Enumerable.SelectMany 方法

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

public:
generic <typename TSource, typename TCollection, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, System::Collections::Generic::IEnumerable<TCollection> ^> ^ collectionSelector, Func<TSource, TCollection, TResult> ^ resultSelector);

public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TCollection,TResult>(this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,System.Collections.Generic.IEnumerable<TCollection>> collectionSelector, Func<TSource,TCollection,TResult> resultSelector);

static member SelectMany : seq<'Source> * Func<'Source, seq<'Collection>> * Func<'Source, 'Collection, 'Result> -> seq<'Result>

<Extension()>
Public Function SelectMany(Of TSource, TCollection, TResult) (source As IEnumerable(Of TSource), collectionSelector As Func(Of TSource, IEnumerable(Of TCollection)), resultSelector As Func(Of TSource, TCollection, TResult)) As IEnumerable(Of TResult)

類型參數

參數

傳回

其IEnumerable<T>元素是對每個元素collectionSelector調用一對多轉換函source數，然後將這些序列元素及其對應的來源元素映射到結果元素的結果。

例外狀況

範例

以下程式碼範例示範如何對 SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 陣列進行一對多投影，並使用結果選擇函數保持來源序列中每個對應元素在最終呼叫 Select的範圍內。

class PetOwner
{
    public string Name { get; set; }
    public List<string> Pets { get; set; }
}

public static void SelectManyEx3()
{
    PetOwner[] petOwners =
        { new PetOwner { Name="Higa",
              Pets = new List<string>{ "Scruffy", "Sam" } },
          new PetOwner { Name="Ashkenazi",
              Pets = new List<string>{ "Walker", "Sugar" } },
          new PetOwner { Name="Price",
              Pets = new List<string>{ "Scratches", "Diesel" } },
          new PetOwner { Name="Hines",
              Pets = new List<string>{ "Dusty" } } };

    // Project the pet owner's name and the pet's name.
    var query =
        petOwners
        .SelectMany(petOwner => petOwner.Pets, (petOwner, petName) => new { petOwner, petName })
        .Where(ownerAndPet => ownerAndPet.petName.StartsWith("S"))
        .Select(ownerAndPet =>
                new
                {
                    Owner = ownerAndPet.petOwner.Name,
                    Pet = ownerAndPet.petName
                }
        );

    // Print the results.
    foreach (var obj in query)
    {
        Console.WriteLine(obj);
    }
}

// This code produces the following output:
//
// {Owner=Higa, Pet=Scruffy}
// {Owner=Higa, Pet=Sam}
// {Owner=Ashkenazi, Pet=Sugar}
// {Owner=Price, Pet=Scratches}

Structure PetOwner
    Public Name As String
    Public Pets() As String
End Structure

Sub SelectManyEx3()
    ' Create an array of PetOwner objects.
    Dim petOwners() As PetOwner =
{New PetOwner With
 {.Name = "Higa", .Pets = New String() {"Scruffy", "Sam"}},
 New PetOwner With
 {.Name = "Ashkenazi", .Pets = New String() {"Walker", "Sugar"}},
 New PetOwner With
 {.Name = "Price", .Pets = New String() {"Scratches", "Diesel"}},
 New PetOwner With
 {.Name = "Hines", .Pets = New String() {"Dusty"}}}

    ' Project an anonymous type that consists of
    ' the owner's name and the pet's name (string).
    Dim query =
petOwners _
.SelectMany(
    Function(petOwner) petOwner.Pets,
    Function(petOwner, petName) New With {petOwner, petName}) _
.Where(Function(ownerAndPet) ownerAndPet.petName.StartsWith("S")) _
.Select(Function(ownerAndPet) _
       New With {.Owner = ownerAndPet.petOwner.Name,
                 .Pet = ownerAndPet.petName
       })

    Dim output As New System.Text.StringBuilder
    For Each obj In query
        output.AppendLine(String.Format("Owner={0}, Pet={1}", obj.Owner, obj.Pet))
    Next

    ' Display the output.
    Console.WriteLine(output.ToString())
End Sub

' This code produces the following output:
'
' Owner=Higa, Pet=Scruffy
' Owner=Higa, Pet=Sam
' Owner=Ashkenazi, Pet=Sugar
' Owner=Price, Pet=Scratches

備註

此方法透過延遲執行實現。 立即回傳值是一個物件，儲存執行動作所需的所有資訊。 此方法所代表的查詢，直到物件被枚舉，無論是直接呼叫其 GetEnumerator 方法，或是使用 C# 中的 foreach，或在 Visual Basic 中使用 For Each，才會執行。

當需要在查詢邏輯中保持 的SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)元素在 的範圍內，以應付呼叫 之後source的查詢邏輯時，此SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)方法非常有用。 請參閱範例章節以了解程式碼範例。 如果 型態TSource的物件與型別TCollection的物件之間存在雙向關係，也就是說，若型別TCollection的物件提供了取得產生它的物件的TSource屬性，則不需要這個過載。SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 相反地，你可以使用SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>)並透過TSource物件導航回物件TCollection。

在查詢表達式語法中，每個 from子句（C#）或 From子句（Visual Basic）後，都會轉換成 SelectMany 的調用。

另請參閱

• from子句（C# 參考）
• From Clause （Visual Basic）

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

public:
generic <typename TSource, typename TCollection, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, System::Collections::Generic::IEnumerable<TCollection> ^> ^ collectionSelector, Func<TSource, TCollection, TResult> ^ resultSelector);

public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TCollection,TResult>(this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,System.Collections.Generic.IEnumerable<TCollection>> collectionSelector, Func<TSource,TCollection,TResult> resultSelector);

static member SelectMany : seq<'Source> * Func<'Source, int, seq<'Collection>> * Func<'Source, 'Collection, 'Result> -> seq<'Result>

<Extension()>
Public Function SelectMany(Of TSource, TCollection, TResult) (source As IEnumerable(Of TSource), collectionSelector As Func(Of TSource, Integer, IEnumerable(Of TCollection)), resultSelector As Func(Of TSource, TCollection, TResult)) As IEnumerable(Of TResult)

類型參數

參數

傳回

其IEnumerable<T>元素是對每個元素collectionSelector調用一對多轉換函source數，然後將這些序列元素及其對應的來源元素映射到結果元素的結果。

例外狀況

備註

此方法透過延遲執行實現。 立即回傳值是一個物件，儲存執行動作所需的所有資訊。 此方法所代表的查詢，直到物件被枚舉，無論是直接呼叫其 GetEnumerator 方法，或是使用 C# 中的 foreach，或在 Visual Basic 中使用 For Each，才會執行。

當需要在查詢邏輯中保持 的SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)元素在 的範圍內，以應付呼叫 之後source的查詢邏輯時，此SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>)方法非常有用。 請參閱範例章節以了解程式碼範例。 如果 型態TSource的物件與型別TCollection的物件之間存在雙向關係，也就是說，若型別TCollection的物件提供了取得產生它的物件的TSource屬性，則不需要這個過載。SelectMany<TSource,TCollection,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TCollection>>, Func<TSource,TCollection,TResult>) 相反地，你可以使用SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>)並透過TSource物件導航回物件TCollection。

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, System::Collections::Generic::IEnumerable<TResult> ^> ^ selector);

public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TResult>(this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,System.Collections.Generic.IEnumerable<TResult>> selector);

static member SelectMany : seq<'Source> * Func<'Source, seq<'Result>> -> seq<'Result>

<Extension()>
Public Function SelectMany(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, IEnumerable(Of TResult))) As IEnumerable(Of TResult)

類型參數

參數

傳回

其 IEnumerable<T> 元素是對輸入序列中每個元素調用一對多轉換函數所產生的結果。

例外狀況

範例

以下程式碼範例示範如何使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 對陣列進行一對多投影。

class PetOwner
{
    public string Name { get; set; }
    public List<String> Pets { get; set; }
}

public static void SelectManyEx1()
{
    PetOwner[] petOwners =
        { new PetOwner { Name="Higa, Sidney",
              Pets = new List<string>{ "Scruffy", "Sam" } },
          new PetOwner { Name="Ashkenazi, Ronen",
              Pets = new List<string>{ "Walker", "Sugar" } },
          new PetOwner { Name="Price, Vernette",
              Pets = new List<string>{ "Scratches", "Diesel" } } };

    // Query using SelectMany().
    IEnumerable<string> query1 = petOwners.SelectMany(petOwner => petOwner.Pets);

    Console.WriteLine("Using SelectMany():");

    // Only one foreach loop is required to iterate
    // through the results since it is a
    // one-dimensional collection.
    foreach (string pet in query1)
    {
        Console.WriteLine(pet);
    }

    // This code shows how to use Select()
    // instead of SelectMany().
    IEnumerable<List<String>> query2 =
        petOwners.Select(petOwner => petOwner.Pets);

    Console.WriteLine("\nUsing Select():");

    // Notice that two foreach loops are required to
    // iterate through the results
    // because the query returns a collection of arrays.
    foreach (List<String> petList in query2)
    {
        foreach (string pet in petList)
        {
            Console.WriteLine(pet);
        }
        Console.WriteLine();
    }
}

/*
 This code produces the following output:

 Using SelectMany():
 Scruffy
 Sam
 Walker
 Sugar
 Scratches
 Diesel

 Using Select():
 Scruffy
 Sam

 Walker
 Sugar

 Scratches
 Diesel
*/

Structure PetOwner
    Public Name As String
    Public Pets() As String
End Structure

Sub SelectManyEx1()
    ' Create an array of PetOwner objects.
    Dim petOwners() As PetOwner =
{New PetOwner With
 {.Name = "Higa, Sidney", .Pets = New String() {"Scruffy", "Sam"}},
 New PetOwner With
 {.Name = "Ashkenazi, Ronen", .Pets = New String() {"Walker", "Sugar"}},
 New PetOwner With
 {.Name = "Price, Vernette", .Pets = New String() {"Scratches", "Diesel"}}}

    ' Call SelectMany() to gather all pets into a "flat" sequence.
    Dim query1 As IEnumerable(Of String) =
petOwners.SelectMany(Function(petOwner) petOwner.Pets)

    Dim output As New System.Text.StringBuilder("Using SelectMany():" & vbCrLf)
    ' Only one foreach loop is required to iterate through
    ' the results because it is a one-dimensional collection.
    For Each pet As String In query1
        output.AppendLine(pet)
    Next

    ' This code demonstrates how to use Select() instead
    ' of SelectMany() to get the same result.
    Dim query2 As IEnumerable(Of String()) =
petOwners.Select(Function(petOwner) petOwner.Pets)
    output.AppendLine(vbCrLf & "Using Select():")
    ' Notice that two foreach loops are required to iterate through
    ' the results because the query returns a collection of arrays.
    For Each petArray() As String In query2
        For Each pet As String In petArray
            output.AppendLine(pet)
        Next
    Next

    ' Display the output.
    Console.WriteLine(output.ToString())
End Sub

' This code produces the following output:
'
' Using SelectMany():
' Scruffy
' Sam
' Walker
' Sugar
' Scratches
' Diesel
'
' Using Select():
' Scruffy
' Sam
' Walker
' Sugar
' Scratches
' Diesel

備註

此方法透過延遲執行實現。 立即回傳值是一個物件，儲存執行動作所需的所有資訊。 此方法所代表的查詢，直到物件被枚舉，無論是直接呼叫其 GetEnumerator 方法，或是使用 C# 中的 foreach，或在 Visual Basic 中使用 For Each，才會執行。

該 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 方法枚舉輸入序列，使用轉換函數將每個元素映射到 IEnumerable<T>，然後枚舉並產生每個此類 IEnumerable<T> 物件的元素。 也就是說，對 的sourceselector每個元素 ，都會被調用，並回傳一串值。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 然後將這個二維集合平面化為一維 IEnumerable<T> ，並返回它。 例如，若查詢使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,IEnumerable<TResult>>) 取得資料庫中每位客戶的訂單（類型為 Order），結果在 C# 中為 IEnumerable<Order>，或在 Visual Basic 中為 IEnumerable(Of Order)。 若查詢改用 Select 取得順序，則順序集合不會合併，結果為 C# 中的 IEnumerable<List<Order>> 或 Visual Basic 中的 IEnumerable(Of List(Of Order))型。

在查詢表達式語法中，每個 from子句（C#）或 From子句（Visual Basic）後，都會轉換成 SelectMany 的調用。

另請參閱

• from子句（C# 參考）
• From Clause （Visual Basic）

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

來源:

SelectMany.cs

public:
generic <typename TSource, typename TResult>
[System::Runtime::CompilerServices::Extension]
 static System::Collections::Generic::IEnumerable<TResult> ^ SelectMany(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, int, System::Collections::Generic::IEnumerable<TResult> ^> ^ selector);

public static System.Collections.Generic.IEnumerable<TResult> SelectMany<TSource,TResult>(this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,int,System.Collections.Generic.IEnumerable<TResult>> selector);

static member SelectMany : seq<'Source> * Func<'Source, int, seq<'Result>> -> seq<'Result>

<Extension()>
Public Function SelectMany(Of TSource, TResult) (source As IEnumerable(Of TSource), selector As Func(Of TSource, Integer, IEnumerable(Of TResult))) As IEnumerable(Of TResult)

類型參數

參數

傳回

其 IEnumerable<T> 元素是對輸入序列中每個元素調用一對多轉換函數所產生的結果。

例外狀況

範例

以下程式碼範例示範如何使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 對陣列進行一對多投影，並使用每個外層元素的索引。

class PetOwner
{
    public string Name { get; set; }
    public List<string> Pets { get; set; }
}

public static void SelectManyEx2()
{
    PetOwner[] petOwners =
        { new PetOwner { Name="Higa, Sidney",
              Pets = new List<string>{ "Scruffy", "Sam" } },
          new PetOwner { Name="Ashkenazi, Ronen",
              Pets = new List<string>{ "Walker", "Sugar" } },
          new PetOwner { Name="Price, Vernette",
              Pets = new List<string>{ "Scratches", "Diesel" } },
          new PetOwner { Name="Hines, Patrick",
              Pets = new List<string>{ "Dusty" } } };

    // Project the items in the array by appending the index
    // of each PetOwner to each pet's name in that petOwner's
    // array of pets.
    IEnumerable<string> query =
        petOwners.SelectMany((petOwner, index) =>
                                 petOwner.Pets.Select(pet => index + pet));

    foreach (string pet in query)
    {
        Console.WriteLine(pet);
    }
}

// This code produces the following output:
//
// 0Scruffy
// 0Sam
// 1Walker
// 1Sugar
// 2Scratches
// 2Diesel
// 3Dusty

Structure PetOwner
    Public Name As String
    Public Pets() As String
End Structure

Sub SelectManyEx2()
    ' Create an array of PetOwner objects.
    Dim petOwners() As PetOwner =
{New PetOwner With
 {.Name = "Higa, Sidney", .Pets = New String() {"Scruffy", "Sam"}},
 New PetOwner With
 {.Name = "Ashkenazi, Ronen", .Pets = New String() {"Walker", "Sugar"}},
 New PetOwner With
 {.Name = "Price, Vernette", .Pets = New String() {"Scratches", "Diesel"}},
 New PetOwner With
 {.Name = "Hines, Patrick", .Pets = New String() {"Dusty"}}}

    ' Project the items in the array by appending the index
    ' of each PetOwner to each pet's name in that petOwner's
    ' array of pets.
    Dim query As IEnumerable(Of String) =
petOwners.SelectMany(Function(petOwner, index) _
                         petOwner.Pets.Select(Function(pet) _
                                                  index.ToString() + pet))

    Dim output As New System.Text.StringBuilder
    For Each pet As String In query
        output.AppendLine(pet)
    Next

    ' Display the output.
    Console.WriteLine(output.ToString())
End Sub

備註

此方法透過延遲執行實現。 立即回傳值是一個物件，儲存執行動作所需的所有資訊。 此方法所代表的查詢，直到物件被枚舉，無論是直接呼叫其 GetEnumerator 方法，或是使用 C# 中的 foreach，或在 Visual Basic 中使用 For Each，才會執行。

該 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 方法枚舉輸入序列，使用轉換函數將每個元素映射到 IEnumerable<T>，然後枚舉並產生每個此類 IEnumerable<T> 物件的元素。 也就是說，對 的sourceselector每個元素 ，都會被調用，並回傳一串值。 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 然後將這個二維集合平面化為一維 IEnumerable<T> ，並返回它。 例如，若查詢使用 SelectMany<TSource,TResult>(IEnumerable<TSource>, Func<TSource,Int32,IEnumerable<TResult>>) 取得資料庫中每位客戶的訂單（類型為 Order），結果在 C# 中為 IEnumerable<Order>，或在 Visual Basic 中為 IEnumerable(Of Order)。 若查詢改用 Select 取得順序，則順序集合不會合併，結果為 C# 中的 IEnumerable<List<Order>> 或 Visual Basic 中的 IEnumerable(Of List(Of Order))型。

第一個 selector 參數代表要處理的元素。 第二個 selector 參數代表該元素在來源序列中以零為基礎的索引。 如果元素順序已知，且你想對某個特定索引的元素做某件事，這會很有用。 如果你想取得一個或多個元素的索引，它也很有用。