c# - 使用 Linq 每次迭代选择多个项目？

Question

每次迭代，此查询都会为每个填充的地址创建一个 EmailRecipient。不经过多次迭代可以吗？

var addedRecipients = (from oldRecip in oldEmailRecipients
                        where !string.IsNullOrWhiteSpace(oldRecip.EmailAddress1)
                        select new EmailRecipient
                            {
                                UserName = oldRecip.UserName,
                                EmailAddress = oldRecip.EmailAddress1
                            }
                ).Union(from oldRecip in oldEmailRecipients
                        where !string.IsNullOrWhiteSpace(oldRecip.EmailAddress2)
                        select new EmailRecipient
                            {
                                UserName = oldRecip.UserName,
                                EmailAddress = oldRecip.EmailAddress2
                            });

Answer 1

您可以使用 SelectMany扩展方法:

var addedRecipients = oldEmailRecipients.SelectMany(e=>
                                         {
                                          var result= new List<EmailRecipient>();
                                          if(!string.IsNullOrWhiteSpace(e.EmailAddress1))
                                          {
                                             result.Add(new EmailRecipient
                                                            {
                                                              UserName = e.UserName,
                                                              EmailAddress = e.EmailAddress1
                                                            });
                                          }
                                          if(!string.IsNullOrWhiteSpace(e.EmailAddress2))
                                          {
                                             result.Add(new EmailRecipient
                                                            {
                                                                UserName = e.UserName,
                                                                EmailAddress = e.EmailAddress2
                                                            });
                                          }
                                          return result;
                                         });

更新

我上面展示的解决方案只适用于 Linq to Objects。您的评论表明您正在使用 EF。一个简单的解决方案是在 SelectMany 之前调用 AsEnumerable 方法以切换到 Linq to Objects，但如果您不先过滤收件人，这可能会损害您的性能。

另一种解决方案可能是在调用 SelectMany 之前仅从您的服务器中选择您首先需要的数据，以便在这种情况下不加载您不需要的其他列:

...Where(...)
  .Select(r=>new{UserName=r.UserName,
                 EmailAddress1=r.EmailAddress1,
                 EmailAddress2=r.EmailAddress2 })
  .AsEnumerable()
  .SelectMany(...);