I am finalizing one of my projects and taking a look over the whole project looking for mistakes, bugs and performance mistakes. I am using MVC. I caught one Don't and that is:
Never put a RenderPartial within a loop. it will drastically slow down your whole server.
Have you run your program through FxCop? It has a set of rules for performance.
Never store a WebControl to Session.
Because it has a reference to the Page object, it ends up storing every control to session.
Don't optimize prematurely. :) If the site is not performant, profile the code to determine where to spend your time.
Don't profile or otherwise judge performance in the debug configuration. The debug configuration isn't intended to be fast, and you may make performance conclusions which are wrong (like the idea that partial views/user controls are slow; this is true in debug configuration but not in release configuration). When you profile to measure performance, you should use the release configuration so that you can see where the real problems are.
Only use try/catch blocks when necessary. They do slow down your application.
EDIT for Clarity: By "necessary" I mean, for catching real errors.
If you can write some code and be proactive to ensure the error won't be thrown do it, as it will be more performant than letting an exception be thrown, then handling it.
Don't use exceptions to control program flow. I don't know who said it first, but I recall the phrase "Exceptions should be exceptional!". They should be for the cases where unforseen issues occur, things that couldn't be tested prior to code executing and throwing them.
The worst example I see all to often is something along these lines...
int i = 0;
try
{
i = int.Parse(txt);
} catch {Exception x) {
// Do nothing, i = 0
}
In C#, objects are always created with new. This alone can be a drawback in a certain perspective. For example, if you create an object inside a loop (meaning that a new object is created for each iteration in the loop) you can slow down your program.
for (int i = 0; i < 1000; ++i)
{
Object o = new Object();
//...
}
Instead create an instance outside the loop. Object o = new Object();
Object o = new Object();
for (int i = 0; i < 1000; ++i)
{
//...
}
Only create an object in a loop if you really have to...
Perhaps doing a bit of C++ would help you understand the mechanics behind and know when and where to optimize your code. Although C++ is a different language there are a lot of things you can apply to other languages once you understand the basic of memory management (new, delete, pointers, dynamic arrays/static arrays, etc.).
Most performance problems are due to disk access or calls across networks.
So be carefull how and how often you access the file system or a database. Do you need to make so many calls across the network, or could you do it in a single call.
One good example:
In this case a better solution may be to write custom code to store and read the value.
Caching would help you improve the performance but you should be careful use it only where it makes sense
DO use static methods - but only if the method is frequently used.
DON'T mark a variable as static unless you really want the variable's value to be the same across all instances (another developer did this and I had fun debugging why we got odd behavior only when multiple users hit the site). This is not for performance reasons, but just good advice.