// Update the demographics for a store, which is stored 
    // in an xml column. 
    string commandText = "UPDATE Sales.Store SET Demographics = @demographics "
        + "WHERE CustomerID = @ID;";
 
    using (SqlConnection connection = new SqlConnection(connectionString))
    {
        SqlCommand command = new SqlCommand(commandText, connection);
        command.Parameters.Add("@ID", SqlDbType.Int);
        command.Parameters["@ID"].Value = customerID;
 
        // Use AddWithValue to assign Demographics.
        // SQL Server will implicitly convert strings into XML.
        command.Parameters.AddWithValue("@demographics", demoXml);
 
        try
        {
            connection.Open();
            Int32 rowsAffected = command.ExecuteNonQuery();
            Console.WriteLine("RowsAffected: {0}", rowsAffected);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
    }

------解决方案--------------------
@是个特殊标记，表示后面跟的是sql参数名，既然有了sql参数名，就要添加sql参数值，cmd.Parameters.Add 方法可以添加一个sql参数值，方法里先指定sql参数名也就是@id，再指定类型，同时这个方法返回创建好的 SqlParameter 实例，对这个实例可以赋sql参数具体值，简便写法就可以直接点Value等于什么什么