Insertar en una base de datos con LINQ

Tienes algunos problemas:

1. You haven't added the model to your project. I don't see any ORM-related files in Solution Explorer for SqlIntegrationWithVs.

2. Once you add the model, given that BloodBank no es propiedad de Register class; thus, it still won't exist in the current context.

   In general, ORMs put table-mapped classes in a separate namespace, for example BloodBankDatabase.BloodBanks, and objects which let you access table data are put in a specific class, so you'll end up with something like:

   using BloodBankDatabase;
   
   using (var context = new DatabaseContext(connectionString))
   {
       var returnedPerson = from loginDetails in BloodBank
                            where loginDetails.Username == usersInput
                            select loginDetails.Username;
   }
   

Also, looking at your code, you don't seem to understand how C# work. For example:

• No deberías llamar cmd.Dispose() dentro de using (SqlCommand cmd = ...) { }, porque using ya hace eso.

• SqlDataReader implementos IDisposable, so you've forgotten the using () { } alrededor.

• You should't catch Exception.

• where Username == usersInput doesn't make sense in this context; are you sure you understand lambda expressions?

• No bool usernameTaken = false;. Instead, declare the variable as close as possible to the location where you use it first.

• Validate your inputs before using them. What happens if userInput is empty? Or extremely long?

¿Cómo puedes mejorar?

1. Stack Oveflow is not a code review website. Once you get your code working (and only then), you may be interested in submitting it to https://codereview.stackexchange.com/

2. You may also be interested in reading a book to learn C#. C# in Depth by Jon Skeet is de facto THE book any C# beginner should read.

3. If you work in a company, insist in doing code reviews for every line of committed code. Try to get more experienced developers to review your code. If not, find someone (a friend, an old colleague) who is ready to review your code.

4. Análisis de código, available in paid versions of Visual Studio, will point out the issues like the forgotten using () { } blocks. This is much easier than checking on MSDN every class you use to see whether it's implementing IDisposable.

5. poliestilo is another tool which may interest you: it won't find errors per se, but will force you to use an uniform style across the code base.

Intenta agregar

using System.Linq;

at the using statements.