# Hashtable.ContainsKey(Object) Metodo

## Definizione

Determina se l'oggetto Hashtable contiene una chiave specifica.Determines whether the Hashtable contains a specific key.

``````public:
virtual bool ContainsKey(System::Object ^ key);``````
``public virtual bool ContainsKey (object key);``
``````abstract member ContainsKey : obj -> bool
override this.ContainsKey : obj -> bool``````
``Public Overridable Function ContainsKey (key As Object) As Boolean``

#### Parametri

key
Object

Chiave da individuare in Hashtable.The key to locate in the Hashtable.

#### Restituisce

`true` se Hashtable contiene un elemento con la chiave specificata; in caso contrario, `false`.`true` if the Hashtable contains an element with the specified key; otherwise, `false`.

#### Eccezioni

`key` è `null`.`key` is `null`.

## Esempi

Nell'esempio seguente viene illustrato come determinare se il Hashtable contiene un elemento specifico.The following example shows how to determine whether the Hashtable contains a specific element.

``````using namespace System;
using namespace System::Collections;
void PrintIndexAndKeysAndValues( Hashtable^ myHT );
int main()
{

// Creates and initializes a new Hashtable.
Hashtable^ myHT = gcnew Hashtable;

// Displays the values of the Hashtable.
Console::WriteLine( "The Hashtable contains the following values:" );
PrintIndexAndKeysAndValues( myHT );

// Searches for a specific key.
int myKey = 2;
Console::WriteLine( "The key \"{0}\" is {1}.", myKey, myHT->ContainsKey( myKey ) ? (String^)"in the Hashtable" : "NOT in the Hashtable" );
myKey = 6;
Console::WriteLine( "The key \"{0}\" is {1}.", myKey, myHT->ContainsKey( myKey ) ? (String^)"in the Hashtable" : "NOT in the Hashtable" );

// Searches for a specific value.
String^ myValue = "three";
Console::WriteLine( "The value \"{0}\" is {1}.", myValue, myHT->ContainsValue( myValue ) ? (String^)"in the Hashtable" : "NOT in the Hashtable" );
myValue = "nine";
Console::WriteLine( "The value \"{0}\" is {1}.", myValue, myHT->ContainsValue( myValue ) ? (String^)"in the Hashtable" : "NOT in the Hashtable" );
}

void PrintIndexAndKeysAndValues( Hashtable^ myHT )
{
int i = 0;
Console::WriteLine( "\t-INDEX-\t-KEY-\t-VALUE-" );
IEnumerator^ myEnum = myHT->GetEnumerator();
while ( myEnum->MoveNext() )
{
DictionaryEntry de =  *safe_cast<DictionaryEntry ^>(myEnum->Current);
Console::WriteLine( "\t[{0}]:\t{1}\t{2}", i++, de.Key, de.Value );
}

Console::WriteLine();
}

/*
This code produces the following output.

The Hashtable contains the following values:
-INDEX- -KEY-   -VALUE-
[0]:    4       four
[1]:    3       three
[2]:    2       two
[3]:    1       one
[4]:    0       zero

The key "2" is in the Hashtable.
The key "6" is NOT in the Hashtable.
The value "three" is in the Hashtable.
The value "nine" is NOT in the Hashtable.

*/
``````
``````using System;
using System.Collections;
public class SamplesHashtable
{

public static void Main()
{
// Creates and initializes a new Hashtable.
var myHT = new Hashtable();

// Displays the values of the Hashtable.
Console.WriteLine("The Hashtable contains the following values:");
PrintIndexAndKeysAndValues(myHT);

// Searches for a specific key.
int myKey = 2;
Console.WriteLine("The key \"{0}\" is {1}.", myKey, myHT.ContainsKey(myKey) ? "in the Hashtable" : "NOT in the Hashtable");
myKey = 6;
Console.WriteLine("The key \"{0}\" is {1}.", myKey, myHT.ContainsKey(myKey) ? "in the Hashtable" : "NOT in the Hashtable");

// Searches for a specific value.
var myValue = "three";
Console.WriteLine("The value \"{0}\" is {1}.", myValue, myHT.ContainsValue( myValue ) ? "in the Hashtable" : "NOT in the Hashtable");
myValue = "nine";
Console.WriteLine("The value \"{0}\" is {1}.", myValue, myHT.ContainsValue( myValue ) ? "in the Hashtable" : "NOT in the Hashtable");
}

public static void PrintIndexAndKeysAndValues(Hashtable myHT)
{
int i = 0;
Console.WriteLine("\t-INDEX-\t-KEY-\t-VALUE-");
foreach (DictionaryEntry de in myHT)
Console.WriteLine(\$"\t[{i++}]:\t{de.Key}\t{de.Value}");
Console.WriteLine();
}
}

/*
This code produces the following output.

The Hashtable contains the following values:
-INDEX- -KEY-   -VALUE-
[0]:    4       four
[1]:    3       three
[2]:    2       two
[3]:    1       one
[4]:    0       zero

The key "2" is in the Hashtable.
The key "6" is NOT in the Hashtable.
The value "three" is in the Hashtable.
The value "nine" is NOT in the Hashtable.

*/
``````
``````Imports System.Collections

Public Class SamplesHashtable

Public Shared Sub Main()

' Creates and initializes a new Hashtable.
Dim myHT As New Hashtable()

' Displays the values of the Hashtable.
Console.WriteLine("The Hashtable contains the following values:")
PrintIndexAndKeysAndValues(myHT)

' Searches for a specific key.
Dim myKey As Integer = 2
Console.Write(\$"The key ""{myKey}"" is ")
If (myHT.ContainsKey(myKey))
Console.WriteLine("in the Hashtable.")
Else
Console.WriteLine("NOT in the Hashtable.")
End If

myKey = 6
Console.Write(\$"The key ""{myKey}"" is ")
If (myHT.ContainsKey(myKey))
Console.WriteLine(" in the Hashtable.")
Else
Console.WriteLine(" NOT in the Hashtable.")
End If

' Searches for a specific value.
Dim myValue As String = "three"
Console.Write("The value ""{0}"" is ", myValue)
If (myHT.ContainsValue(myValue))
Console.WriteLine(" in the Hashtable.")
Else
Console.WriteLine(" NOT in the Hashtable.")
End If

myValue = "nine"
Console.Write(\$"The value ""{myValue}"" is ")
If (myHT.ContainsValue(myValue))
Console.WriteLine(" in the Hashtable.")
Else
Console.WriteLine(" NOT in the Hashtable.")
End If

End Sub

Public Shared Sub PrintIndexAndKeysAndValues(myHT As Hashtable)
Dim i As Integer = 0
Console.WriteLine(vbTab + "-INDEX-" + vbTab + "-KEY-" + vbTab + "-VALUE-")
For Each de As DictionaryEntry In myHT
Console.WriteLine(vbTab + "[{0}]:" + vbTab + "{1}" + vbTab + "{2}", i, de.Key, de.Value)
i += 1
Next
Console.WriteLine()
End Sub

End Class

' This code produces the following output.
'
' The Hashtable contains the following values:
'         -INDEX- -KEY-   -VALUE-
'         [0]:    4       four
'         [1]:    3       three
'         [2]:    2       two
'         [3]:    1       one
'         [4]:    0       zero
'
' The key "2" is in the Hashtable.
' The key "6" is  NOT in the Hashtable.
' The value "three" is  in the Hashtable.
' The value "nine" is  NOT in the Hashtable.

``````

## Commenti

Questo metodo si comporta esattamente come Contains.This method behaves exactly as Contains.

Questo metodo è un'operazione di `O(1)`.This method is an `O(1)` operation.

A partire da .NET Framework 2,0, questo metodo usa i metodi Equals e CompareTo degli oggetti della raccolta su `item` per determinare se l'elemento esiste.Starting with the .NET Framework 2.0, this method uses the collection's objects' Equals and CompareTo methods on `item` to determine whether item exists. Nelle versioni precedenti del .NET Framework, questa determinazione è stata eseguita utilizzando i metodi Equals e CompareTo del parametro `item` negli oggetti della raccolta.In the earlier versions of the .NET Framework, this determination was made by using the Equals and CompareTo methods of the `item` parameter on the objects in the collection.