将连接字符串作为参数传递给Code First Entity Framework中的构造函数

Question

I'm passing the Connection String to my Entity Classes via parameter in the constructor, like this:

public class Site : ISite
{
    private readonly string _connectionString;

    public Site(string connectionString)
    {
        _connectionString = connectionString;
    }

An Exception is thrown when I query the Database like this:

var site = db.Sites.FirstOrDefault(s => s.Name.Equals(name));

The Exception:

InvalidOperationException: The class 'AuctionSite.Site' has no parameterless constructor.

According to the post I found on this thread the documentation states that:

There must be a parameterless constructor

If I create a parameterless constructor the Connection String won't be set and when I create a context inside a method, for example:

public IEnumerable<IUser> GetUsers()
{
    using (var db = new SiteContext(_connectionString))
    {
        return db.Users;
    }
}

_connectionString will be null and will fail when calling base constructor of DbContext.

How should I pass the Connection String to my Entity Classes if not through the constructor?

Is there a better way to do this? Maybe by materializing the Entity with my connection string as a parameter or with a better query?

Answer 1

There is nothing that stops you from having two constructors:

 public Site(string connectionString)
 {
     _connectionString = connectionString;
 }

 public Site()
 {
 }

Answer 2

It looks like you might be confusing Site the entity and SiteContext the DbContext. Site the entity needs the to have a parameterless constructor. SiteContext is the thing which deals with the connection string. The entities do not need to know about the connection string, in fact, they shouldn't know they are related to the database at all.

Additionally the code you have

public IEnumerable<IUser> GetUsers()
{
    using (var db = new SiteContext(_connectionString))
    {
        return db.Users;
    }
}

Will set the connection string just fine. The constructor which matches the arguments, in this case a string, is the one which is called.