Make it easier to use semantic datatypes with EF core.

A semantic data type is a struct or class with a single member of a primitive type. For example:

struct Email
{
   public string Value {get; set;}
   // other code, but no other public properties or fields
}

I use semantic data types to prevent stupid programming errors like assigning an email address to a user code or assigning a employeeId to a studentId. I get the errors at compile time.

EmployeeId employeeId = 3;
StudentId studentId = employeeId ; // gives compiler error

int employeeId = 3;
int studentId = employeeId ; // compiles but give an error at run-time or causes unexpected behavior.

However for each data type I have to make a bit of plumbing code to make it work with EF core and with Json.

I want to get rid of or the plumbing code perhaps using an extra data annotation or extra function in the context:

[UseSingleValue]
struct Email
{
   public string Value {get; set;}
}

or (but not preferred by me)

entity.Property(e => e.Email).UseSingleValue<Email>(v => v.Value)

So EF core knows that a property of type Email really is a string stored in property Value and generates the plumbing code it self.

[lakeman]

You could visit every property in your OnModelCreating, eg modelBuilder.Model.GetEntityTypes().Select(t => t.GetProperties()) and create a converter based on reflection information.

[roji]

Seems like the rest of this is about value objects, so we can probably close as a dup of #9906.