As far as lines of code goes, this is pretty succinct, in fact as far as FizzBuzz in WinForms goes this is about as minimal as you can go.
Definitely need comments, yes in many cases your logic is self documenting, but it is hard to review code for bugs if we assume that the current logic and behaviour is by-design. It is likewise hard to reuse code if we have to keep inspecting the code to analyse if a function really is going to do what we need it to do. Make sure the summary comments viewed in IntelliSense are direct, use remarks to add additional information that we don't need to know every time the tooltips are displayed.
Separate Concerns - Keep Business Logic out of UI
To be flexible, separate your FizzBuzz logic out of the code-behind for your form. I know that was the easy way to display the app in this forum, and we forgive you for taking that shortcut for our benefit ^-^ but in code review we can't be lazy about these things, we don't want readers to learn these bad habits.
With FizzBuzz as it's own class we could easily re-use it in all sorts of different scenarios without having to copy and paste the code around. The overall benefit to code reuse is that there is less logic to test, review and importantly, maintain.
Collate Useful Functions in Static Classes
If you have complete methods that perform very specific functions and do not reference any member level resources from outside that method, then you should mark them as static and collate them in utility classes so that they can be easily located and re-used.
/// Collection of utility functions that might be useful for re-use
public static class Utilities
/// Generates a sequence of values including the start and end points
/// <para>Note: supports both increasing and decreasing ranges</para>
/// <param name="startPoint">First value in the sequence</param>
/// <param name="endPoint">Last value in the sequence</param>
public static IEnumerable<int> Sequence(int startPoint, int endPoint)
return Enumerable.Range(Math.Min(startPoint, endPoint), Math.Abs(startPoint - endPoint) + 1);
/// Tests that an integer value can be divided evenly by the divisor
/// <param name="number">Value to test</param>
/// <param name="divisor">Number of 'groups' to divide the Value into</param>
/// <returns>True if the number can be divided evenly by the divisor</returns>
public static bool IsDivisible(int number, int divisor)
return (number % divisor).Equals(0);
Re-use Objects instead of re-creating them
If you have a method like GetRules() that always returns the same collection, then it should be provided in a singleton pattern, or at the very least declared once as a member level collection. The next evolution of FizzBuzz
from your current start is to do it for large sequences, which will could lead to parallel processing. It's an obvious evolution, to prepare for this some effort should be made to ensure that the rules collection really is immutable and if possible thread-safe.
Creating the array object for every test is simpler than thread-safety, but it is an expensive and lazy habit to get into especially if you try this with larger arrays.
Try something like:
/// Thread-safe collection of FizzBuzz rules
/// <list type="#">
/// <listheader>Fizz Buzz Rules</listheader>
/// <item>If a number is divisible by 3, then we should output 'Fizz'</item>
/// <item>If a number is divisible by 5, then we should output 'Buzz'</item>
/// <item>If a number is divisible by Both 3 and 5, then we should output 'FizzBuzz'</item>
/// <para>You could extend this concept by adding additional rules to just Fizz and Buzz </para>
/// <para>This collection is very specific to FizzBuzz but is public to allow for additional use cases byt the caller</para>
public static readonly ConcurrentDictionary<int, string> Rules = new ConcurrentDictionary<int, string>
 = "Fizz",
 = "Buzz",
Consider Simple over Fancy or Uber efficient
Implementing a Linq Aggregate using StringBuilder is a pretty ambiguous, and in this manner it could be ever so slightly more inefficient than simply using String.Join
- Unless you have serious performance issues go with succinct LINQ expressions keep the logic managable.
A serious code junky will admire your mastery of LINQ and Lambdas but if you are charging dollars per hour spent coding and debugging try not to be too abstract if a simpler option is available
So your aggregate:
StringBuilder result = Rules.Where(rule => IsDivisible(number, rule.Key))
.Aggregate(new StringBuilder(), (stringBuilder, rule) => stringBuilder.Append(rule.Value));
easier to read and arguably more efficient:
return string.Join("", Rules
.Where(rule => IsDivisible(number, rule.Key))
.Select(rule => rule.Value));
Here's the argument: String.Join vs. StringBuilder: which is faster?
Redundant Logic Loops
I would normally question the pattern of checking if the number is in the rules and then if it is re-iterating the rules to build the string. For this implementation I've decided I like it, but in general we should not re-iterate lists multiple times if we don't need to.
I like the implementation here because it does make the code more readable, your intentions are very clear.
"I am going to get my number returned if it is not in the rules, otherwise ...magic will happen" ^-^
The other concession that I give is that the second iteration will only happen a very small fraction of the times that the first iteration is completed and the size of our rules list is very small so there is very little benefit.
This is the best alternative that I could come up with:
var output = string.Join("", Rules
.Where(rule => Utilities.IsDivisible(number, rule.Key))
.Select(rule => rule.Value));
return output.Length > 0 ? output : number.ToString();