【C#9 .NET5】读取数据库，写入xml csv json文件

获取所有类别及其相关产品的查询

创建四个包含序列化类别和产品的文件。

Mian程序：

  //导出类别和产品数据 到 xml  csv   json文件
        static void Main(string[] args)
        {
            WriteLine("Creating four files containing serialized categories and products...");

            using (var db = new Northwind())
            {
                //获取所有类别及其相关产品的查询 a query to get all categories and their related products 
                IQueryable<Category> cats = db.Categories.Include(c => c.Products);

                /*创建四个包含序列化类别和产品的文件。
                 Creating four files containing serialized categories and products...
                 categories-and-products-using-attibutes.xml contains 8,486 bytes.
                 categories-and-products-using-elements.xml contains 16,440 bytes.
                 categories-and-products.csv contains 6,686 bytes.
                 categories-and-products.json contains 14,146 bytes.
                 */
                GenerateXmlFile(cats);
                GenerateXmlFile(cats, useAttributes: false);
                GenerateCsvFile(cats);
                GenerateJsonFile(cats);
            }
        }

写入xml文件

        // we want to easily show the difference between outputting
        // XML using elements or attributes so we will define a 
        // delegate to reference the two different methods.
        //我们希望轻松显示使用元素或属性输出 XML 之间的区别，
        //因此我们将定义一个委托来引用这两种不同的方法。
        private delegate void WriteDataDelegate(string name, string value);

        private static void GenerateXmlFile(IQueryable<Category> cats, bool useAttributes = true)
        {
            string which = useAttributes ? "attibutes" : "elements";

            string xmlFile = $"categories-and-products-using-{which}.xml";

            using (FileStream xmlStream = File.Create(
              Combine(CurrentDirectory, xmlFile)))
            {
                using (XmlWriter xml = XmlWriter.Create(xmlStream,
                  new XmlWriterSettings { Indent = true }))
                {

                    WriteDataDelegate writeMethod;

                    if (useAttributes)
                    {   //在派生类中重写时，写出具有指定本地名称和值的属性。
                        writeMethod = xml.WriteAttributeString;
                    }
                    else // use elements
                    {
                        writeMethod = xml.WriteElementString;//写入具有指定本地名称和值的元素。
                    }

                    xml.WriteStartDocument();
                    xml.WriteStartElement("categories");//根节点

                    foreach (Category c in cats)
                    {
                        xml.WriteStartElement("category");      //类别
                        writeMethod("id", c.CategoryID.ToString());
                        writeMethod("name", c.CategoryName);
                        writeMethod("desc", c.Description);
                        writeMethod("product_count", c.Products.Count.ToString());
                        xml.WriteStartElement("products");

                        foreach (Product p in c.Products)
                        {
                            xml.WriteStartElement("product");

                            writeMethod("id", p.ProductID.ToString());
                            writeMethod("name", p.ProductName);
                            writeMethod("cost", p.Cost.Value.ToString());
                            writeMethod("stock", p.Stock.ToString());
                            writeMethod("discontinued", p.Discontinued.ToString());

                            xml.WriteEndElement(); // </product>
                        }
                        xml.WriteEndElement(); // </products>
                        xml.WriteEndElement(); // </category>       //类别
                    }
                    xml.WriteEndElement(); // </categories>     //结束根节点元素
                }
            }

            WriteLine("{0} contains {1:N0} bytes.",
              arg0: xmlFile,
              arg1: new FileInfo(xmlFile).Length);
        }

写入csv文件：

        //生成CSV文件
        private static void GenerateCsvFile(IQueryable<Category> cats)
        {
            string csvFile = "categories-and-products.csv";

            using (FileStream csvStream = File.Create(Combine(CurrentDirectory, csvFile)))
            {
                using (var csv = new StreamWriter(csvStream))
                {

                    csv.WriteLine("CategoryID,CategoryName,Description,ProductID,ProductName,Cost,Stock,Discontinued");

                    foreach (Category c in cats)
                    {
                        foreach (Product p in c.Products)
                        {
                            csv.Write("{0},\"{1}\",\"{2}\",", // 类别ID，类别名，描述
                              arg0: c.CategoryID.ToString(),
                              arg1: c.CategoryName,
                              arg2: c.Description);

                            csv.Write("{0},\"{1}\",{2},",// 产品id,产品名，单价
                              arg0: p.ProductID.ToString(),
                              arg1: p.ProductName,
                              arg2: p.Cost.Value.ToString());

                            csv.WriteLine("{0},{1}",// 库存 、停产
                              arg0: p.Stock.ToString(),
                              arg1: p.Discontinued.ToString());
                        }
                    }
                }
            }//自动释放文件流

            WriteLine("{0} contains {1:N0} bytes.",
              arg0: csvFile,
              arg1: new FileInfo(csvFile).Length);
        }

写入json文件：

       //生成json格式文件
        private static void GenerateJsonFile(IQueryable<Category> cats)
        {
            string jsonFile = "categories-and-products.json";

            using (FileStream jsonStream = File.Create(Combine(CurrentDirectory, jsonFile)))
            {//使用指定的流和自定义选项初始化 System.Text.Json.Utf8JsonWriter 类的新实例以将输出写入。
                using (var json = new Utf8JsonWriter(jsonStream,
                  new JsonWriterOptions { Indented = true }))//true 格式化 JSON 输出； false 表示没有任何额外的空白。 默认为false。
                {
                    json.WriteStartObject();
                    json.WriteStartArray("categories");//使用指定为字符串的属性名称写入 JSON 数组的开头作为键。

                    foreach (Category c in cats)//遍历类别
                    {
                        json.WriteStartObject();

                        json.WriteNumber("id", c.CategoryID);
                        json.WriteString("name", c.CategoryName);
                        json.WriteString("desc", c.Description);
                        json.WriteNumber("product_count", c.Products.Count);

                        json.WriteStartArray("products");

                        foreach (Product p in c.Products)
                        {
                            json.WriteStartObject();//

                            json.WriteNumber("id", p.ProductID);
                            json.WriteString("name", p.ProductName);
                            json.WriteNumber("cost", p.Cost.Value);
                            json.WriteNumber("stock", p.Stock.Value);
                            json.WriteBoolean("discontinued", p.Discontinued);

                            json.WriteEndObject(); // product
                        }
                        json.WriteEndArray(); // products
                        json.WriteEndObject(); // category
                    }
                    json.WriteEndArray(); // categories
                    json.WriteEndObject();
                }
            }

            WriteLine("{0} contains {1:N0} bytes.",
              arg0: jsonFile,
              arg1: new FileInfo(jsonFile).Length);
        }

附：数据库对应的EF CORE 类：

数据库上下文类：

using Microsoft.EntityFrameworkCore;

namespace Packt.Shared
{
  // this manages the connection to the database
  public class Northwind : DbContext
  {
    // these properties map to tables in the database 
    public DbSet<Category> Categories { get; set; }
    public DbSet<Product> Products { get; set; }

    protected override void OnConfiguring(
      DbContextOptionsBuilder optionsBuilder)
    {
      string path = System.IO.Path.Combine(
        System.Environment.CurrentDirectory, "Northwind.db");
        
      optionsBuilder.UseSqlite($"Filename={path}");
    }

    protected override void OnModelCreating(
      ModelBuilder modelBuilder)
    {
      // example of using Fluent API instead of attributes
      // to limit the length of a category name to under 15
      modelBuilder.Entity<Category>()
        .Property(category => category.CategoryName)
        .IsRequired() // NOT NULL
        .HasMaxLength(15);

      // added to "fix" the decimal support in SQLite
      modelBuilder.Entity<Product>()
        .Property(product => product.Cost)
        .HasConversion<double>();
    }
  }
}

类别-类：

using System.Collections.Generic;
using System.ComponentModel.DataAnnotations.Schema;

namespace Packt.Shared
{
  public class Category
  {
    // these properties map to columns in the database 
    public int CategoryID { get; set; }

    public string CategoryName { get; set; }

    [Column(TypeName = "ntext")]
    public string Description { get; set; }

    // defines a navigation property for related rows
    public virtual ICollection<Product> Products { get; set; }

    public Category()
    {
      // to enable developers to add products to a Category we must
      // initialize the navigation property to an empty list 
      this.Products = new List<Product>();
    }
  }
}

产品类：

using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;

namespace Packt.Shared
{
  public class Product
  {
    public int ProductID { get; set; }

    [Required]
    [StringLength(40)]
    public string ProductName { get; set; }

    [Column("UnitPrice", TypeName = "money")]
    public decimal? Cost { get; set; }

    [Column("UnitsInStock")]
    public short? Stock { get; set; }

    public bool Discontinued { get; set; }

    // these two define the foreign key relationship
    // to the Categories table
    public int CategoryID { get; set; }

    public virtual Category Category { get; set; }
  }
}

数据库：