SqlCommand 类 (System.Data.SqlClient)

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public ref class SqlCommand sealed : System::Data::Common::DbCommand, ICloneable

public ref class SqlCommand sealed : System::Data::Common::DbCommand

public ref class SqlCommand sealed : System::ComponentModel::Component, ICloneable, IDisposable, System::Data::IDbCommand

public ref class SqlCommand sealed : System::Data::Common::DbCommand, ICloneable, IDisposable

public sealed class SqlCommand : System.Data.Common.DbCommand, ICloneable

public sealed class SqlCommand : System.Data.Common.DbCommand

public sealed class SqlCommand : System.ComponentModel.Component, ICloneable, IDisposable, System.Data.IDbCommand

public sealed class SqlCommand : System.Data.Common.DbCommand, ICloneable, IDisposable

type SqlCommand = class
    inherit DbCommand
    interface ICloneable

type SqlCommand = class
    inherit DbCommand

type SqlCommand = class
    inherit Component
    interface IDbCommand
    interface IDisposable
    interface ICloneable

type SqlCommand = class
    inherit DbCommand
    interface IDbCommand
    interface ICloneable
    interface IDisposable

Public NotInheritable Class SqlCommand
Inherits DbCommand
Implements ICloneable

Public NotInheritable Class SqlCommand
Inherits DbCommand

Public NotInheritable Class SqlCommand
Inherits Component
Implements ICloneable, IDbCommand, IDisposable

Public NotInheritable Class SqlCommand
Inherits DbCommand
Implements ICloneable, IDisposable

Object

DbCommand

SqlCommand

以下示例创建一个 SqlConnection 、一个 SqlCommand 和一个 SqlDataReader 。该示例读取数据，并将其写入控制台。最后，该示例关闭 SqlDataReader 代码块，然后在 SqlConnection 退出 Using 代码块时关闭该示例。


   private static void ReadOrderData(string connectionString)
    string queryString =
        "SELECT OrderID, CustomerID FROM dbo.Orders;";
    using (SqlConnection connection = new SqlConnection(
               connectionString))
        SqlCommand command = new SqlCommand(
            queryString, connection);
        connection.Open();
        using(SqlDataReader reader = command.ExecuteReader())
            while (reader.Read())
                Console.WriteLine(String.Format("{0}, {1}",
                    reader[0], reader[1]));
   
    Public Sub ReadOrderData(ByVal connectionString As String)
    Dim queryString As String = _
        "SELECT OrderID, CustomerID FROM dbo.Orders;"
    Using connection As New SqlConnection(connectionString)
        Dim command As New SqlCommand(queryString, connection)
        connection.Open()
        Dim reader As SqlDataReader = command.ExecuteReader()
            While reader.Read()
                Console.WriteLine(String.Format("{0}, {1}", _
                    reader(0), reader(1)))
            End While
        Finally
            ' Always call Close when done reading.
            reader.Close()
        End Try
    End Using
End Sub
    
     以下示例演示如何创建和执行不同类型的 SqlCommand 对象。
    
    
     首先，必须通过执行以下脚本来创建示例数据库：
    
    USE [master]  
CREATE DATABASE [MySchool]  
USE [MySchool]  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
CREATE procedure [dbo].[CourseExtInfo] @CourseId int  
select c.CourseID,c.Title,c.Credits,d.Name as DepartmentName  
from Course as c left outer join Department as d on c.DepartmentID=d.DepartmentID  
where c.CourseID=@CourseId  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
create procedure [dbo].[DepartmentInfo] @DepartmentId int,@CourseCount int output  
select @CourseCount=Count(c.CourseID)  
from course as c  
where c.DepartmentID=@DepartmentId  
select d.DepartmentID,d.Name,d.Budget,d.StartDate,d.Administrator  
from Department as d  
where d.DepartmentID=@DepartmentId  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
Create PROCEDURE [dbo].[GetDepartmentsOfSpecifiedYear]   
@Year int,@BudgetSum money output  
BEGIN  
        SELECT @BudgetSum=SUM([Budget])  
  FROM [MySchool].[dbo].[Department]  
  Where YEAR([StartDate])=@Year   
SELECT [DepartmentID]  
      ,[Name]  
      ,[Budget]  
      ,[StartDate]  
      ,[Administrator]  
  FROM [MySchool].[dbo].[Department]  
  Where YEAR([StartDate])=@Year  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
CREATE TABLE [dbo].[Course]([CourseID] [nvarchar](10) NOT NULL,  
[Year] [smallint] NOT NULL,  
[Title] [nvarchar](100) NOT NULL,  
[Credits] [int] NOT NULL,  
[DepartmentID] [int] NOT NULL,  
 CONSTRAINT [PK_Course] PRIMARY KEY CLUSTERED   
[CourseID] ASC,  
[Year] ASC  
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
CREATE TABLE [dbo].[Department]([DepartmentID] [int] IDENTITY(1,1) NOT NULL,  
[Name] [nvarchar](50) NOT NULL,  
[Budget] [money] NOT NULL,  
[StartDate] [datetime] NOT NULL,  
[Administrator] [int] NULL,  
 CONSTRAINT [PK_Department] PRIMARY KEY CLUSTERED   
[DepartmentID] ASC  
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
CREATE TABLE [dbo].[Person]([PersonID] [int] IDENTITY(1,1) NOT NULL,  
[LastName] [nvarchar](50) NOT NULL,  
[FirstName] [nvarchar](50) NOT NULL,  
[HireDate] [datetime] NULL,  
[EnrollmentDate] [datetime] NULL,  
 CONSTRAINT [PK_School.Student] PRIMARY KEY CLUSTERED   
[PersonID] ASC  
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
CREATE TABLE [dbo].[StudentGrade]([EnrollmentID] [int] IDENTITY(1,1) NOT NULL,  
[CourseID] [nvarchar](10) NOT NULL,  
[StudentID] [int] NOT NULL,  
[Grade] [decimal](3, 2) NOT NULL,  
 CONSTRAINT [PK_StudentGrade] PRIMARY KEY CLUSTERED   
[EnrollmentID] ASC  
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]) ON [PRIMARY]  
SET ANSI_NULLS ON  
SET QUOTED_IDENTIFIER ON  
create view [dbo].[EnglishCourse]  
select c.CourseID,c.Title,c.Credits,c.DepartmentID  
from Course as c join Department as d on c.DepartmentID=d.DepartmentID  
where d.Name=N'English'  
INSERT [dbo].[Course] ([CourseID], [Year], [Title], [Credits], [DepartmentID]) VALUES (N'C1045', 2012, N'Calculus', 4, 7)  
INSERT [dbo].[Course] ([CourseID], [Year], [Title], [Credits], [DepartmentID]) VALUES (N'C1061', 2012, N'Physics', 4, 1)  
INSERT [dbo].[Course] ([CourseID], [Year], [Title], [Credits], [DepartmentID]) VALUES (N'C2021', 2012, N'Composition', 3, 2)  
INSERT [dbo].[Course] ([CourseID], [Year], [Title], [Credits], [DepartmentID]) VALUES (N'C2042', 2012, N'Literature', 4, 2)  
SET IDENTITY_INSERT [dbo].[Department] ON   
INSERT [dbo].[Department] ([DepartmentID], [Name], [Budget], [StartDate], [Administrator]) VALUES (1, N'Engineering', 350000.0000, CAST(0x0000999C00000000 AS DateTime), 2)  
INSERT [dbo].[Department] ([DepartmentID], [Name], [Budget], [StartDate], [Administrator]) VALUES (2, N'English', 120000.0000, CAST(0x0000999C00000000 AS DateTime), 6)  
INSERT [dbo].[Department] ([DepartmentID], [Name], [Budget], [StartDate], [Administrator]) VALUES (4, N'Economics', 200000.0000, CAST(0x0000999C00000000 AS DateTime), 4)  
INSERT [dbo].[Department] ([DepartmentID], [Name], [Budget], [StartDate], [Administrator]) VALUES (7, N'Mathematics', 250024.0000, CAST(0x0000999C00000000 AS DateTime), 3)  
SET IDENTITY_INSERT [dbo].[Department] OFF  
SET IDENTITY_INSERT [dbo].[Person] ON   
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (1, N'Hu', N'Nan', NULL, CAST(0x0000A0BF00000000 AS DateTime))  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (2, N'Norman', N'Laura', NULL, CAST(0x0000A0BF00000000 AS DateTime))  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (3, N'Olivotto', N'Nino', NULL, CAST(0x0000A0BF00000000 AS DateTime))  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (4, N'Anand', N'Arturo', NULL, CAST(0x0000A0BF00000000 AS DateTime))  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (5, N'Jai', N'Damien', NULL, CAST(0x0000A0BF00000000 AS DateTime))  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (6, N'Holt', N'Roger', CAST(0x000097F100000000 AS DateTime), NULL)  
INSERT [dbo].[Person] ([PersonID], [LastName], [FirstName], [HireDate], [EnrollmentDate]) VALUES (7, N'Martin', N'Randall', CAST(0x00008B1A00000000 AS DateTime), NULL)  
SET IDENTITY_INSERT [dbo].[Person] OFF  
SET IDENTITY_INSERT [dbo].[StudentGrade] ON   
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (1, N'C1045', 1, CAST(3.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (2, N'C1045', 2, CAST(3.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (3, N'C1045', 3, CAST(2.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (4, N'C1045', 4, CAST(4.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (5, N'C1045', 5, CAST(3.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (6, N'C1061', 1, CAST(4.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (7, N'C1061', 3, CAST(3.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (8, N'C1061', 4, CAST(2.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (9, N'C1061', 5, CAST(1.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (10, N'C2021', 1, CAST(2.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (11, N'C2021', 2, CAST(3.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (12, N'C2021', 4, CAST(3.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (13, N'C2021', 5, CAST(3.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (14, N'C2042', 1, CAST(2.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (15, N'C2042', 2, CAST(3.50 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (16, N'C2042', 3, CAST(4.00 AS Decimal(3, 2)))  
INSERT [dbo].[StudentGrade] ([EnrollmentID], [CourseID], [StudentID], [Grade]) VALUES (17, N'C2042', 5, CAST(3.00 AS Decimal(3, 2)))  
SET IDENTITY_INSERT [dbo].[StudentGrade] OFF  
ALTER TABLE [dbo].[Course]  WITH CHECK ADD  CONSTRAINT [FK_Course_Department] FOREIGN KEY([DepartmentID])  
REFERENCES [dbo].[Department] ([DepartmentID])  
ALTER TABLE [dbo].[Course] CHECK CONSTRAINT [FK_Course_Department]  
ALTER TABLE [dbo].[StudentGrade]  WITH CHECK ADD  CONSTRAINT [FK_StudentGrade_Student] FOREIGN KEY([StudentID])  
REFERENCES [dbo].[Person] ([PersonID])  
ALTER TABLE [dbo].[StudentGrade] CHECK CONSTRAINT [FK_StudentGrade_Student]  
接下来，编译并执行以下内容：
using System;  
using System.Data;  
using System.Data.SqlClient;  
using System.Threading.Tasks;  
class Program {  
   static class SqlHelper {  
      // Set the connection, command, and then execute the command with non query.  
      public static Int32 ExecuteNonQuery(String connectionString, String commandText,  
          CommandType commandType, params SqlParameter[] parameters) {  
         using (SqlConnection conn = new SqlConnection(connectionString)) {  
            using (SqlCommand cmd = new SqlCommand(commandText, conn)) {  
               // There're three command types: StoredProcedure, Text, TableDirect. The TableDirect   
               // type is only for OLE DB.    
               cmd.CommandType = commandType;  
               cmd.Parameters.AddRange(parameters);  
               conn.Open();  
               return cmd.ExecuteNonQuery();  
      // Set the connection, command, and then execute the command and only return one value.  
      public static Object ExecuteScalar(String connectionString, String commandText,  
          CommandType commandType, params SqlParameter[] parameters) {  
         using (SqlConnection conn = new SqlConnection(connectionString)) {  
            using (SqlCommand cmd = new SqlCommand(commandText, conn)) {  
               cmd.CommandType = commandType;  
               cmd.Parameters.AddRange(parameters);  
               conn.Open();  
               return cmd.ExecuteScalar();  
      // Set the connection, command, and then execute the command with query and return the reader.  
      public static SqlDataReader ExecuteReader(String connectionString, String commandText,  
          CommandType commandType, params SqlParameter[] parameters) {  
         SqlConnection conn = new SqlConnection(connectionString);  
         using (SqlCommand cmd = new SqlCommand(commandText, conn)) {  
            cmd.CommandType = commandType;  
            cmd.Parameters.AddRange(parameters);  
            conn.Open();  
            // When using CommandBehavior.CloseConnection, the connection will be closed when the   
            // IDataReader is closed.  
            SqlDataReader reader = cmd.ExecuteReader(CommandBehavior.CloseConnection);  
            return reader;  
   static void Main(string[] args) {  
      String connectionString = "Data Source=(local);Initial Catalog=MySchool;Integrated Security=True;Asynchronous Processing=true;";  
      CountCourses(connectionString, 2012);  
      Console.WriteLine();  
      Console.WriteLine("Following result is the departments that started from 2007:");  
      GetDepartments(connectionString, 2007);  
      Console.WriteLine();  
      Console.WriteLine("Add the credits when the credits of course is lower than 4.");  
      AddCredits(connectionString, 4);  
      Console.WriteLine();  
      Console.WriteLine("Please press any key to exit...");  
      Console.ReadKey();  
   static void CountCourses(String connectionString, Int32 year) {  
      String commandText = "Select Count([CourseID]) FROM [MySchool].[dbo].[Course] Where Year=@Year";  
      SqlParameter parameterYear = new SqlParameter("@Year", SqlDbType.Int);  
      parameterYear.Value = year;  
      Object oValue = SqlHelper.ExecuteScalar(connectionString, commandText, CommandType.Text, parameterYear);  
      Int32 count;  
      if (Int32.TryParse(oValue.ToString(), out count))  
         Console.WriteLine("There {0} {1} course{2} in {3}.", count > 1 ? "are" : "is", count, count > 1 ? "s" : null, year);  
   // Display the Departments that start from the specified year.  
   static void GetDepartments(String connectionString, Int32 year) {  
      String commandText = "dbo.GetDepartmentsOfSpecifiedYear";  
      // Specify the year of StartDate  
      SqlParameter parameterYear = new SqlParameter("@Year", SqlDbType.Int);  
      parameterYear.Value = year;  
      // When the direction of parameter is set as Output, you can get the value after   
      // executing the command.  
      SqlParameter parameterBudget = new SqlParameter("@BudgetSum", SqlDbType.Money);  
      parameterBudget.Direction = ParameterDirection.Output;  
      using (SqlDataReader reader = SqlHelper.ExecuteReader(connectionString, commandText,  
          CommandType.StoredProcedure, parameterYear, parameterBudget)) {  
         Console.WriteLine("{0,-20}{1,-20}{2,-20}{3,-20}", "Name", "Budget", "StartDate",  
             "Administrator");  
         while (reader.Read()) {  
            Console.WriteLine("{0,-20}{1,-20:C}{2,-20:d}{3,-20}", reader["Name"],  
                reader["Budget"], reader["StartDate"], reader["Administrator"]);  
      Console.WriteLine("{0,-20}{1,-20:C}", "Sum:", parameterBudget.Value);  
   // If credits of course is lower than the certain value, the method will add the credits.  
   static void AddCredits(String connectionString, Int32 creditsLow) {  
      String commandText = "Update [MySchool].[dbo].[Course] Set Credits=Credits+1 Where Credits<@Credits";  
      SqlParameter parameterCredits = new SqlParameter("@Credits", creditsLow);  
      Int32 rows = SqlHelper.ExecuteNonQuery(connectionString, commandText, CommandType.Text, parameterCredits);  
      Console.WriteLine("{0} row{1} {2} updated.", rows, rows > 1 ? "s" : null, rows > 1 ? "are" : "is");  
	创建实例 SqlCommand 时，读/写属性将设置为其初始值。 有关这些值的列表，请参阅 SqlCommand 构造函数。
SqlCommand在SQL Server数据库中执行命令的方法如下：
BeginExecuteNonQuery
启动由此SqlCommand描述的 Transact-SQL 语句或存储过程的异步执行，通常执行 INSERT、DELETE、UPDATE 和 SET 语句等命令。 要调用的每个调用都必须与完成操作的调用BeginExecuteNonQueryEndExecuteNonQuery配对，通常位于单独的线程上。
BeginExecuteReader
启动由此描述SqlCommand的 Transact-SQL 语句或存储过程的异步执行，并从服务器检索一个或多个结果集。 要调用的每个调用都必须与完成操作的调用BeginExecuteReaderEndExecuteReader配对，通常位于单独的线程上。
BeginExecuteXmlReader
启动此 SqlCommand 描述的 Transact-SQL 语句或存储过程的异步执行。 要调用的每个调用都必须与调用配对，该调用BeginExecuteXmlReader``EndExecuteXmlReader完成操作（通常位于单独的线程上）并返回对象XmlReader。
ExecuteReader
执行返回行的命令。 为了提高性能，ExecuteReader请使用 Transact-SQL sp_executesql 系统存储过程调用命令。 因此，如果用于执行 Transact-SQL SET 语句等命令，ExecuteReader则可能没有所需的效果。
ExecuteNonQuery
执行 Transact-SQL INSERT、DELETE、UPDATE 和 SET 语句等命令。
ExecuteScalar
检索单个值 (例如，从数据库) 聚合值。
ExecuteXmlReader
将 CommandText 发送到 Connection，并生成一个 XmlReader 对象。
可以重置 CommandText 属性并重复使用 SqlCommand 该对象。 但是，必须先关闭 SqlDataReader 才能执行新的或上一个命令。
如果由执行 a 的方法生成，SqlExceptionSqlCommand则当严重级别为 19 或更少时，该SqlConnection级别将保持打开状态。 当严重级别为 20 或更大时，服务器通常会关闭该 SqlConnection级别。 但是，用户可以重新打开连接并继续操作。
.NET Framework数据提供程序不支持无名称（也称为序号）参数SQL Server。
		  提供程序应执行该方法对 ExecuteReader 重载提供非默认的实现。
默认实现调用同步 ExecuteReader() 方法并返回已完成任务，以便阻止调用线程。 如果传递到已取消的取消标记，则默认实现将返回已取消的任务。 ExecuteReader 引发的异常将通过返回的任务异常属性传递。
该方法可用于请求操作之前接受取消标记。 实现可能会忽略该请求。
		(继承自   DbCommand)
		  一个异步版本 ExecuteNonQuery()，它对其连接对象执行命令，返回受影响的行数。
通过 CancellationToken.None 调用 ExecuteNonQueryAsync(CancellationToken)。
		(继承自   DbCommand)
		  这是 ExecuteNonQuery() 的异步版本。 提供程序应使用合适的实现进行重写。 可选择性忽略取消标记。
默认实现调用同步 ExecuteNonQuery() 方法并返回已完成任务，以便阻止调用线程。 如果传递到已取消的取消标记，则默认实现将返回已取消的任务。  ExecuteNonQuery() 引发的异常将通过任务异常属性传递。
在返回的任务完成前，不要调用 DbCommand 对象的其他方法和属性。
		(继承自   DbCommand)
		  一个异步版本，该版本 ExecuteReader针对其连接执行命令，返回 DbDataReader 可用于访问结果的命令。
通过 CancellationToken.None 调用 ExecuteDbDataReaderAsync(CommandBehavior, CancellationToken)。
		(继承自   DbCommand)
		  一个异步版本，该版本 ExecuteReader针对其连接执行命令，返回 DbDataReader 可用于访问结果的命令。
调用 ExecuteDbDataReaderAsync(CommandBehavior, CancellationToken).
		(继承自   DbCommand)
		  一个异步版本，该版本 ExecuteReader针对其连接执行命令，返回 DbDataReader 可用于访问结果的命令。
调用 ExecuteDbDataReaderAsync(CommandBehavior, CancellationToken).
		(继承自   DbCommand)
		ExecuteReader(CommandBehavior) 的异步版本，它可以将 CommandText 发送到 Connection，并生成 SqlDataReader。
取消标记可用于在命令超时超过前请求放弃操作。  将通过返回的任务对象报告异常。
		  异步版本的 ExecuteScalar()命令执行该命令，并返回第一个返回结果集中第一行的第一列。 将忽略所有其他列、行和结果集。
通过 CancellationToken.None 调用 ExecuteScalarAsync(CancellationToken)。
		(继承自   DbCommand)
		  这是 ExecuteScalar() 的异步版本。 提供程序应使用合适的实现进行重写。 可选择性忽略取消标记。
默认实现调用同步 ExecuteScalar() 方法并返回已完成任务，以便阻止调用线程。 如果传递到已取消的取消标记，则默认实现将返回已取消的任务。 ExecuteScalar 引发的异常将通过返回的任务异常属性传递。
在返回的任务完成前，不要调用 DbCommand 对象的其他方法和属性。
		(继承自   DbCommand)