C# | Add key and value into OrderedDictionary collection

OrderedDictionary.Add(Object, Object) method is used to add an entry with the specified key and value into the OrderedDictionary collection with the lowest available index.

Syntax:

public void Add (object key, object value);

Parameters:

key : The key of the entry to add.
value : The value of the entry to add. This value can be null.

Exceptions :

NotSupportedException : If the OrderedDictionary collection is read-only.
ArgumentException : If an element with the same key already exists in the OrderedDictionary collection.

Below given are some examples to understand the implementation in a better way:

Example 1:

// C# code to add key and value 
// into OrderedDictionary 
using System; 
using System.Collections; 
using System.Collections.Specialized; 
  
class GFG { 
  
    // Driver method 
    public static void Main() 
    { 
  
        // Creating a orderedDictionary named myDict 
        OrderedDictionary myDict = new OrderedDictionary(); 
  
        // Adding key and value in myDict 
        myDict.Add("key1", "value1"); 
        myDict.Add("key2", "value2"); 
        myDict.Add("key3", "value3"); 
        myDict.Add("key4", "value4"); 
        myDict.Add("key5", "value5"); 
  
        // Displaying the number of key/value 
        // pairs in myDict 
        Console.WriteLine(myDict.Count); 
  
        // Displaying the key/value pairs in myDict 
        foreach(DictionaryEntry de in myDict) 
            Console.WriteLine(de.Key + " --> " + de.Value); 
    } 
} 

Output:

5
key1 --> value1
key2 --> value2
key3 --> value3
key4 --> value4
key5 --> value5

Example 2:

// C# code to add key and value 
// into OrderedDictionary 
using System; 
using System.Collections; 
using System.Collections.Specialized; 
  
class GFG { 
  
    // Driver method 
    public static void Main() 
    { 
  
        // Creating a orderedDictionary named myDict 
        OrderedDictionary myDict = new OrderedDictionary(); 
  
        // Adding key and value in myDict 
        myDict.Add("key1", "value1"); 
        myDict.Add("key2", "value2"); 
  
        // This should raise "ArgumentException" 
        // as an element with the same key already 
        // exists in the OrderedDictionary collection. 
        myDict.Add("key2", "value3"); 
        myDict.Add("key4", "value4"); 
        myDict.Add("key5", "value5"); 
  
        // Displaying the number of key/value 
        // pairs in myDict 
        Console.WriteLine(myDict.Count); 
  
        // Displaying the key/value pairs in myDict 
        foreach(DictionaryEntry de in myDict) 
            Console.WriteLine(de.Key + " --> " + de.Value); 
    } 
} 

Runtime Error:

Unhandled Exception:
System.ArgumentException: Item has already been added. Key in dictionary: ‘key2’ Key being added: ‘key2’

Note: A key cannot be null, but a value can be.

Reference:

https://docs.microsoft.com/en-us/dotnet/api/system.collections.specialized.ordereddictionary.add?view=netframework-4.7.2

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