c# - 使用 linq 选择匿名委托(delegate)

Question

我知道有办法做到这一点，但我一直在用头撞墙试图找出答案。这很好用:

private GenericRecord CreateGeneric(GenericRecord g, Member m)
{
    g.Member = m;
    return g;
}

public IList<GenericRecord> ReportFromDatabase(DateTime startDate, DateTime endDate)
{
    List<GenericRecord> returnRecords = new List<GenericRecord>();

    returnRecords.AddRange(from r in pjRepository.Records
                           join m in memberRepository.Members on r.SID equals m.MemberId.ToString()
                           where r.TransactionDate >= startDate && r.TransactionDate <= endDate
                           select CreateGeneric((GenericRecord)r, m));

    return returnRecords;
}

但我知道有一种方法可以在没有 CreateGeneric 函数的情况下做到这一点。如何选择内联委托(delegate)函数？

returnRecords.AddRange(from r in pjRepository.Records
                       join m in memberRepository.Members on r.SID equals m.MemberId.ToString()
                       where r.TransactionDate >= startDate && r.TransactionDate <= endDate
                       select (delegate
                       {
                           GenericRecord g = (GenericRecord)r;
                           g.Member = m;
                           return g;
                       }));

这给了我这个异常(exception):

The type of the expression in the select clause is incorrect. Type inference failed in the call to 'Select'.

编辑:又一次失败的尝试

returnRecords.AddRange((from r in pjRepository.Records
                        join m in memberRepository.Members on r.SID equals m.MemberId.ToString()
                        where r.TransactionDate >= startDate && r.TransactionDate <= endDate
                        select new { r, m }).Select(x =>
                        {
                            GenericRecord g = (GenericRecord)x.r;
                            g.Member = x.m;
                            return g;
                        }));

这给了我:

A lambda expression with a statement body cannot be converted to an expression tree.

Answer 1

尝试:

returnRecords.AddRange((from r in pjRepository.Records
                            join m in memberRepository.Members on r.SID equals m.MemberId.ToString()
                            where r.TransactionDate >= startDate && r.TransactionDate <= endDate
                            select new { r, m }).AsEnumerable().Select(x =>
                            {
                                GenericRecord g = (GenericRecord)x.r;
                                g.Member = x.m;
                                return g;
                            }));

关键的区别在于 AsEnumerable() 函数。这需要一个 IQueryable 并返回一个 IEnumerable，它在幕后强制由 Linq 提供程序计算表达式树。这可以防止 Linq 库尝试将第二个 Select 的 lambda 转换为表达式树的一部分(它说不能)；第二个 Select 将对通过评估 IQueryable 表达式树生成的实际 im-memory 集合执行转换。由于无论如何都需要在语句结束之前对查询进行评估(因此所有元素都可以添加到 returnRecords)，因此应该没有显着的性能差异。