If SQL Express is running, it will be used.
The NuGet package will then update the configuration file by setting the default database server that Code First uses when creating a connection by convention. During installation, the EntityFramework NuGet package checks which database server is available.
Visual Studio 2010 includes SQL Express by default and Visual Studio 2012 and later includes LocalDB. If both are installed, SQL Express will be used. In this example DbContext uses the namespace qualified name of your derived context class-Demo.EF.BloggingContext-as the database name and creates a connection string for this database using either SQL Express or LocalDB. C# will call base class parameterless constructor by default If you have not done any other configuration in your application, then calling the parameterless constructor on DbContext will cause DbContext to run in Code First mode with a database connection created by convention. Use Code First with connection by convention The following fragments show some of the ways the DbContext constructors can be used. Whether the context will use calculate a model using Code First or load a model created with the EF Designer.How the context will connect to a database - that is, how a connection string is found/used.This derived class will call one of the constructors on the base DbContext class to control: Typically an Entity Framework application uses a class derived from DbContext. Models created with Code First and the EF Designer are both covered in this topic. This topic covers how Entity Framework discovers which database connection to use, and how you can change it.