C# 可以把数据转换成字典处理

var customers = await _db.Db.Queryable<Customer>()
    .Where(a => projectNumberIds.Contains(a.Id))
    .Select(a => new { a.Id, a.Name }).ToListAsync();

var customersById = customers.ToDictionary(c => c.Id, c => c.Name);

foreach (var code in pCodes)
{
    ProductStockPrintOutputDto dto = new();
    dto.ApplyTime = productStock.CreationTime;
    dto.MesCode = order.MesCode;
    dto.BatchNumber = order.BatchNumber;

    //从字典获取
    if (int.TryParse(order.ProjectNumber, out int pnId) &&
        customersById.TryGetValue(pnId, out string projectName))
        dto.ProjectNumber = projectName;

    result.Add(dto);
}