在 C# 中连接到 SQL Server

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

你应该像这样分配对 SqlCommand 对象的连接

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

或

  SqlCommand command = new SqlCommand("YourQuery",con);

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

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

或

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

2：创建 SqlCommand 对象并将命令文本（查询）和连接字符串附加到该对象

SqlCommand command = new SqlCommand("Select * from Products",con);

或

SqlCommand command = new SqlCommand();
command.Connection = con;
command.CommandText ="Select * from Products";

您还可以指定命令类型

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

那么你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();
 } 

仅供参考： 尝试最终阻止的替代方法，确保数据库连接关闭，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
    }
}

请参阅此处的 SqlCommand 类的 MSDN 文档，https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlcommand.aspx，并查找用于执行 INSERT、UPDATE 和 DELETE 语句的 ExecuteNonQuery（） 方法。 然后查找可用于执行返回单个值的 SELECT 语句的 ExecuteScalar（） 方法。 您可以使用 ExecuteReader（） 为 SELECT 语句返回返回多个列的 SqlDataReader。

不初始化sqlcommand连接，初始化方法是：

command.Connection=con;

这是完整的代码：

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=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;
    }
}