// Structure projects like C# solutions
src/
├── main.rs          // Program.cs equivalent
├── lib.rs           // Library entry point
├── models/          // Like Models/ folder in C#
│   ├── mod.rs
│   ├── user.rs
│   └── product.rs
├── services/        // Like Services/ folder
│   ├── mod.rs
│   ├── user_service.rs
│   └── product_service.rs
├── controllers/     // Like Controllers/ (for web apps)
├── repositories/    // Like Repositories/
└── utils/          // Like Utilities/

// Create a common Result type for your application
pub type AppResult<T> = Result<T, AppError>;

#[derive(Error, Debug)]
pub enum AppError {
    #[error("Database error: {0}")]
    Database(#[from] sqlx::Error),
    
    #[error("HTTP error: {0}")]
    Http(#[from] reqwest::Error),
    
    #[error("Validation error: {message}")]
    Validation { message: String },
    
    #[error("Business logic error: {message}")]
    Business { message: String },
}

// Use throughout your application
pub async fn create_user(data: CreateUserRequest) -> AppResult<User> {
    validate_user_data(&data)?;  // Returns AppError::Validation
    let user = repository.create_user(data).await?;  // Returns AppError::Database
    Ok(user)
}

// Structure tests like C# unit tests
#[cfg(test)]
mod tests {
    use super::*;
    use rstest::*;  // For parameterized tests like C# [Theory]
    
    #[test]
    fn test_basic_functionality() {
        // Arrange
        let input = "test data";
        
        // Act
        let result = process_data(input);
        
        // Assert
        assert_eq!(result, "expected output");
    }
    
    #[rstest]
    #[case(1, 2, 3)]
    #[case(5, 5, 10)]
    #[case(0, 0, 0)]
    fn test_addition(#[case] a: i32, #[case] b: i32, #[case] expected: i32) {
        assert_eq!(add(a, b), expected);
    }
    
    #[tokio::test]  // For async tests
    async fn test_async_functionality() {
        let result = async_function().await;
        assert!(result.is_ok());
    }
}

// [ERROR] Don't try to implement inheritance
// Instead of:
// struct Manager : Employee  // This doesn't exist in Rust

// [OK] Use composition with traits
trait Employee {
    fn get_salary(&self) -> u32;
}

trait Manager: Employee {
    fn get_team_size(&self) -> usize;
}

// [ERROR] Don't use unwrap() everywhere (like ignoring exceptions)
let value = might_fail().unwrap();  // Can panic!

// [OK] Handle errors properly
let value = match might_fail() {
    Ok(v) => v,
    Err(e) => {
        log::error!("Operation failed: {}", e);
        return Err(e.into());
    }
};

// [ERROR] Don't clone everything (like copying objects unnecessarily)
let data = expensive_data.clone();  // Expensive!

// [OK] Use borrowing when possible
let data = &expensive_data;  // Just a reference

// [ERROR] Don't use RefCell everywhere (like making everything mutable)
struct Data {
    value: RefCell<i32>,  // Interior mutability - use sparingly
}

// [OK] Prefer owned or borrowed data
struct Data {
    value: i32,  // Simple and clear
}

// [ERROR] C# habit: cloning strings to pass around
fn greet(name: String) {
    println!("Hello, {name}");
}

let user_name = String::from("Alice");
greet(user_name.clone());  // unnecessary allocation
greet(user_name.clone());  // and again

// [OK] Borrow instead — zero allocation
fn greet(name: &str) {
    println!("Hello, {name}");
}

let user_name = String::from("Alice");
greet(&user_name);  // borrows
greet(&user_name);  // borrows again — no cost

// [ERROR] The "I'll fix this later" trap
let config = std::fs::read_to_string("config.toml").unwrap();
let port: u16 = config_value.parse().unwrap();
let conn = db_pool.get().await.unwrap();

// [OK] Propagate with ? in application code
let config = std::fs::read_to_string("config.toml")?;
let port: u16 = config_value.parse()?;
let conn = db_pool.get().await?;

// [OK] Use expect() only when failure is truly a bug
let home = std::env::var("HOME")
    .expect("HOME environment variable must be set");  // documents the invariant

// [ERROR] Trying to mutate while iterating (C# foreach + modify pattern)
let mut items = vec![1, 2, 3, 4, 5];
for item in &items {
    if *item > 3 {
        items.push(*item * 2);  // ERROR: can't borrow items as mutable
    }
}

// [OK] Collect first, then mutate
let extras: Vec<i32> = items.iter()
    .filter(|&&x| x > 3)
    .map(|&x| x * 2)
    .collect();
items.extend(extras);

// [ERROR] Returning a reference to a local (C# returns references freely via GC)
fn get_greeting() -> &str {
    let s = String::from("hello");
    &s  // ERROR: s is dropped at end of function
}

// [OK] Return owned data
fn get_greeting() -> String {
    String::from("hello")  // caller owns it
}

// [ERROR] Nested null-checking style from C#
fn process(input: Option<String>) -> Option<usize> {
    match input {
        Some(s) => {
            if !s.is_empty() {
                match s.parse::<usize>() {
                    Ok(n) => {
                        if n > 0 {
                            Some(n * 2)
                        } else {
                            None
                        }
                    }
                    Err(_) => None,
                }
            } else {
                None
            }
        }
        None => None,
    }
}

// [OK] Flatten with combinators
fn process(input: Option<String>) -> Option<usize> {
    input
        .filter(|s| !s.is_empty())
        .and_then(|s| s.parse::<usize>().ok())
        .filter(|&n| n > 0)
        .map(|n| n * 2)
}