C# Language Idioms - Effective C# - page 12

The final advantage of using const over readonly is performance: Known
constant values can generate slightly more efficient code than the variable
accesses necessary for readonly values. However, any gains are slight and
should be weighed against the decreased flexibility. Be sure to profile per-
formance differences before giving up the flexibility.
Yo u ' l l e n c o u n t e r s i m i l a r t r a d e o f f s b e t w e e n r u n t i m e a n d c o m p i l e - t i m e p r o -
cessing of constant values when you use named and optional parameters.
The default values for optional parameters are placed in the call site just
like the default value for compile-time constants (those declared with
const ). Like working with readonly and const values, you'll want to be
careful with changes to the values of optional parameters. (See Item 10.)
const must be used when the value must be available at compile time:
attribute parameters and enum definitions, and those rare times when you
mean to define a value that does not change from release to release. For
everything else, prefer the increased flexibility of readonly constants.
Item 3: Prefer the is or as Operators to Casts
By embracing C#, you've embraced strong typing. That is almost always a
good thing. Strong typing means you expect the compiler to find type mis-
matches in your code. That also means your applications do not need to
perform as much type checking at runtime. But sometimes, runtime type
checking is unavoidable. There will be times in C# when you write func-
tions that take object parameters because the framework defines the
method signature for you. You likely need to attempt to cast those objects
to other types, either classes or interfaces. You've got two choices: Use the
as operator or force the compiler to bend to your will using a cast. You
also have a defensive variant: You can test a conversion with is and then
use as or casts to convert it.
The correct choice is to use the as operator whenever you can because it
is safer than blindly casting and is more efficient at runtime. The as and
is operators do not perform any user-defined conversions. They succeed
only if the runtime type matches the sought type; they never construct a
new object to satisfy a request.
Ta k e a l o o k a t a n e x a m p l e . Yo u w r i t e a p i e c e o f c o d e t h a t n e e d s t o c o n v e r t
an arbitrary object into an instance of MyType. You could write it this way: