Initial capacity of collection types, e.g. Dictionary, List

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If the default values are not documented, the reason is likely that the optimal initial capacity is an implementation detail and subject to change between framework versions. That is, you shouldn’t write code that assumes a certain default value.

The constructor overloads with a capacity are for cases in which you know better than the class what number of items are to be expected. For example, if you create a collection of 50 values and know that this number will never increase, you can initialize the collection with a capacity of 50, so it won’t have to resize if the default capacity is lower.

That said, you can determine the default values using Reflector. For example, in .NET 4.0 (and probably previous versions as well),

  • a List<T> is initialized with a capacity of 0. When the first item is added, it is reinitialized to a capacity of 4. Subsequently, whenever the capacity is reached, the capacity is doubled.

  • a Dictionary<T> is intialized with a capacity of 0 as well. But it uses a completely different algorithm to increase the capacity: it increases the capacity always to prime numbers.

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