Great question! Let’s break down how these two switch syntaxes differ in real-world C# development, even though they behave identically at runtime:

• Syntax Cleanliness & Readability
  The second approach (case < 12:) is a simplified relational pattern introduced in C# 9.0. It cuts out the redundant int x declaration since you don’t actually use that variable in your case logic. This makes the code shorter, cleaner, and easier to scan—no extra visual noise to distract from the actual condition you’re checking. For simple range checks like this, it’s definitely more intuitive.

• Variable Capture Use Cases
  The first syntax (case int x when hour < 12:) declares a variable x that captures the value being switched on (in this case, hour). While you don’t need it here, if your case logic required using that captured value directly (e.g., case int x when x % 2 == 0), this syntax lets you do that without re-referencing the original variable. The simplified syntax doesn’t support this directly—if you needed to capture the value, you’d have to use case var x when x < 12 instead (which is still cleaner than the first version if you need the variable).

• Version Compatibility
  This is a key practical difference. The simplified < 12 syntax only works in C# 9.0 or newer. If your project needs to support older C# versions (like 8.0 or earlier), you’ll have to stick with the first approach, since pattern matching with when clauses was introduced back in C# 7.0.

• Team & Project Norms
  Sometimes the choice comes down to your team’s coding standards. If you’re working on a legacy codebase where consistency with existing code matters more than cutting-edge syntax, you might stick with the first option. For new projects using modern C#, though, the simplified syntax is generally preferred for its brevity.

• Runtime Performance
  Just to confirm: there’s no difference in how these run. The compiler generates identical IL code for both approaches, so you won’t see any performance gains or losses between them.

At the end of the day, the simplified syntax is the way to go for modern projects unless you have a specific reason to use the older, more verbose version (like compatibility or needing to capture the matched variable).

内容的提问来源于stack exchange，提问作者Nika