How to create a DbContext from just a connection string?

Time needed: 10 minutes

How to instantiate an Entity Framework Core database context using DbContextOptionsBuilder

1. Add dependencies

   You’ll need to add these dependencies – and hence the NuGet packages “Microsoft.EntityFrameworkCore” and “Microsoft.EntityFrameworkCore.SqlServer“.
   
   Then, add your using statements, like in the example below:
   
   // EF dependencies:
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.SqlServer;

// and of course, something like this: using YourNamespace.Data.Entities;
   Note – in newer versions of .NET Core CLI , you won’t be needing the Microsoft.EntityFrameworkCore.SqlServer reference there (you will still need the nuget package though!)
   
   Caveat: Tooling might be a bit “meh” here!
   
   Interestingly, in some versions of Visual Studio, Intellisense bugs out and tries to remove the usings. See this post for more information about that:
   How to access Entity Framework Core’s DbContext in an Azure Function?
   
   But that’s enough of that. Now…

2. Configure your Database Context

   You’ll need to add a suitable constructor for our next step to your dbcontext class – somewhat like below:
   
   using Microsoft.EntityFrameworkCore;

namespace DataLayer
{
public class ApplicationDbContext : DbContext
{
// entities etc omitted

public ApplicationDbContext(DbContextOptions options) : base(options) { }

}
}

3. Enough – show me the code!

   Okay – not too complicated or anything. See below for my example code:
   
   using Microsoft.EntityFrameworkCore;
using System;

namespace YourNamespace
{
public class YourClass
{
private string _connectionString = "Comes from your configuration file";

public YourClass()
{
// In this example
// "ApplicationDbContext" is my DbContext class
var options = new DbContextOptionsBuilder<ApplicationDbContext>().UseSqlServer(_connectionString).Options;
// With the options generated above, we can then just construct a new DbContext class

using (var ctx = new ApplicationDbContext(options))
{
// Your code here
}

}
}
}
   Copy-paste it and give it a go!
4. What’s next?

   Actually, there’s no step 3. It should be done with that! You just add whatever DB-querying code you might have inside the block where you’re using the context.
   
   This piece of code should successfully connect to a MS SQL Database, and perform whatever cool stuff it needs to do!