是否可以组合这两个linq查询

虽然大多数其他答案都在向您展示如何使用相同的代码在EPL和EPA之间切换查询，但我相信您要问的是如何在单个查询中获得这两个值。

static IEnumerable<BuyerEarning> GetBuyerEPA()
{
    var collectTo = DateTime.Now.AddDays(-1).DayEnd();
    var collectFrom = collectTo.AddDays(-29).DayStart();
    using (var uow = new UnitOfWork(ConnectionString.PaydayLenders))
    {
        var r = new Repository<MatchHistory>(uow.Context);
        return r.Find()
            .Where(x =>
                x.CreatedOn <= collectTo &&
                x.CreatedOn >= collectFrom)
            .GroupBy(x => new
            {
                x.BuyerId,
                x.TreeId,
                x.TierId
            })
            .Select(x => new BuyerEarning(
                x.Key.BuyerId,
                x.Key.TreeId,
                x.Key.TierId,
                x.Average(y => y.Commission), //EPL
                x.Where(y => y.ResultTypeId == (int)MatchResultType.Accepted)
                    .Average(y => y.Commission)) //EPA
            .ToList();
    }
}

您可以创建以谓词为参数的辅助方法：

static IEnumerable<BuyerEarning> GetXXXX(Func<MatchHistory, bool> predicate = null)
{
    var collectTo = DateTime.Now.AddDays(-1).DayEnd();
    var collectFrom = collectTo.AddDays(-29).DayStart();
    using (var uow = new UnitOfWork(ConnectionString.PaydayLenders))
    {
        var r = new Repository<MatchHistory>(uow.Context);
        var filtered = r.Find()
                        .Where(x.CreatedOn <= collectTo && x.CreatedOn >= collectFrom);
        if(predicate != null)
            filtered = filtered.Where(predicate);
        return filtered 
            .GroupBy(x => new
            {
                x.BuyerId,
                x.TreeId,
                x.TierId
            })
            .ToList()
            .Select(x => new BuyerEarning(
                x.Key.BuyerId,
                x.Key.TreeId,
                x.Key.TierId,
                x.Average(y => y.Commission)))
            .ToList();
    }
}

然后在每个原始方法中使用它：

static IEnumerable<BuyerEarning> GetBuyerEPA()
{
    return GetXXXX(x => x.ResultTypeId == (int)MatchResultType.Accepted);
}
static IEnumerable<BuyerEarning> GetBuyerEPL()
{
    return GetXXXX();
}

Btw，为什么在最终投影之前使用ToList()？它使得Average计算由应用程序（使用LINQ to Objects）执行，这比由SQL Server执行的效率低得多。

为什么不重构Where子句并将其作为变量传入？

var epaFilter = new Func<MatchHistory, bool>(x => x.ResultTypeId == (int)MatchResultType.Accepted &&      x.CreatedOn <= collectTo &&                    x.CreatedOn >= collectFrom);
var eplFilter = new Func<MatchHistory, bool>(x => x.CreatedOn <= collectTo && x.CreatedOn >= collectFrom);        
private static IEnumerable<MatchHistory> GetBuyerByFilter(Func<MatchHistory,Boolean> filter)
{
    var collectTo = DateTime.Now.AddDays(-1).DayEnd();
    var collectFrom = collectTo.AddDays(-29).DayStart();
    using (var uow = new UnitOfWork(ConnectionString.PaydayLenders))
    {
        var r = new Repository<MatchHistory>(uow.Context);
        return r.Find()
            .Where(filter)
            .GroupBy(x => new
            {
                x.BuyerId,
                x.TreeId,
                x.TierId
            })
            .ToList()
            .Select(x => new BuyerEarning(
                x.Key.BuyerId,
                x.Key.TreeId,
                x.Key.TierId,
                x.Average(y => y.Commission)))
            .ToList();
    }
}

看起来你想要这个：

static IEnumerable<BuyerEarning> GetBuyer(bool acceptedOnly)
{
    var collectTo = DateTime.Now.AddDays(-1).DayEnd();
    var collectFrom = collectTo.AddDays(-29).DayStart();
    using (var uow = new UnitOfWork(ConnectionString.PaydayLenders))
    {
        var r = new Repository<MatchHistory>(uow.Context);
        IQueryable<MatchHistory> results = r.Find()
            .Where(x =>
                x.CreatedOn <= collectTo &&
                x.CreatedOn >= collectFrom);
        if (acceptedOnly)
        {
            results = results
                .Where(x => x.ResultTypeId == (int)MatchResultType.Accepted);
        }
        return results
            .GroupBy(x => new
            {
                x.BuyerId,
                x.TreeId,
                x.TierId
            })
            .ToList()
            .Select(x => new BuyerEarning(
                x.Key.BuyerId,
                x.Key.TreeId,
                x.Key.TierId,
                x.Average(y => y.Commission)))
            .ToList();
    }
}