An unused using has no impact to the runtime performance of your application.
It can affect the performance of the IDE and the overall compilation phase. The reason why is that it creates an additional namespace in which name resolution must occur. However these tend to be minor and shouldn't have a noticeable impact on your IDE experience for most scenarios.
It can also affect the performance of evaluating expressions in the debugger for the same reasons.
No, it's just a compile-time/coding style thing. .NET binaries use fully qualified names under the hood.
The following link A good read on why to remove unused references explains how it be useful to remove unused references from the application.
Below are the some excerpts from the link:
By removing any unused references in your application, you are
preventing the CLR
from loading the unused referenced modules at
runtime. Which means that you will reduce the startup time of your
application, because it takes time to load each module and avoids
having the compiler load metadata that will never be used. You may
find that depending on the size of each library, your startup time
is noticeably reduced. This isn't to say that your application will
be faster once loaded, but it can be pretty handy to know that your
startup time might get reduced.
Another benefit of removing any unused references is that you will
reduce the risk of conflicts with namespaces. For example, if you
have both System.Drawing
and System.Web.UI.WebControls
referenced,
you might find that you get conflicts when trying to reference the
Image
class. If you have using directives in your class that match
these references, the compiler can't tell which of the ones to use.
If you regularly use autocomplete when developing, removing unused
namespaces will reduce the number of autocompletion values in your
text editor as you type.
No effect on execution speed, but there may be some slight effect on compilation speed/intellisense as there are more potential namespaces to search for the proper class. I wouldn't worry too much about it, but you can use the Organize Usings menu item to remove and sort the using statements.
Code that does not execute does not affect the performance of a program.
No, there are several process involved when compiling a program. When the compiler start looking for references (classes, methods) it will use only the ones used on the code. The using directive only tells the compiler where to look. A lot of unused using statement could maybe have a performance issue but just at compile time. At runtime, all the outside code is properly linked or included as part of the binary.
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