How to get IdentityUser name using SqlDataReader

I have this model:

public class Model
{
     public IdentityUser UserId { get; set; }
     public string data { get; set; }

     public decimal data2 { get; set; }
     public DateTime data3 { get; set; }
}

And this function:

using (SqlConnection connection = new SqlConnection(connectionString))
{
     connection.Open();

     string sql = "SELECT UserId, data, data2, data3 FROM Table;

     using (SqlCommand command = new SqlCommand(sql, connection))
     {
         using (SqlDataReader reader = await command.ExecuteReaderAsync())
         {
             while (reader.Read())
             {
                 Model test = new Model
                     {
                         UserId = reader.GetString(0) // error, can not convert from Identity to string
                         data = reader.GetString(1),
                         data2 = reader.GetDecimal(2),
                         data3 = reader.GetDateTime(3)
                     };

                 list.Add(test);
             }
         }
     }
}

Before this, I populated the data from the table and was able to display the data for a specific user using

command.Parameters.AddWithValue("@UserId", userId);

and I obtained the userId in the function:

protected override async Task OnInitializedAsync()
{
    var authState = await AuthenticationStateProvider.GetAuthenticationStateAsync();
    var user = authState.User;

    if (user != null)
    {
        userId = user.Identity.Name;
    }
}

I want to display on a Blazor page a table with all data from that table but I cannot get the UserId because I don't know how to read it using SqlDataReader.