c# - 当我对 POCO 实体执行 toString 时，我收到 DataReader 打开错误

Question

这是我第一次使用 Entity Framework Code First，我正在尝试这样做toString()：

public class User
{
    public int UserId { get; set; }
    public string Name { get; set; }
    public int Age { get; set; }
    public string City { get; set; }
    public string Country { get; set; }
    public string Email { get; set; }
    public string InterestIn { get; set; }

    public virtual ICollection<User> Friends { get; set; }
    public virtual ICollection<User> FromWhomIsFriend { get; set; }

    public override string ToString()
    {
        string output = string.Empty;

        output = String.Format("UserId: {0}, Name: {1}, Age: {2}, City: {3}, " +
            "Country: {4}, Email: {5}, InterestIn: {6}, Friends: {7}",
            UserId, Name, Age, City, Country, Email, InterestIn, Friends);

        return output;
    }
}

当我这样做时，toString()我收到此错误：

'System.Data.EntityCommandExecutionException'
"There is already an open datareader associated with this command you must close it first."

我在这里做：

private static void showUsers()
{
    using (var context = new AdnLineContext())
    {
        var users = from u in context.Users
                    select u;
        foreach (User user in users)
        {
            Console.WriteLine(user.ToString());
        }
    }
}

我不明白，为什么我会得到它？我该如何解决？

Answer 1

尝试使用 .ToList() 强制执行查询（默认情况下，查询的执行将推迟到您枚举它）：

private static void showUsers()
{
    using (var context = new AdnLineContext())
    {
        var users = context.Users.ToList();

        foreach (User user in users)
        {
            Console.WriteLine(user.ToString());
        }
    }
}

如果失败，您也可以尝试添加MultipleActiveResultSets=True到您的连接字符串配置，例如：

<add name="YourContext" connectionString="Data Source=.;Initial Catalog=YourDb;Integrated Security=True;MultipleActiveResultSets=True" providerName="System.Data.SqlClient" />