Why is it okay that this struct is mutable? When are mutable structs acceptable?

Question

Eric Lippert told me I should \"try to always make value types immutable\", so I figured I should try to always make value types immutable.

But, I just found this i

Answer 1

Actually, if you search for all classes containing BitVector in the .NET framework, you'll find a bunch of these beasts :-)

• System.Collections.Specialized.BitVector32 (the sole public one...)
• System.Web.Util.SafeBitVector32 (thread safe)
• System.Web.Util.SimpleBitVector32
• System.Runtime.Caching.SafeBitVector32 (thread safe)
• System.Configuration.SafeBitVector32 (thread safe)
• System.Configuration.SimpleBitVector32

And if you look here were resides the SSCLI (Microsoft Shared Source CLI, aka ROTOR) source of System.Configuration.SimpleBitVector32, you'll find this comment:

//
// This is a cut down copy of System.Collections.Specialized.BitVector32. The
// reason this is here is because it is used rather intensively by Control and
// WebControl. As a result, being able to inline this operations results in a
// measurable performance gain, at the expense of some maintainability.
//
[Serializable()]
internal struct SimpleBitVector32

I believe this says it all. I think the System.Web.Util one is more elaborate but built on the same grounds.