11. From and Into Traits — C# → Rust | Rust Training by Microsoft

Type Conversions in Rust

What you'll learn: From/Into traits vs C#'s implicit/explicit operators, TryFrom/TryInto for fallible conversions, FromStr for parsing, and idiomatic string conversion patterns.

Difficulty: 🟡 Intermediate

C# uses implicit/explicit conversions and casting operators. Rust uses the From and Into traits for safe, explicit conversions.

C# Conversion Patterns

// C# implicit/explicit conversions
public class Temperature
{
    public double Celsius { get; }
    
    public Temperature(double celsius) { Celsius = celsius; }
    
    // Implicit conversion
    public static implicit operator double(Temperature t) => t.Celsius;
    
    // Explicit conversion
    public static explicit operator Temperature(double d) => new Temperature(d);
}

double temp = new Temperature(100.0);  // implicit
Temperature t = (Temperature)37.5;     // explicit

Rust From and Into

#[derive(Debug)]
struct Temperature {
    celsius: f64,
}

impl From<f64> for Temperature {
    fn from(celsius: f64) -> Self {
        Temperature { celsius }
    }
}

impl From<Temperature> for f64 {
    fn from(temp: Temperature) -> f64 {
        temp.celsius
    }
}

fn main() {
    // From
    let temp = Temperature::from(100.0);
    
    // Into (automatically available when From is implemented)
    let temp2: Temperature = 37.5.into();
    
    // Works in function arguments too
    fn process_temp(temp: impl Into<Temperature>) {
        let t: Temperature = temp.into();
        println!("Temperature: {:.1}°C", t.celsius);
    }
    
    process_temp(98.6);
    process_temp(Temperature { celsius: 0.0 });
}

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Rule of thumb: Implement From, and you get Into for free. Callers can use whichever reads better.

TryFrom for Fallible Conversions

use std::convert::TryFrom;

impl TryFrom<i32> for Temperature {
    type Error = String;
    
    fn try_from(value: i32) -> Result<Self, Self::Error> {
        if value < -273 {
            Err(format!("Temperature {}°C is below absolute zero", value))
        } else {
            Ok(Temperature { celsius: value as f64 })
        }
    }
}

fn main() {
    match Temperature::try_from(-300) {
        Ok(t) => println!("Valid: {:?}", t),
        Err(e) => println!("Error: {}", e),
    }
}

String Conversions

// ToString via Display trait
impl std::fmt::Display for Temperature {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{:.1}°C", self.celsius)
    }
}

// Now .to_string() works automatically
let s = Temperature::from(100.0).to_string(); // "100.0°C"

// FromStr for parsing
use std::str::FromStr;

impl FromStr for Temperature {
    type Err = String;
    
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let s = s.trim_end_matches("°C").trim();
        let celsius: f64 = s.parse().map_err(|e| format!("Invalid temp: {}", e))?;
        Ok(Temperature { celsius })
    }
}

let t: Temperature = "100.0°C".parse().unwrap();

Exercises

🏋️ Exercise: Currency Converter (click to expand)

Create a Money struct that demonstrates the full conversion ecosystem:

Money { cents: i64 } (stores value in cents to avoid floating-point issues)
Implement From<i64> (treats input as whole dollars → cents = dollars * 100)
Implement TryFrom<f64> — reject negative amounts, round to nearest cent
Implement Display to show "$1.50" format
Implement FromStr to parse "$1.50" or "1.50" back into Money
Write a function fn total(items: &[impl Into<Money> + Copy]) -> Money that sums values

🔑 Solution

use std::fmt;
use std::str::FromStr;

#[derive(Debug, Clone, Copy)]
struct Money { cents: i64 }

impl From<i64> for Money {
    fn from(dollars: i64) -> Self {
        Money { cents: dollars * 100 }
    }
}

impl TryFrom<f64> for Money {
    type Error = String;
    fn try_from(value: f64) -> Result<Self, Self::Error> {
        if value < 0.0 {
            Err(format!("negative amount: {value}"))
        } else {
            Ok(Money { cents: (value * 100.0).round() as i64 })
        }
    }
}

impl fmt::Display for Money {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "${}.{:02}", self.cents / 100, self.cents.abs() % 100)
    }
}

impl FromStr for Money {
    type Err = String;
    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let s = s.trim_start_matches('$');
        let val: f64 = s.parse().map_err(|e| format!("{e}"))?;
        Money::try_from(val)
    }
}

fn main() {
    let a = Money::from(10);                       // $10.00
    let b = Money::try_from(3.50).unwrap();         // $3.50
    let c: Money = "$7.25".parse().unwrap();        // $7.25
    println!("{a} + {b} + {c}");
}