C#中ArrayList的Capacity属性疑问：添加元素后容量变化原因解析

Let's break down exactly what's happening with your ArrayList's capacity here—it all comes down to how ArrayList handles internal resizing in C#.

Step 1: Initial Capacity of listInt #

When you created listInt and added 3 elements (9,10,11), the output showed Capacity Before: 4. That makes perfect sense because:

• The default ArrayList starts with a capacity of 0. When you add the first element, it automatically resizes to a capacity of 4 (this is the default initial growth threshold for ArrayList).
• Since you only added 3 elements, you didn't hit the capacity limit yet, so it stayed at 4.

Step 2: What Happens When You Call AddRange(list2) #

You added 3 more elements from list2 to listInt, bringing the total number of elements to 6. Now the current capacity (4) can't hold all 6 elements, so ArrayList triggers a resize. Here's the core rule behind this:

If the required capacity exceeds the current capacity, the new capacity is set to the larger value between twice the current capacity or the exact required capacity.

In your scenario:

• Current capacity: 4
• Required capacity after adding 3 elements: 3 (existing) + 3 (new) = 6
• Twice the current capacity is 8, which is larger than 6. So the capacity gets updated to 8.

Quick Test to Confirm the Logic #

If you want to validate this further, try adding more elements to listInt after the AddRange call:

listInt.Add(12); // Total elements: 7, Capacity stays at 8
listInt.Add(13); // Total elements: 8, Capacity still 8
listInt.Add(14); // Total elements: 9, Capacity jumps to 16 (twice 8)

As a side note: ArrayList is a non-generic collection and is largely replaced by type-safe alternatives like List<T> (e.g., List<int> or List<string>) in modern C#. List<T> follows nearly identical resizing logic but avoids boxing/unboxing overhead and type errors.

内容的提问来源于stack exchange，提问作者Shailesh Prajapati