为什么Uri是类而不是结构？

Question

System.Uri seems to be a perfect candidate for being a struct but the good folks at Microsoft decided to make it a class . It is clearly a value object -> there is no sense in having two different instances of " https://stackoverflow.com " and so it has struct-y equality rules. I can't see any setters on the public API either, so it is immutable.

Is there some implementation detail which dictates it must be a class ?

Answer 1

A struct would always have a default constructor (where all fields would be initialized to their default values). This could cause issues, for example, with some of the internal fields.

One example is that the existing constructor ensures that m_String can't be null - https://referencesource.microsoft.com/#system/net/system/UriExt.cs,39 . If you made this a struct, you can't easily achieve that in C#. So, everywhere you read from m_String you'd need to add a null check (which is not required if it is a class).

Additionally, as others have pointed out, the docs have other guidance on choosing when to use struct :

✓ CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

X AVOID defining a struct unless the type has all of the following characteristics:

It logically represents a single value, similar to primitive types (int, double, etc.).

It has an instance size under 16 bytes.

It is immutable.

It will not have to be boxed frequently.

The 16 byte requirement, in particular, is likely to be problematic for Uri .