列舉非泛型字典的元素。
命名空間: System.Collections
組件: mscorlib (在 mscorlib.dll 中)
<ComVisibleAttribute(True)> _
Public Interface IDictionaryEnumerator
Inherits IEnumerator
Dim instance As IDictionaryEnumerator
[ComVisibleAttribute(true)]
public interface IDictionaryEnumerator : IEnumerator
[ComVisibleAttribute(true)]
public interface class IDictionaryEnumerator : IEnumerator
/** @attribute ComVisibleAttribute(true) */
public interface IDictionaryEnumerator extends IEnumerator
ComVisibleAttribute(true)
public interface IDictionaryEnumerator extends IEnumerator
C# 語言的 foreach 陳述式 (在 Visual Basic 中為 for each) 會隱藏列舉值的複雜度。因此,建議您使用 foreach,而不要直接使用列舉值。
列舉值可以用來讀取集合中的資料,但是無法用來修改基礎集合。
列舉值一開始會位於集合中第一個元素之前,Reset 方法也會將列舉值帶回這個位置。在這個位置,呼叫 Current 屬性會擲回例外狀況。因此,在讀取 Current 的值之前,必須呼叫 MoveNext 方法,以將列舉值前移至集合的第一個元素。
Current 會傳回相同的物件直到呼叫 MoveNext 或 Reset。MoveNext 會將 Current 設定為下一個元素。
如果 MoveNext 傳遞集合結尾,列舉值就會置於集合的最後一個項目之後,然後 MoveNext 傳回 false。當列舉值位於這個位置時,後續的 MoveNext 呼叫也都會傳回 false。如果最後呼叫 MoveNext 傳回 false,則呼叫 Current 會擲回例外狀況。若要將 Current 再次設定為集合的第一個項目,您可以先呼叫 Reset,然後再呼叫 MoveNext。
只要集合保持不變,列舉值就會保持有效。如果已對集合做變更,例如加入、修改或刪除元素,則列舉值將永遠無效,而再次呼叫 MoveNext 或 Reset 會擲回 InvalidOperationException。如果在 MoveNext 和 Current 之間修改集合,Current 會傳回所設定的元素,即使列舉值已經無效。
列舉值沒有集合的獨佔存取權,因此,列舉一個集合在本質上並不是執行緒安全的程序。即使集合經過同步化,其他的執行緒仍可修改該集合,使列舉值擲回例外狀況。若要保證列舉過程的執行緒安全,您可以在整個列舉過程中鎖定集合,或攔截由其他執行緒的變更所造成的例外狀況。
實作者注意事項
繼承自 IEnumerator 的 Current 屬性會傳回 Object (它是一個 boxed DictionaryEntry),與 Entry 屬性的傳回值類似。
這個程式碼範例顯示如何定義實作 IDictionaryEnumerator 介面的字典列舉值。
Private Class SimpleDictionaryEnumerator
Implements IDictionaryEnumerator
' A copy of the SimpleDictionary object's key/value pairs.
Dim items() As DictionaryEntry
Dim index As Integer = -1
Public Sub New(ByVal sd As SimpleDictionary)
' Make a copy of the dictionary entries currently in the SimpleDictionary object.
items = New DictionaryEntry(sd.Count - 1) {}
Array.Copy(sd.items, 0, items, 0, sd.Count)
End Sub
' Return the current item.
Public ReadOnly Property Current() As Object Implements IDictionaryEnumerator.Current
Get
ValidateIndex()
Return items(index)
End Get
End Property
' Return the current dictionary entry.
Public ReadOnly Property Entry() As DictionaryEntry Implements IDictionaryEnumerator.Entry
Get
Return Current
End Get
End Property
' Return the key of the current item.
Public ReadOnly Property Key() As Object Implements IDictionaryEnumerator.Key
Get
ValidateIndex()
Return items(index).Key
End Get
End Property
' Return the value of the current item.
Public ReadOnly Property Value() As Object Implements IDictionaryEnumerator.Value
Get
ValidateIndex()
Return items(index).Value
End Get
End Property
' Advance to the next item.
Public Function MoveNext() As Boolean Implements IDictionaryEnumerator.MoveNext
If index < items.Length - 1 Then
index = index + 1
Return True
End If
Return False
End Function
' Validate the enumeration index and throw an exception if the index is out of range.
Private Sub ValidateIndex()
If index < 0 Or index >= items.Length Then
Throw New InvalidOperationException("Enumerator is before or after the collection.")
End If
End Sub
' Reset the index to restart the enumeration.
Public Sub Reset() Implements IDictionaryEnumerator.Reset
index = -1
End Sub
End Class
Public Function GetEnumerator() As IDictionaryEnumerator Implements IDictionary.GetEnumerator
'Construct and return an enumerator.
Return New SimpleDictionaryEnumerator(Me)
End Function
' ICollection Members
Public ReadOnly Property IsSynchronized() As Boolean Implements IDictionary.IsSynchronized
Get
Return False
End Get
End Property
Public ReadOnly Property SyncRoot() As Object Implements IDictionary.SyncRoot
Get
Throw New NotImplementedException()
End Get
End Property
Public ReadOnly Property Count() As Integer Implements IDictionary.Count
Get
Return ItemsInUse
End Get
End Property
Public Sub CopyTo(ByVal array As Array, ByVal index As Integer) Implements IDictionary.CopyTo
Throw New NotImplementedException()
End Sub
' IEnumerable Members
Public Function GetEnumerator1() As IEnumerator Implements IEnumerable.GetEnumerator
' Construct and return an enumerator.
Return Me.GetEnumerator()
End Function
End Class
private class SimpleDictionaryEnumerator : IDictionaryEnumerator
{
// A copy of the SimpleDictionary object's key/value pairs.
DictionaryEntry[] items;
Int32 index = -1;
public SimpleDictionaryEnumerator(SimpleDictionary sd)
{
// Make a copy of the dictionary entries currently in the SimpleDictionary object.
items = new DictionaryEntry[sd.Count];
Array.Copy(sd.items, 0, items, 0, sd.Count);
}
// Return the current item.
public Object Current { get { ValidateIndex(); return items[index]; } }
// Return the current dictionary entry.
public DictionaryEntry Entry
{
get { return (DictionaryEntry) Current; }
}
// Return the key of the current item.
public Object Key { get { ValidateIndex(); return items[index].Key; } }
// Return the value of the current item.
public Object Value { get { ValidateIndex(); return items[index].Value; } }
// Advance to the next item.
public Boolean MoveNext()
{
if (index < items.Length - 1) { index++; return true; }
return false;
}
// Validate the enumeration index and throw an exception if the index is out of range.
private void ValidateIndex()
{
if (index < 0 || index >= items.Length)
throw new InvalidOperationException("Enumerator is before or after the collection.");
}
// Reset the index to restart the enumeration.
public void Reset()
{
index = -1;
}
}
public IDictionaryEnumerator GetEnumerator()
{
// Construct and return an enumerator.
return new SimpleDictionaryEnumerator(this);
}
#endregion
#region ICollection Members
public bool IsSynchronized { get { return false; } }
public object SyncRoot { get { throw new NotImplementedException(); } }
public int Count { get { return ItemsInUse; } }
public void CopyTo(Array array, int index) { throw new NotImplementedException(); }
#endregion
#region IEnumerable Members
IEnumerator IEnumerable.GetEnumerator()
{
// Construct and return an enumerator.
return ((IDictionary)this).GetEnumerator();
}
#endregion
}
private:
ref class SimpleDictionaryEnumerator : public IDictionaryEnumerator
{
// A copy of the SimpleDictionary object's key/value pairs.
private:
array<DictionaryEntry^>^ items;
private:
int index;
public:
SimpleDictionaryEnumerator(SimpleDictionary^ sd)
{
// Make a copy of the dictionary entries currently in the
// SimpleDictionary object.
items = gcnew array<DictionaryEntry^>(sd->Count);
Array::Copy(sd->items, 0, items, 0, sd->Count);
index = -1;
}
// Return the current item.
public:
virtual property Object^ Current
{
Object^ get()
{
ValidateIndex();
return items[index];
}
}
// Return the current dictionary entry.
public:
virtual property DictionaryEntry Entry
{
DictionaryEntry get()
{
return (DictionaryEntry) Current;
}
}
// Return the key of the current item.
public:
virtual property Object^ Key
{
Object^ get()
{
ValidateIndex();
return items[index]->Key;
}
}
// Return the value of the current item.
public:
virtual property Object^ Value
{
Object^ get()
{
ValidateIndex();
return items[index]->Value;
}
}
// Advance to the next item.
public:
virtual bool MoveNext()
{
if (index < items->Length - 1)
{
index++;
return true;
}
return false;
}
// Validate the enumeration index and throw an exception if
// the index is out of range.
private:
void ValidateIndex()
{
if (index < 0 || index >= items->Length)
{
throw gcnew InvalidOperationException
("Enumerator is before or after the collection.");
}
}
// Reset the index to restart the enumeration.
public:
virtual void Reset()
{
index = -1;
}
};
public:
virtual IDictionaryEnumerator^ GetEnumerator()
{
// Construct and return an enumerator.
return gcnew SimpleDictionaryEnumerator(this);
}
#pragma endregion
#pragma region ICollection Members
public:
virtual property bool IsSynchronized
{
bool get()
{
return false;
}
}
public:
virtual property Object^ SyncRoot
{
Object^ get()
{
throw gcnew NotImplementedException();
}
}
public:
virtual property int Count
{
int get()
{
return itemsInUse;
}
}
public:
virtual void CopyTo(Array^ array, int index)
{
throw gcnew NotImplementedException();
}
#pragma endregion
#pragma region IEnumerable Members
virtual IEnumerator^ IEnumerable_GetEnumerator()
= IEnumerable::GetEnumerator
{
// Construct and return an enumerator.
return ((IDictionary^)this)->GetEnumerator();
}
#pragma endregion
};
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