C# | Check whether a Hashtable contains a specific key or not

Hashtable.Contains(Object) Method is used to check whether the Hashtable contains a specific key or not.

Syntax:

public virtual bool Contains (object key);

Here, key is the Key of Object type which is to be located in the Hashtable.

Return Value: This method returns true if the Hashtable contains an element with the specified key otherwise returns false.

Exception: This method will give ArgumentNullException if the key is null.

Note:

• Hashtable.ContainsKey(Object) Method is also used to check whether the Hashtable contains a specific key or not. This method behaves same as Contains() method.
• Contains method implements IDictionary.Contains. It behaves exactly as ContainsKey and this method is an O(1) operation.

Below programs illustrate the use of above-discussed method:

Example 1:

// C# code to check whether the Hashtable
// contains a specific key or not
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("g", "geeks");
        myTable.Add("c", "c++");
        myTable.Add("d", "data structures");
        myTable.Add("q", "quiz");

        // Checking if Hashtable contains
        // the key "Brazil"
        Console.WriteLine(myTable.Contains("d"));
    }
}

Output:

True

Example 2:

// C# code to check whether the Hashtable
// contains a specific key or not
using System;
using System.Collections;

class GFG {

    // Driver code
    public static void Main()
    {

        // Creating a Hashtable
        Hashtable myTable = new Hashtable();

        // Adding elements in Hashtable
        myTable.Add("1", "C");
        myTable.Add("2", "C++");
        myTable.Add("3", "Java");
        myTable.Add("4", "Python");

        // Checking if Hashtable contains
        // the key null. It will give exception
        // ArgumentNullException
        Console.WriteLine(myTable.Contains(null));
    }
}

Runtime Error:

Unhandled Exception:
System.ArgumentNullException: Key cannot be null.
Parameter name: key

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.collections.hashtable.contains?view=netframework-4.7.2