Design a key value data structure support Get/Fetch/Delete/RandomDelete all in O(1)

Implement a Key-Value pair datastucture such that there is:
    O(1) insertion                    void insert(K, V)
    O(1) fetching                     
    O(1) deletion                    
    O(1) fetching of Random element  
    
    
    
Test cases:
1. Unit tests to cover all APIs
2. We try delete a key not exist, 
3. Performance test case to verify its O(1)  
4. Track memory
5. 
    
public class KeyValueStore
{
    private Dictionary<string, int> dict = new Dictionary<string, int>();
    private Tuple<string, SomeType> [] array = new Tuple<string, SomeType>[1000]; 
    private int current = 0;
     
    public void Insert(string key, SomeType value)
    {
        if (!dict.ContainsKey(key))
        {
            this.array[current] = new Tuple<string, SomeType>(key, value);
            dict[key] = current;
            current++;
        }
        else
        {
            this.array[dict[key]] = new Tuple<string, SomeType>(key, value);
        }
    }
    
    public SomeType Fetch(string key)
    {
        if (!dict.ContainsKey(key))
        {
            throw new KeyNotFoundException(key);
        }
        else
        {
            return this.array[dict[key]].Item2;
        }
    }
    
    public void Delete(string key)
    {
        if (dict.ContainsKey(key))
        {
            int toRemoveIndex = dict[key];
            int lastElementIndex = this.current - 1;
            
            // swap last element with the element to remove
            this.array[toRemoveIndex] = this.array[lastElementIndex];
            dict.Remove(key);
            
            // we need to update the dict for the last element
            string key = this.array[lastElementIndex].Item1;
            dict[key] = toRemoveIndex;
            this.current--;
        }
    }
    
    public SomeType FetchRandom()
    {
        if (current == 0)
        {
            throw new ValueNotFoundException();
        }
        else
        {
            int random = Random(0, current);
            return this.array[random].Item2;
        }
    }
}