本文基于efcore8.0，参考文档。
假设需要重写efcore或者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

public class MyMethodCallTranslator:IMethodCallTranslator
{
    private readonly SqlServerSqlExpressionFactory _sqlExpressionFactory;
    public MyMethodCallTranslator(SqlServerSqlExpressionFactory sqlExpressionFactory)
    {
        _sqlExpressionFactory=sqlExpressionFactory
    }
    public virtual SqlExpression Translate(SqlExpression instance, MethodInfo method, IReadOnlyList<SqlExpression> arguments,IDiagnosticsLogger<DbLoggerCategory.Query> logger)
    {
        //你需要的转换代码，并返回你的SqlExpression
        return null;
    }
}
public class MyMethodCallTranslatorPlugin:IMethodCallTranslatorPlugin
{
    public IEnumerable<IMethodCallTranslator> Translators{get;}
    //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
    public MyMethodCallTranslatorPlugin(ISqlExpressionFactory sqlExpressionFactory)
    {
        Translators=new List<IMethodCallTranslator>()
        {
            new MyMethodCallTranslator((SqlServerSqlExpressionFactory)sqlExpressionFactory)
        }
    }
}
public class MyDbContextOptionsExtension:IDbContextOptionsExtension
{
    private DbContextOptionsExtensionInfo? _info;
    public virtual void ApplyServices(IServiceCollection services)
    {
        //坑2：必须用new EntityFrameworkRelationalServicesBuilder(services)再添加服务，不能直接用services添加
        new EntityFrameworkRelationalServicesBuilder(services)
            .TryAdd<IMethodCallTranslatorPlugin, MyMethodCallTranslatorPlugin>();
    }
    public virtual DbContextOptionsExtensionInfo Info
        => _info ??= new ExtensionInfo(this);
    public virtual void Validate(IDbContextOptions options)
    {
    }
    private sealed class ExtensionInfo(IDbContextOptionsExtension extension) : DbContextOptionsExtensionInfo(extension)
    {
        public override bool IsDatabaseProvider => false;

        public override int GetServiceProviderHashCode() => 0;

        public override bool ShouldUseSameServiceProvider(DbContextOptionsExtensionInfo other)
            => other is ExtensionInfo;

        public override void PopulateDebugInfo(IDictionary<string, string> debugInfo)
        {
        }

        public override string LogFragment => "";
    }
}
public static class MySqlServerDbContextOptionsBuilder Extensions
{
    //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
    //关键是SqlServerDbContextOptionsBuilder不是从DbContextOptionsBuilder继承的，看名称似乎是继承的，其实不是
    public static SqlServerDbContextOptionsBuilder Use...(this SqlServerDbContextOptionsBuilder optionsBuilder)
    {
        var coreOptionsBuilder = ((IRelationalDbContextOptionsBuilderInfrastructure)optionsBuilder).OptionsBuilder;

        var extension = coreOptionsBuilder.Options.FindExtension<MyDbContextOptionsExtension>()
            ?? new MyDbContextOptionsExtension();

        ((IDbContextOptionsBuilderInfrastructure)coreOptionsBuilder).AddOrUpdateExtension(extension);

        return optionsBuilder;
    }
}

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继承的，看名称似乎是继承的，其实不是

作者：Rick Carter
出处：http://pains.cnblogs.com/
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