紀錄一下目前解法( EF Core 3.1 適用，其他版本尚未試過 )

1. 實作 IDbContextTransactionManager：Begin/Commit 時計算層數，最外層才真的 Commit

   
public class NestedTransactionManager : IDbContextTransactionManager
{
    readonly ISqlServerConnection _sqlServerConnection;

    public NestedTransactionManager(ISqlServerConnection sqlServerConnection)
    {
        // Dependency inject ISqlServerConnection, ISqlServerConnection is original IDbContextTransactionManager in EF Core 3.1 .
        _sqlServerConnection = sqlServerConnection;
    }

    int Layer = 0;

    public IDbContextTransaction CurrentTransaction
        => _sqlServerConnection.CurrentTransaction;

    public IDbContextTransaction BeginTransaction()
    {
        if (Layer++ == 0)
            _sqlServerConnection.BeginTransaction();
        return new NestedTransation(this, Layer);
    }

    public async Task<IDbContextTransaction> BeginTransactionAsync(CancellationToken cancellationToken = default)
    {
        if (Layer++ == 0)
            await _sqlServerConnection.BeginTransactionAsync(cancellationToken);
        return new NestedTransation(this, Layer);
    }

    public void CommitTransaction()
    {
        if (Layer-- <= 1)
            _sqlServerConnection.CommitTransaction();
    }

    public Task CommitTransactionAsync(CancellationToken cancellationToken = default)
        => Layer-- <= 1
        ? _sqlServerConnection.CurrentTransaction.CommitAsync(cancellationToken)
        : Task.CompletedTask;
        
    public void ResetState()
        => _sqlServerConnection.ResetState();

    public Task ResetStateAsync(CancellationToken cancellationToken = default)
        => _sqlServerConnection.ResetStateAsync(cancellationToken);

    public void RollbackTransaction()
        => _sqlServerConnection.RollbackTransaction();
}

class NestedTransation : IDbContextTransaction
{
    readonly NestedTransactionManager _manager;

    readonly int _layer;

    public NestedTransation(NestedTransactionManager manager, int layer)
    {
        _manager = manager;
        _layer = layer;
    }

    bool Commited => _layer > _manager.Layer;

    public Guid TransactionId
        => Transaction.TransactionId;

    IDbContextTransaction Transaction
        => _manager.CurrentTransaction;

    public void Commit()
        => _manager.CommitTransaction();

    public Task CommitAsync(CancellationToken cancellationToken = default)
        => _manager.CommitTransactionAsync(cancellationToken);

    public void Dispose()
    {
        if (!Commited && Transaction != null)
            Transaction.Dispose();
    }

    public ValueTask DisposeAsync()
        => !Commited && Transaction != null
        ? Transaction.DisposeAsync()
        : default;

    public void Rollback()
        => Transaction.Rollback();

    public Task RollbackAsync(CancellationToken cancellationToken = default)
        => Transaction.RollbackAsync(cancellationToken);
}	

2. Startup 取代 Service

services.AddDbContext<AppDbContext>(options =>
{
    ...
    options.ReplaceService<IDbContextTransactionManager, NestedTransactionManager>();
});