C# | Add key and value into StringDictionary

Last Updated : 01 Feb, 2019

StringDictionary.Add(String, String) method is used to add an entry with the specified key and value into the StringDictionary.

Syntax:

public virtual void Add (string key, string value);

Parameters:

key: It is the key of the entry which is to be added.
value: It is the value of the entry which is to be added. The value can be null.

Exceptions:

ArgumentNullException : If the key is null.
ArgumentException : It is an entry with the same key already exists in the StringDictionary.
NotSupportedException : If the StringDictionary is read-only.

Below programs illustrate the use of StringDictionary.Add(String, String) method:

Example 1:

// C# code to add key and value 
// into the StringDictionary 
using System; 
using System.Collections; 
using System.Collections.Specialized; 
  
class GFG { 
  
    // Driver code 
    public static void Main() 
    { 
  
        // Creating a StringDictionary named myDict 
        StringDictionary myDict = new StringDictionary(); 
  
        // Adding key and value into the StringDictionary 
        myDict.Add("A", "Apple"); 
        myDict.Add("B", "Banana"); 
        myDict.Add("C", "Cat"); 
        myDict.Add("D", "Dog"); 
        myDict.Add("E", "Elephant"); 
  
        // Displaying the keys and values in StringDictionary 
        foreach(DictionaryEntry dic in myDict) 
        { 
            Console.WriteLine(dic.Key + "  " + dic.Value); 
        } 
    } 
} 

Output:

d  Dog
b  Banana
c  Cat
e  Elephant
a  Apple

Example 2:

// C# code to add key and value 
// into the StringDictionary 
using System; 
using System.Collections; 
using System.Collections.Specialized; 
  
class GFG { 
  
    // Driver code 
    public static void Main() 
    { 
  
        // Creating a StringDictionary named myDict 
        StringDictionary myDict = new StringDictionary(); 
  
        // Adding key and value into the StringDictionary 
        myDict.Add("A", "Apple"); 
        myDict.Add("B", "Banana"); 
        myDict.Add("C", "Cat"); 
        myDict.Add("D", "Dog"); 
  
        // It should raise "ArgumentException" 
        // as an entry with the same key 
        // already exists in the StringDictionary. 
        myDict.Add("C", "Code"); 
  
        // Displaying the keys and values in StringDictionary 
        foreach(DictionaryEntry dic in myDict) 
        { 
            Console.WriteLine(dic.Key + "  " + dic.Value); 
        } 
    } 
} 

Runtime Error:

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

Note:

The key is handled in a case-insensitive manner i.e, it is translated to lowercase before it is added to the string dictionary.
This method is an O(1) operation.

Reference:

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

C# | Check if the StringDictionary contains a specific key

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