c# - 如何使用 BeginExecuteReader

Question

美好的一天。

请帮助我了解如何使用通用列表使用 SqlCommand 类的三种方法 BeginExecuteReader()。我用 BeginExecuteReader 做了一个方法，但我不知道这是否是最好的使用方式

public class Empresa {
    public Empresa() {
        PkEmpresa = -1;
        CodigoEmpresa = "";
        Descripcion = "";
        PkCategoriaEmpresa = -1;
    }

    public int PkEmpresa { get; set; }
    public string CodigoEmpresa { get; set; }
    public string Descripcion { get; set; }
    public int PkCategoriaEmpresa { get; set; }

    public Empresa ShallowCopy() {
        return (Empresa)this.MemberwiseClone();
    }
}



public class AsyncronousDAL {
    private static string getConexion() {
        return "Data Source=DATABASE;Initial Catalog=DATA_BASE;Integrated Security=True;Asynchronous Processing=True";
    }

    public static List<Empresa> ConsultaAsincrona() {
        List<Empresa> _resultados = new List<Empresa>();

        using (SqlConnection conexion = new SqlConnection(getConexion())) {
            using (SqlCommand commando = new SqlCommand("[dbo].[pruebaAsync]", conexion)) {
                commando.CommandType = System.Data.CommandType.StoredProcedure;
                conexion.Open();
                IAsyncResult resultado = commando.BeginExecuteReader();
                using (SqlDataReader reader = commando.EndExecuteReader(resultado)) {
                    while (reader.Read()) {
                        _resultados.Add(new Empresa() {
                            PkEmpresa = Convert.ToInt32(reader["PkEmpresa"]),
                            CodigoEmpresa = reader["CodigoEmpresa"].ToString(),
                            Descripcion = reader["Descripcion"].ToString(),
                            PkCategoriaEmpresa = Convert.ToInt32(reader["PkCategoriaEmpresa"])
                        });
                    }
                }
            }
        }

        return _resultados;
    }
}

Answer 1

如果您不熟悉异步模式，网上有很多教程和示例。这是旧的，但仍然相关:http://msdn.microsoft.com/en-us/library/aa719595(v=vs.71).aspx

当您调用 BeginExecuteReader 时，该工作将最终被推送到工作线程，从而允许您的主线程继续执行。当您调用 EndExecuteReader 时，这将导致您的主线程阻塞，直到该任务完成。

如果您立即调用 EndExecuteReader - 您实际上并没有获得任何好处(事实上，您引入了额外的开销)。

看看这里的例子:http://msdn.microsoft.com/en-us/library/7szdt0kc.aspx

The BeginExecuteReader method returns immediately, but until the code executes the corresponding EndExecuteReader method call, it must not execute any other calls that start a synchronous or asynchronous execution against the same SqlCommand object. Calling the EndExecuteReader before the command's execution is completed cause the SqlCommand object to block until the execution is finished.

这是代码的相关部分:

            // Although it is not required that you pass the 
            // SqlCommand object as the second parameter in the 
            // BeginExecuteReader call, doing so makes it easier
            // to call EndExecuteReader in the callback procedure.
            AsyncCallback callback = new AsyncCallback(HandleCallback);
            command.BeginExecuteReader(callback, command);