From Pluralsight/Building a Data-driving ASP.NET Core 6 Blazor Server Application

Implement a Database with EF Core in an ASP.NET Core Blazor Web App

create model classes

Data/Models/Employee.cs

public class Employee
{
    public int Id { get; set; } // Primary Key
    [Required] // even though it's nullable, this property is required
    [StringLength(50)]
    public string? FirstName { get; set; }
    [Required]
    [StringLength(50)]
    public string? LastName { get; set; }
    public bool IsDeveloper { get; set; }
    [Required] // an Employee MUST be in a Department
    public int? DepartmentId { get; set; } // Foreign Key
    public Department? Department { get; set; }
}

Data/Models/Department.cs

public class Department
{
    public int Id { get; set; }
    [Required]
    [StringLength(50)]
    public string? Name { get; set; }
    public List<Employee> Employees { get; set; } = new();
}

Implement a `DbContext`

dotnet add package microsoft.entityframeworkcore.sqlserver

Data/EmployeeManagerDbContext.cs

public class EmployeeManagerDbContext : DbContext
{
    /*
    In previous versions of C#, Employees would throw a null warning here. To resolve this,  
    you would use the Set method of the DbContext base class to set Employees to a default value:  
    public DbSet<Employees> Employees => Set<Employee>();  
    */
    public DbSet<Employee> Employees { get; set; }
    public DbSet<Department> Departments { get; set; }

    public EmployeeManagerDbContext(DbContextOptions<EmployeeManagerDbContext> options) : base(options)
    {
    }
}

Register the `DbContext` in DI

Program.cs

builder.Services.AddDbContext<EmployeeManagerDbContext>(
    opt => opt.UseSqlServer(builder.Configuration.GetConnectionString("EmployeeManagerDb")));

Set the Connection String in `appsettings.json`

appsettings.json

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "ConnectionStrings": {
    // localDb = SQL Server development database
    // MSSQLLocalDb = username
    "EmployeeManagerDb": "Data Source=(localDb)\\MSSQLLocalDb;Initial Catalog=EmployeeManagerDb"
  }
}

execute a migration

add a migration

dotnet add package microsoft.entityframeworkcore.tools

PM> add-migration InitialCreate

This creates:

Migrations/EmployeeManagerDbContextSnapshot — a snapshot of the current database state
Migrations/###_InitialCreate.cs — contains:
- Up method — creates the database tables
- Down method — drops the tables

run the migration

PM> update-database

confirm the database exists

Visual Studio > View > SQL Server Object Explorer

Using the `DbContext` in a Component

Pages/EmployeeOverview.razor

@page "/employees/list"
@using Microsoft.EntityFrameworkCore; @* ToArrayAsync *@
@using WiredBrainCoffee.EmployeeManager.Data.Models;
@using WiredBrainCoffee.EmployeeManager.Data;
<!-- Inject the DbContext into this Component: --> 
@inject EmployeeManagerDbContext Context

<PageTitle>Employees</PageTitle>
<h1>Employees</h1>

TODO: List the employees here.

@code
{
    private Employee[]? Employees { get; set; }

    protected override async Task OnInitializedAsync()
    {
        // The DbContext has a DbSet<Employee> that we use to set the Employees property.
        // The Include method specifies related entities to include in the query.
        // In this case, since Employee has a Department property, that it a related entity we need to include.
        Employees = await Context.Employees.Include(emp => emp.Department).ToArrayAsync();
    }
}

using dbcontextfactory

By injecting a DbContext into a Component, the lifetime of the DbContext is now tightly coupled to the Component. In Blazor Server, since the client (browser) is connected to the server via a SignalR connection, the Component is not re-created for each request; the Component lives as long as the client is using it. The DbContext is designed for a short lifespan.

To avoid this problem, inject an IDbContextFactory into the Component instead. It can be used to inject a short-lived context. IDbContextFactory registers both an IDbContextFactory (short-lived) and a DbContext (long-lived) as services: Program.cs

builder.Services.AddDbContextFactory<EmployeeManagerDbContext>(
    opt => opt.UseSqlServer(builder.Configuration.GetConnectionString("EmployeeManagerDb")));