实现读写分离可以使用AOP（面向切面编程）思想，通过划分事务的读写属性，将其分别交由读写数据库实例来处理。下面是实现读写分离的代码示例：

```
using  System;
using  System.Linq;
using  System.Transactions;
using  System.Collections.Generic;
using  Microsoft.EntityFrameworkCore;
using  Microsoft.EntityFrameworkCore.Diagnostics;

namespace  DataAccessLayer
{
        public  class  MainDbContext  :  DbContext
        {
                private  readonly  string  _readConnectionString;
                private  readonly  string  _writeConnectionString;
                private  readonly  List<string>  _readEntityNames;
 
                public  MainDbContext(
                        string  readConnectionString,
                        string  writeConnectionString,
                        List<string>  readEntityNames)
                {
                        _readConnectionString  =  readConnectionString;
                        _writeConnectionString  =  writeConnectionString;
                        _readEntityNames  =  readEntityNames;
                }
 
                protected  override  void  OnConfiguring(DbContextOptionsBuilder  optionsBuilder)
                {
                        optionsBuilder.ConfigureWarnings(
                                warnings  =>  warnings.Throw(CoreEventId.FirstWithoutOrderByAndFilterWarning));
 
                        //  读写分离
                        optionsBuilder.UseSqlServer(_writeConnectionString)
                                .EnableSensitiveDataLogging();
                        base.OnConfiguring(optionsBuilder);
                }
 
                protected  override  void  OnModelCreating(ModelBuilder  modelBuilder)
                {
                        //  全部从主库读写
                        modelBuilder.Model.SetReadOnly(false);
                        base.OnModelCreating(modelBuilder);
                }
 
                public  override  int  SaveChanges()
                {
                        //  事务范围内打开连接
                        using  (var  transactionScope  =  new  TransactionScope(TransactionScopeOption.Required,
                                new  TransactionOptions  {  IsolationLevel  =  IsolationLevel.ReadUncommitted,  Timeout  =  TimeSpan.FromSeconds(30)  }))
                        {
                                //  划分事务
                                var  writeEntities  =  ChangeTracker.Entries()
                                        .Where(e  =>  e.State  ==  EntityState.Added  ||  e.State  ==  EntityState.Modified  ||  e.State  ==  EntityState.Deleted)
                                        .Select(e  =>  e.Entity.GetType().Name).Distinct().Except(_readEntityNames)
                                        .ToList();
                                var  readEntities  =  ChangeTracker.Entries()
                                        .Where(e  =>  e.State  ==  EntityState.Unchanged)
                                        .Select(e  =>  e.Entity.GetType().Name).Distinct().Intersect(_readEntityNames)
                                        .ToList();
 
                                //  读写分离处理
                                bool  isRead  =  false;
                                var  connection  =  this.Database.GetDbConnection();
                                connection.Open();
 
                                foreach  (var  entity  in  writeEntities)
                                {
                                        this.Database.UseTransaction(null);
                                        isRead  =  false;
                                }
 
                                foreach  (var  entity  in  readEntities)
                                {
                                        this.Database.UseTransaction(Transaction.Current);
                                        isRead  =  true;
                                }
 
                                //提交事务
                                var  result  =  base.SaveChanges();
                                transactionScope.Complete();
                                return  result;
                        }
                }
        }
}
```

在代码中，MainDbContext继承自DbContext类，包含了读、写两个连接字符串，以及需要读取的表名的列表。在OnConfiguring()方法中，通过调用UseSqlServer()方法来配置写连接字符串。在SaveChanges()方法中，通过划分事务的读写属性，将其分别交由读写数据库实例来处理。这样就实现了基于AOP思想的读写分离。