EntityFramework Core筆記(4)_延遲加載(執行)

EntityFramework Core筆記(4)_延遲加載(執行)_IQueryable跟IEnumerable差異

- 4月 17, 2022

延遲加載(執行):

屬於預設情況，當你查詢

一個集合就延遲，等到真正要使用存取該資料才去DB抓。

預設從DbContext存取的表資料回傳只要還沒去call .ToList() 或是 .ToArray() 等方法

都是回傳IQueryable<T> 都還不會真正跟DB有交握。

優點:

就在於可以降低阻塞，提升性能

需注意:

加載資料不能夠在DbContext被回收之後才運行

延遲加載(執行)-測試程式

using EFCoreDbFirst_ConsoleApp.Models;
using System;
using System.Linq;

namespace EFCoreDbFirst_ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                using (CustomerDbContext context = new CustomerDbContext())
                {
                    //Lazy Loading
                    IQueryable<Commodity> commodities = context.Commodities.Where(item => item.Id == 20001);

                    Console.WriteLine("開始存取");
                    foreach(var item in commodities)
                    {
                        Console.WriteLine(item.Title);
                    }
                }
            }
            catch (Exception ex)
            {
                throw;
            }
        }
    }
}

所以可觀察到跑至foreach存取時才真正觸發底層查詢SQL語句跟DB交握

加載資料不能夠在DbContext被回收之後才運行-測試程式

所以跳脫出using區域後該context 就已經被釋放記憶體了

之後要再存取就會失效。

IQueryable跟IEnumerable差異

IQueryable 跟 IEnumerable 默認都是Lazy Loading

以下是測試程式

using EFCoreDbFirst_ConsoleApp.Models;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace EFCoreDbFirst_ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                //IQueryable 跟 IEnumerable 默認都是Lazy Loading
                IQueryable<Commodity> commodities = null;
                using (CustomerDbContext context = new CustomerDbContext())
                {
                    //Lazy Loading
                    commodities = context.Commodities.Where(item => item.Id == 20001).AsQueryable();
                    Console.WriteLine("透過IQueryable存取");
                    foreach (var item in commodities)
                    {
                        Console.WriteLine(item.Title);
                    }
                }
                IEnumerable<Commodity> commoditiesEnum = null;
                using (CustomerDbContext context = new CustomerDbContext())
                {
                    //Lazy Loading
                    commoditiesEnum = context.Commodities.Where(item => item.Id == 20001).AsEnumerable();
                    Console.WriteLine("透過IEnumerable存取");
                    foreach (var item in commoditiesEnum)
                    {
                        Console.WriteLine(item.Title);
                    }
                }

            }
            catch (Exception ex)
            {
                throw;
            }
        }
    }
}

都是到foreach開始遍歷存取後才去真正撈DB

若有多條件情況時

IQueryable(表達式查詢) : 表達式目錄樹拼裝

無論有多少where拼接都是合併在一起後才丟入DB做查詢

若有多個條件通常建議採用IQueryable

會多包含 IQueryProvider (表達式目錄樹轉換成SQL語句的驅動)

#region 組件 System.Linq.Expressions, Version=4.2.2.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a
// C:\Program Files\dotnet\packs\Microsoft.NETCore.App.Ref\3.1.0\ref\netcoreapp3.1\System.Linq.Expressions.dll
#endregion

using System.Collections;
using System.Linq.Expressions;

namespace System.Linq
{
    //
    // 摘要:
    //     Provides functionality to evaluate queries against a specific data source wherein
    //     the type of the data is not specified.
    public interface IQueryable : IEnumerable
    {
        //
        // 摘要:
        //     Gets the type of the element(s) that are returned when the expression tree associated
        //     with this instance of System.Linq.IQueryable is executed.
        //
        // 傳回:
        //     A System.Type that represents the type of the element(s) that are returned when
        //     the expression tree associated with this object is executed.
        Type ElementType { get; }
        //
        // 摘要:
        //     Gets the expression tree that is associated with the instance of System.Linq.IQueryable.
        //
        // 傳回:
        //     The System.Linq.Expressions.Expression that is associated with this instance
        //     of System.Linq.IQueryable.
        Expression Expression { get; }
        //
        // 摘要:
        //     Gets the query provider that is associated with this data source.
        //
        // 傳回:
        //     The System.Linq.IQueryProvider that is associated with this data source.
        IQueryProvider Provider { get; }
    }
}

IEnumerable(迭代器):

則是在之前的where會直接去撈DB 再之後的則是去內存(緩存)做存取

namespace System.Collections.Generic
{
    //
    // 摘要:
    //     Exposes the enumerator, which supports a simple iteration over a collection of
    //     a specified type.
    //
    // 類型參數:
    //   T:
    //     The type of objects to enumerate.
    public interface IEnumerable<out T> : IEnumerable
    {
        //
        // 摘要:
        //     Returns an enumerator that iterates through the collection.
        //
        // 傳回:
        //     An enumerator that can be used to iterate through the collection.
        IEnumerator<T> GetEnumerator();
    }
}

多條件情況-測試程式

using EFCoreDbFirst_ConsoleApp.Models;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace EFCoreDbFirst_ConsoleApp
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                //IQueryable 跟 IEnumerable 默認都是Lazy Loading
                IQueryable<Commodity> commodities = null;
                using (CustomerDbContext context = new CustomerDbContext())
                {
                    //Lazy Loading
                    commodities = context.Commodities
                                    .Where(item => item.Id == 20001)
                                    .AsQueryable()
                                    .Where(item => item.Title.Contains("test"))
                                    .Where(item => item.Title.Contains("1"));

                    Console.WriteLine("透過IQueryable存取");
                    foreach (var item in commodities)
                    {
                        Console.WriteLine(item.Title);
                    }
                }
                IEnumerable<Commodity> commoditiesEnum = null;
                using (CustomerDbContext context = new CustomerDbContext())
                {
                    //Lazy Loading
                    commoditiesEnum = context.Commodities
                                                .Where(item => item.Id == 20001)
                                                .AsEnumerable()
                                                .Where(item => item.Title.Contains("test"))
                                                .Where(item => item.Title.Contains("1"));
                    Console.WriteLine("透過IEnumerable存取");
                    foreach (var item in commoditiesEnum)
                    {
                        Console.WriteLine(item.Title);
                    }
                }

            }
            catch (Exception ex)
            {
                throw;
            }
        }
    }
}

運行效果可觀察到

IQueryable(表達式查詢)

無論有多少where拼接都是合併在一起後才丟入DB存取的

IEnumerable(迭代器):

則是在AsEnumerable()之前的where會直接去撈DB

再之後的則是去內存(緩存)做存取，而非DB即時最新抓取。

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EntityFramework Core筆記(4)_延遲加載(執行)_IQueryable跟IEnumerable差異

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