C# 在C中连接到SQL Server

我是一个C语言的初学者。我正在尝试开发一个连接到数据库的应用程序，并执行诸如插入、删除、更新和获取之类的典型操作

数据库连接出错。我正在使用SQL Server 2012，在那里我创建了一个名为company的数据库

这是我的代码：

namespace DAL
{
    public class DAL
    {
        public const string CADENA_CONEXION = "Data Source=localhost;" +
            "Initial Catalog=Company" +
            "Integrated Security=false" +
            "UID=root PWD=root";
        public SqlConnection con;
        public SqlCommand command;

    public DAL()
    {
        con = new SqlConnection();
        con.ConnectionString = CADENA_CONEXION;
    }

    public Boolean addEmployee(Employee emp)
    {
        try
        {
            /*String sqlInsertString = "INSERT INTO Employee (FirstName, LastName, ID, " + 
           "Designation) VALUES ("+e.firstName+","+ e.lastName+","+e.empCode+","+e.designation+")";*/
            string sqlInsertString =
                "INSERT INTO Employee (FirstName, LastName, ID, " +
                "Designation) VALUES (@firstName, @lastName, @ID, @designation)";
            command = new SqlCommand();
            command.Connection.Open();
            command.CommandText = sqlInsertString;

            SqlParameter firstNameparam = new SqlParameter("@firstName", emp.FirstName);
            SqlParameter lastNameparam = new SqlParameter("@lastName", emp.LastName);
            SqlParameter IDparam = new SqlParameter("@ID", emp.EmpCode);
            SqlParameter designationParam = new SqlParameter("@designation", emp.Designation);

            command.Parameters.AddRange(new SqlParameter[]{
            firstNameparam,lastNameparam,IDparam,designationParam});

            command.ExecuteNonQuery();
            command.Connection.Close();
            return true;
        }
        catch (Exception ex)
        {
            return false;
            throw;
        }

        return true;
    }
}

错误是什么？我在这一行遇到一个例外：

command.Connection.Open();

提前谢谢

SqlConnection con = new SqlConnection("Your Connection String Goes here");

您应该像这样将连接分配给SqlCommand对象

SqlCommand command = new SqlCommand();
command.Connection = con;

或

执行命令的一些重要步骤

1：创建SqlConnection对象并为该对象分配连接字符串

或

2：创建SqlCommand对象，并将您的查询和连接字符串分配给该对象

或

您还可以指定CommandType

command.CommandType =CommandType.Text; 
/* if you are executing storedprocedure CommandType Will be
  => CommandType.StoredProcedure; */

然后您可以像这样执行命令

 try 
 {
  con.Open();
  int TotalRowsAffected = command.ExecuteNonQuery();
 }
 catch(Exeception ex)
 {
   MessageBox.Show(ex.Message);
 }
 finaly
 {
  con.Close();
 } 

---

不初始化sqlcommand连接，初始化方式为：

command.Connection=con;

这是您的完整代码：

namespace DAL
{
    public class DAL
    {
        public const string CADENA_CONEXION = "Data Source=localhost;" +
            "Initial Catalog=Company" +
            "Integrated Security=false" +
            "UID=root PWD=root";
        public SqlConnection con;
        public SqlCommand command;

    public DAL()
    {
        con = new SqlConnection();
        con.ConnectionString = CADENA_CONEXION;
    }

    public Boolean addEmployee(Employee emp)
    {
        try
        {
            /*String sqlInsertString = "INSERT INTO Employee (FirstName, LastName, ID, " + 
           "Designation) VALUES ("+e.firstName+","+ e.lastName+","+e.empCode+","+e.designation+")";*/
            string sqlInsertString =
                "INSERT INTO Employee (FirstName, LastName, ID, " +
                "Designation) VALUES (@firstName, @lastName, @ID, @designation)";
            command = new SqlCommand();
            command.Connection=con;
            command.Connection.Open();
            command.CommandText = sqlInsertString;

            SqlParameter firstNameparam = new SqlParameter("@firstName", emp.FirstName);
            SqlParameter lastNameparam = new SqlParameter("@lastName", emp.LastName);
            SqlParameter IDparam = new SqlParameter("@ID", emp.EmpCode);
            SqlParameter designationParam = new SqlParameter("@designation", emp.Designation);

            command.Parameters.AddRange(new SqlParameter[]{
            firstNameparam,lastNameparam,IDparam,designationParam});

            command.ExecuteNonQuery();
            command.Connection.Close();
            return true;
        }
        catch (Exception ex)
        {
            return false;
            throw;
        }

        return true;
    }
}

---

仅供参考：确保数据库连接关闭的try-finally块的替代方法是使用using语句，例如：

using (SqlConnection connection = new SqlConnection(
       connectionString))
{
    try
    {
        SqlCommand command = new SqlCommand(queryString, connection);
        command.Connection.Open();
        command.ExecuteNonQuery();
    }
    catch (InvalidOperationException)
    {
        //log and/or rethrow or ignore
    }
    catch (SqlException)
    {
        //log and/or rethrow or ignore
    }
    catch (ArgumentException)
    {
        //log and/or rethrow or ignore
    }
}

---

请参阅此处SqlCommand类的MSDN文档，并查找用于执行INSERT、UPDATE和DELETE语句的方法。然后查找可用于执行返回单个值的SELECT语句的方法。您可以使用为返回多列的SELECT语句返回SqlDataReader。

异常消息是什么？当我看到您的连接字符串时，就好像您想连接到Mysql数据库而不是SQL Server数据库，您能给出更多说明吗先打开连接，然后创建命令并将连接分配给命令！！command.Connection.Open将失败并出现NullReferenceException，因为您尚未将SqlConnection对象分配给该命令的Connection属性，但您正在尝试对其调用Open。您编写的所有更改我都已完成。现在，我得到这个异常消息：用户的init会话错误=“”使用这个连接字符串：public const string CADENA_CONEXION=Data Source=localhost；+初始目录=公司+集成安全=真；我已修改了连接字符串，但收到了相同的消息例外请检查连接字符串中的主机名例如：sql server管理中的FATHI-PC是的，这方面有错误。我解决了，但问题是一样的。我也有同样的例外。

command.CommandType =CommandType.Text; 
/* if you are executing storedprocedure CommandType Will be
  => CommandType.StoredProcedure; */

 try 
 {
  con.Open();
  int TotalRowsAffected = command.ExecuteNonQuery();
 }
 catch(Exeception ex)
 {
   MessageBox.Show(ex.Message);
 }
 finaly
 {
  con.Close();
 } 

command.Connection=con;

namespace DAL
{
    public class DAL
    {
        public const string CADENA_CONEXION = "Data Source=localhost;" +
            "Initial Catalog=Company" +
            "Integrated Security=false" +
            "UID=root PWD=root";
        public SqlConnection con;
        public SqlCommand command;

    public DAL()
    {
        con = new SqlConnection();
        con.ConnectionString = CADENA_CONEXION;
    }

    public Boolean addEmployee(Employee emp)
    {
        try
        {
            /*String sqlInsertString = "INSERT INTO Employee (FirstName, LastName, ID, " + 
           "Designation) VALUES ("+e.firstName+","+ e.lastName+","+e.empCode+","+e.designation+")";*/
            string sqlInsertString =
                "INSERT INTO Employee (FirstName, LastName, ID, " +
                "Designation) VALUES (@firstName, @lastName, @ID, @designation)";
            command = new SqlCommand();
            command.Connection=con;
            command.Connection.Open();
            command.CommandText = sqlInsertString;

            SqlParameter firstNameparam = new SqlParameter("@firstName", emp.FirstName);
            SqlParameter lastNameparam = new SqlParameter("@lastName", emp.LastName);
            SqlParameter IDparam = new SqlParameter("@ID", emp.EmpCode);
            SqlParameter designationParam = new SqlParameter("@designation", emp.Designation);

            command.Parameters.AddRange(new SqlParameter[]{
            firstNameparam,lastNameparam,IDparam,designationParam});

            command.ExecuteNonQuery();
            command.Connection.Close();
            return true;
        }
        catch (Exception ex)
        {
            return false;
            throw;
        }

        return true;
    }
}

using (SqlConnection connection = new SqlConnection(
       connectionString))
{
    try
    {
        SqlCommand command = new SqlCommand(queryString, connection);
        command.Connection.Open();
        command.ExecuteNonQuery();
    }
    catch (InvalidOperationException)
    {
        //log and/or rethrow or ignore
    }
    catch (SqlException)
    {
        //log and/or rethrow or ignore
    }
    catch (ArgumentException)
    {
        //log and/or rethrow or ignore
    }
}