Pergunta do https://brasildotnet.slack.com/archives/C0R3GPZMJ/p1503324783000429

andrepaggi.cs

interface IConnectionString
{
    string ObterConnectionDoClienteString();
}

class ConnectionString : IConnectionString
{
    private Dicitionary<string, string> _connectionStrings = new Dicitionary<string, string>();

    public string ObterConnectionDoClienteString()
    {
        var cliente = HttpContext.Current.Request.Url.SubDominio;
        if(_connectionStrings.HasKey(cliente))
            return _connectionStrings[cliente];

        //Criar connectionstring
        var novaConnectionStringCriada = CriarConnectionStringParaCliente(cliente);

        //Salvar no dictionary
        _connectionStrings.Add(cliente, novaConnectionStringCriada);

        return novaConnectionStringCriada;
    }

    private string CriarConnectionStringParaCliente(string cliente)
    {
        //Seu codigo
    }
}

class ContextoDoEf : DbContext
{
    public ContextoDoEf(IConnectionString connectionString)
        : base(connectionString.ObterConnectionDoClienteString())
    {

    }
}

class RepositorioDapper
{
    private readonly string _connectionString;
    
    public RepositorioDapper(IConnectionString connectionString)
    {
        _connectionString = connectionString.ObterConnectionDoClienteString();
    }
}