Question

Wiki said it used to make class,interface uniquely identifier , how about object (actual instance) ??

When work with SQL,i also see the GUID for ID field (table user,..etc in database aspnetdb in asp.net MVC template project)

So I want to clearly understand the GUID usage, which case should use it , and is it really unique ,

Any explain appreciated

thank

Answer 1

For a good overview of what a GUID is, check out our good friend Wikipedia: GUID.

and is it really unique

GUIDs generated from the same machine are virtually guaranteed to be unique. You have an infinitesimally small chance of generating the same one twice on the same machine. Arguably you have a tiny chance of generating two GUIDs the same out in the wider world, but that chance is still small and the chances of those two GUIDs ever meeting are also pretty small. In fact you probably have a greater chance of the Large Hadron Collider generating a black hole that swallows the Earth than you would having two identical GUIDs meeting somewhere on a network.

Because of this, some people like to use it as the primary key for database tables. Personally i don’t like to do this because:

• an auto-incrementing integer gives me enough uniqueness to be able to use it as a primary key
• GUIDs are a massive PITA to deal with when you are writing SQL queries.

Wiki said it used to make class,interface uniquely identifier

If you need an identifier that is unique across several disparate areas (like hives in a registry), then GUIDs are a good solution. In this particular case they are being used to identify a type. A concrete instance could also internally use a GUID identifier, but this is really only useful for data objects.