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What is the difference between an array and a List<T> in C#?

C# Collections Basic Fresher1–2 Yrs Conceptual 1 min read

Quick Answer

An array is a fixed-size, contiguous block of memory — fast, low overhead, but you can't add or remove elements. A List<T> wraps an array and grows automatically (doubling its internal capacity when full), so it supports Add/Remove at the cost of a little overhead and occasional reallocation.

An array is fixed size — you allocate it once and you can't grow it. A List<T> is backed by an array but manages resizing for you: when it fills up it allocates a bigger array, usually double, and copies. So arrays are leaner and I use them when the size is fixed or in performance-critical code, and List<T> is my default everywhere else because I usually don't know the count up front. If I do know it, I pass the capacity to the List constructor so it doesn't have to keep regrowing.

  • Use an array when the size is known and fixed, in hot paths, or for interop; multidimensional data.
  • Use a List<T> for everything else — the default general-purpose collection.
  • List<T> has Count (elements) and Capacity (allocated slots); set the capacity in the constructor if you know the size to avoid regrowth.
  • Both are index-accessible in O(1); insert/remove in the middle of either is O(n).
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