多线程WCF与ADO.. NET和SQL Server连接会抛出“当读取器关闭时，试图调用Read无效”

我有一个连接到SQL Server数据库的WCF服务。

WCF代码:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall, ConcurrencyMode = ConcurrencyMode.Multiple)]
[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class MyWebService : IMyWebService
{
    public IList<IDictionary<string, object>> Select()
    {
        return nee MyRepository().Select("SELECT A, B, C FROM TABLE_T");
    }
}

库代码:

public IList<IDictionary<string, object>> Select(string sqlQuery, params SqlParameter[] pars)
{
    IList<IDictionary<string, object>> list = new List<IDictionary<string, object>>();
    using (SqlCommand command = new SqlCommand(sqlQuery, Connection))
    {
        command.Parameters.AddRange(pars);
        if (command.Connection.State == ConnectionState.Closed)
            command.Connection.Open();
        using (SqlDataReader reader = command.ExecuteReader())
        {
            while (reader.Read())
            {
                IDictionary<string, object> keyPair = new Dictionary<string, object>();
                for (int i = 0; i < reader.FieldCount; i++)
                    keyPair.Add(reader.GetName(i), reader.GetValue(i));
                list.Add(keyPair);
            }
        }
        command.Parameters.Clear();
        _connection.Dispose();
           _connection = null;
    }
    return list;
}

当只有一台机器调用时可以正常工作，但是当两台或两台以上的机器调用时，我得到一个错误

reader关闭时调用Read无效

和SQL Server连接失败，直到重启。

PS:我真正的web服务代码执行更多的操作，如插入和更新。对于每个操作，我都创建一个到数据库服务器的新连接，并且总是在命令周围加上using。

更新1:(在评论中请求)

我的连接代码:

private SqlConnection _connection = null;
protected SqlConnection Connection
{
    get
    {
        try
        {
            if (string.IsNullOrWhiteSpace(_connectionString))
                throw new NotImplementedException("Missing _connectionString");
            if (_connection == null)
                _connection = new SqlConnection(_connectionString);
            if (_connection.State == ConnectionState.Closed)
                _connection.ConnectionString = _connectionString;
            _connection.Open();
            return _connection;
        }
        catch (Exception e)
        {
            _lastConnectErrorMessage = e.Message;
            throw e;
        }
    }
}