我们在使用EF Core的时候，很多时候需要在Visual Studio的输出窗口中知道EF Core在后台生成的SQL语句是什么，这个需求可以通过自定义EF Core的ILoggerFactory和ILogger类来实现：

首先定义一个实现了ILogger接口的类EFLogger，主要目的是将EF Core生成的Log信息输出到Visual Studio的输出窗口：

然后定义一个实现了ILoggerFactory接口的类EFLoggerFactory，用于创建上面定义的EFLogger类的实例：

using Microsoft.Extensions.Logging;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;

namespace Model
{
    class EFLogger : ILogger
    {
        protected string categoryName;
        public EFLogger(string categoryName)
        {
            this.categoryName = categoryName;
        }
        public IDisposable BeginScope<TState>(TState state)
        {
            return null;
        }

        public bool IsEnabled(LogLevel logLevel)
        {
            return true;
        }

        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
        {
            if (logLevel != LogLevel.Information)
            {
                return;
            }
            //通过Debugger.Log方法来将EF Core生成的Log信息输出到Visual Studio的输出窗口
            Debugger.Log(0, categoryName, "=============================== EF Core log started ===============================\r\n");
            Debugger.Log(0, categoryName, formatter(state, exception) + "\r\n");
            Debugger.Log(0, categoryName, "=============================== EF Core log finished ===============================\r\n");
        }
    }

    public class EFLoggerFactory : ILoggerFactory
    {
        public void AddProvider(ILoggerProvider provider)
        {
           
        }

        public ILogger CreateLogger(string categoryName)
        {
            return new EFLogger(categoryName);//创建EFLogger类的实例
        }

        public void Dispose()
        {
           
        }
    }
}

最后在DbContext的OnConfiguring方法中，调用optionsBuilder.UseLoggerFactory来将EFLoggerFactory类的实例注入给EF Core，这样所有DbContext的Log信息，都会由EFLogger类输出到Visual Studio的输出窗口了。

特别要注意的是： static readonly EFLoggerFactory loggerFactory = new EFLoggerFactory();  这里一定要用静态类

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            //配置数据库连接地址--测试库
            if (string.IsNullOrWhiteSpace(SomConfig.OMSConfig))
            {
                string str = System.Environment.CurrentDirectory;
                FileStream file = new FileStream(str + "\\Config.json", FileMode.Open);
                byte[] byData = new byte[(int)file.Length];//构建一个缓冲池
                file.Seek(0, SeekOrigin.Begin);
                file.Read(byData, 0, (int)file.Length);
                var HaveRead = System.Text.Encoding.UTF8.GetString(byData).Substring(1);
                var array = JArray.Parse(HaveRead)[0];
                SomConfig.DataBase = array.Value<string>("DataBase");
                SomConfig.DataBaseConfig();
                SomConfig.Port = array.Value<string>("Port");
                file.Close();
            }
            if (!optionsBuilder.IsConfigured)
            {
                optionsBuilder.UseLoggerFactory(loggerFactory);
            }
            optionsBuilder.UseSqlServer(SomConfig.OMSConfig);
        }