一個簡單的LINQ查詢

Question

這是情況：我有大量的數據存儲在'mysamples'作爲下面的數據結構。我需要做一些關於數據的查詢和報告工作，而我是Linq的新手。首先，我需要編寫一個Linq腳本來提取所有記錄的每月年度報告。它將總結'mysamples'中所有樣本的所有樣本點（當年，按月分類）的'值'。我可以很容易地在vb.net代碼中做到這一點，但是我意識到Linq是適合這種應用的正確工具。任何人都可以給我一些開始。謝謝。

Public Structure sample 
    Public Property id As String 
    Public datapoints As List(Of sampledatapoint) 
End Structure 

Public Structure sampledatapoint 
    Public time As DateTime 
    Public value As Decimal 
End Structure 

Public mysamples As New List(Of sample) 

Answer 1

Dim sampleQ1 as New sample() With {.id ="2016Q1", .datapoints = New List(Of sampledatapoint)()} 
    sampleQ1.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 1, 1), .value = 1000}) 
    sampleQ1.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 1, 10), .value = 2000}) 
    sampleQ1.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 2, 1), .value = 500}) 
    sampleQ1.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 3, 7), .value = 750}) 

    Dim sampleQ2 as New sample() With {.id ="2016Q2", .datapoints = New List(Of sampledatapoint)()} 
    sampleQ2.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 4, 4), .value = 800}) 
    sampleQ2.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2016, 4, 5), .value = 150}) 

    Dim sampleQ4 as New sample() With {.id ="2015Q4", .datapoints = New List(Of sampledatapoint)()} 
    sampleQ4.datapoints.Add(New sampledatapoint() With {.time = New DateTime(2015, 10, 20), .value = 666}) 

    Dim mysamples As New List(Of sample) 
    mysamples.Add((sampleQ1)) 
    mysamples.Add((sampleQ2)) 
    mysamples.Add((sampleQ4)) 

    Dim q = From sdp In mysamples.SelectMany(Function(x) x.datapoints).Where(Function(x) x.time.Year = DateTime.Now.Year) 
      Group By grp = sdp.time.Month Into Group 
      Select New With {.Month = grp, .Sum = Group.Sum(Function(x) x.value)} 

    For Each monthlySum In q 
     Console.WriteLine("Month: {0}, Sum: {1}", monthlySum.Month, monthlySum.Sum) 
    Next 

    'Month: 1, Sum: 3000 
    'Month: 2, Sum: 500 
    'Month: 3, Sum: 750 
    'Month: 4, Sum: 950 

SelectMany的伎倆。它將所有樣本的所有抽樣點平展成一個列表。 Group By保持不變。

此外：是否有任何理由爲什麼您使用struct而不是class？我建議至少爲sample使用一個類，因爲它既不代表單個值也不包含值類型。