual Studio 2019(Supports .Net Core 3.0 plus) or  Visual Studio Code . Create A Typed Client: In .Net Core using the Http

GraphQL API Integration In Asp.Net Core Application

Introduction: GraphQL is a query language for your API and a server-side runtime for executing queries by using a type system you define for your data. GraphQL can be integrated into any framework like ASP.NET, Java, NestJs, etc and it isn't tied to any specific database or storage engine and is instead backed by your existing code and data. How GraphQL API Different From Rest API: GraphQL exposes a single end-point or route for the entire application, regardless of its responses or actions. HTTP-POST is the only Http verb recommended by the GraphQL. The client applications (consumers of API) can give instructions to GraphQL API about what type of properties to be returned in the response. Building Blocks Of GraphQL API: The main building blocks of GraphQL API is Schemas and Types.  A 'Schema' in GrpahQL API describes the functionality available to the clients connect to API. Schema mostly consists of GraphQL Object Types, Queries, Mutations, etc. T

ASP.NET Core Web API Versioning

Introduction: An iteration and evolutionary changes of an ASP.NET Core Web API is handled by Versioning. Versioning of an API gives confidence to the clients which consumes API for a long time. Any changes or development of an API will be accessible using the new version and it won't cause issues to the clients consuming the old version of API. When To Use Versioning: Any API response changes. Developing an API by implementing testing levels like 'Alpha', 'Beta', and 'RC' versions before releasing Production. Deprecating an API which means API going to be removed or upgraded by a version within a short period. Versioning Types: Query String Versioning Url Path Versioning Media Type Versioning API Version Nuget: To Configure versioning to AspNet Core Web API Microsoft provided a library(Microsoft.AspNetCore.Mvc.Versioning). So to use the versioning library please install NuGet below.              Install-Package Microsoft.A

Blazor Server CRUD Operations

Introduction: Blazor Server is a web framework to develop server-side single-page applications. Blazor is made up of components with the combinations on C#, Html, CSS.  Blazor Server is production-ready from the .Net Core 3.0.  Blazor Server Working Mechanism: Blazor Server is a very light-weight web development framework.  In Blazor Server, not all code gets downloaded to the client browsers. Blazor Server made of components these components can be a block of code or page with respective navigation.  Blazor server application communicates with the server with a SignalR background connection which is inbuilt functionality. Application click,  form submission, change events, application page navigation every operation is carried out by the SignalR connection by communicating with the server.  Blazor updates the Html DOM very gently on every data update without any overhead. Blazor Server application maintains a nice intelligent tree structure to update the required inform