关于EFCore插件API使用中踩过的坑

本文基于efcore8.0，参考文档。
假设须要重写efcore大概特定数据库的方法的SQL转换，按上面文档及其示例代码，本例中我们大概就是提供下面几个接口的实现。

• IMethodCallTranslator
  
• IMethodCallTranslatorPlugin
  
• IDbContextOptionsExtension
  
• 以及特定数据库的DbContextOptionsBuilder（如SqlServerDbContextOptionsBuilder）扩展方法
  

后面再说说踩过的坑（总结）！
示例

1. public class MyMethodCallTranslator:IMethodCallTranslator
2. {
3.     private readonly SqlServerSqlExpressionFactory _sqlExpressionFactory;
4.     public MyMethodCallTranslator(SqlServerSqlExpressionFactory sqlExpressionFactory)
5.     {
6.         _sqlExpressionFactory=sqlExpressionFactory
7.     }
8.     public virtual SqlExpression Translate(SqlExpression instance, MethodInfo method, IReadOnlyList<SqlExpression> arguments,IDiagnosticsLogger<DbLoggerCategory.Query> logger)
9.     {
10.         //你需要的转换代码，并返回你的SqlExpression
11.         return null;
12.     }
13. }
14. public class MyMethodCallTranslatorPlugin:IMethodCallTranslatorPlugin
15. {
16.     public IEnumerable<IMethodCallTranslator> Translators{get;}
17.     //坑1：这里注入时不能用SqlServerSqlExpressionFactory，无法解析SqlServerSqlExpressionFactory，只能ISqlExpressionFactory注入后再强转成SqlServerSqlExpressionFactory
18.     public MyMethodCallTranslatorPlugin(ISqlExpressionFactory sqlExpressionFactory)
19.     {
20.         Translators=new List<IMethodCallTranslator>()
21.         {
22.             new MyMethodCallTranslator((SqlServerSqlExpressionFactory)sqlExpressionFactory)
23.         }
24.     }
25. }
26. public class MyDbContextOptionsExtension:IDbContextOptionsExtension
27. {
28.     private DbContextOptionsExtensionInfo? _info;
29.     public virtual void ApplyServices(IServiceCollection services)
30.     {
31.         //坑2：必须用new EntityFrameworkRelationalServicesBuilder(services)再添加服务，不能直接用services添加
32.         new EntityFrameworkRelationalServicesBuilder(services)
33.             .TryAdd<IMethodCallTranslatorPlugin, MyMethodCallTranslatorPlugin>();
34.     }
35.     public virtual DbContextOptionsExtensionInfo Info
36.         => _info ??= new ExtensionInfo(this);
37.     public virtual void Validate(IDbContextOptions options)
38.     {
39.     }
40.     private sealed class ExtensionInfo(IDbContextOptionsExtension extension) : DbContextOptionsExtensionInfo(extension)
41.     {
42.         public override bool IsDatabaseProvider => false;
44.         public override int GetServiceProviderHashCode() => 0;
46.         public override bool ShouldUseSameServiceProvider(DbContextOptionsExtensionInfo other)
47.             => other is ExtensionInfo;
49.         public override void PopulateDebugInfo(IDictionary<string, string> debugInfo)
50.         {
51.         }
53.         public override string LogFragment => "";
54.     }
55. }
56. public static class MySqlServerDbContextOptionsBuilder Extensions
57. {
58.     //坑3：这里要用具体数据库的...DbContextOptionsBuilder，用DbContextOptionsBuilder也行，但是就无法注入具体的ISqlExpressionFactory
59.     //关键是SqlServerDbContextOptionsBuilder不是从DbContextOptionsBuilder继承的，看名称似乎是继承的，其实不是
60.     public static SqlServerDbContextOptionsBuilder Use...(this SqlServerDbContextOptionsBuilder optionsBuilder)
61.     {
62.         var coreOptionsBuilder = ((IRelationalDbContextOptionsBuilderInfrastructure)optionsBuilder).OptionsBuilder;
64.         var extension = coreOptionsBuilder.Options.FindExtension<MyDbContextOptionsExtension>()
65.             ?? new MyDbContextOptionsExtension();
67.         ((IDbContextOptionsBuilderInfrastructure)coreOptionsBuilder).AddOrUpdateExtension(extension);
69.         return optionsBuilder;
70.     }
71. }

复制代码

总结

1. ISqlExpressionFactory

注入时不能用SqlServerSqlExpressionFactory，无法解析SqlServerSqlExpressionFactory，只能ISqlExpressionFactory注入后再强转成SqlServerSqlExpressionFactory
2. EntityFrameworkRelationalServicesBuilder

必须用new EntityFrameworkRelationalServicesBuilder(services)再添加服务，不能直接用services添加
3. DbContextOptionsBuilder

这里要用具体数据库的...DbContextOptionsBuilder，用DbContextOptionsBuilder也行，但是就无法注入具体的ISqlExpressionFactory
关键是SqlServerDbContextOptionsBuilder不是从DbContextOptionsBuilder继续的，看名称似乎是继续的，其实不是

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