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A Hash Table is a data structure that maps "Keys" to "Values." It is the most powerful tool for performance because it provides O(1) average search time. In .NET, this is implemented as Dictionary<K, V>.
When you add a key (e.g., "Sandeep"), the table runs a Hash Function that converts the string into an integer (e.g., index 4). It then stores the value at that specific array index. This is why lookup is instant—you don't search; you jump directly to the index.
What if two different strings (e.g., "Sandeep" and "Admin") produce the same hash index? This is a Collision.
Q: "Why should you NEVER use a mutable object (like a List) as a Dictionary Key?"
Architect Answer: "Because if the object changes while it is in the dictionary, its **Hash Code** will also change. When you try to look up the key later, the dictionary will look at the *new* hash index, find nothing (or someone else), and tell you the key doesn't exist. You have effectively 'lost' the data in memory. ALWAYS use **Immutable** types like Strings or Records as dictionary keys."
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On this page
1. The Hashing Process 2. Handling Collisions 4. Interview Mastery