Threading

The Rust standard library supports threading, synchronisation and concurrency. Also the language itself and the standard library do have basic support for the concepts, a lot of additional functionality is provided by crates and will not be covered in this document.

The following lists approximate mapping of threading types and methods in .NET to Rust:

.NET	Rust
`Thread`	`std::thread::thread`
`Thread.Start`	`std::thread::spawn`
`Thread.Join`	`std::thread::JoinHandle`
`Thread.Sleep`	`std::thread::sleep`
`ThreadPool`	-
`Mutex`	`std::sync::Mutex`
`Semaphore`	-
`Monitor`	`std::sync::Mutex`
`ReaderWriterLock`	`std::sync::RwLock`
`AutoResetEvent`	`std::sync::Condvar`
`ManualResetEvent`	`std::sync::Condvar`
`Barrier`	`std::sync::Barrier`
`CountdownEvent`	`std::sync::Barrier`
`Interlocked`	`std::sync::atomic`
`Volatile`	`std::sync::atomic`
`ThreadLocal`	`std::thread_local`

Launching a thread and waiting for it to finish works the same way in C#/.NET and Rust. Below is a simple C# program that creates a thread (where the thread prints some text to standard output) and then waits for it to end:

using System;
using System.Threading;

var thread = new Thread(() => Console.WriteLine("Hello from a thread!"));
thread.Start();
thread.Join(); // wait for thread to finish

The same code in Rust would be as follows:

use std::thread;

fn main() {
    let thread = thread::spawn(|| println!("Hello from a thread!"));
    thread.join().unwrap(); // wait for thread to finish
}

Creating and initializing a thread object and starting a thread are two different actions in .NET whereas in Rust both happen at the same time with thread::spawn.

In .NET, it's possible to send data as an argument to a thread:

#nullable enable

using System;
using System.Text;
using System.Threading;

var t = new Thread(obj =>
{
    var data = (StringBuilder)obj!;
    data.Append(" World!");
});

var data = new StringBuilder("Hello");
t.Start(data);
t.Join();

Console.WriteLine($"Phrase: {data}");

However, a more modern or terser version would use closures:

using System;
using System.Text;
using System.Threading;

var data = new StringBuilder("Hello");

var t = new Thread(obj => data.Append(" World!"));

t.Start();
t.Join();

Console.WriteLine($"Phrase: {data}");

In Rust, there is no variation of thread::spawn that does the same. Instead, the data is passed to the thread via a closure:

use std::thread;

fn main() {
    let data = String::from("Hello");
    let handle = thread::spawn(move || {
        let mut data = data;
        data.push_str(" World!");
        data
    });
    println!("Phrase: {}", handle.join().unwrap());
}

A few things to note:

The move keyword is required to move or pass the ownership of data to the closure for the thread. Once this is done, it's no longer legal to continue to use the data variable of main, in main. If that is needed, data must be copied or cloned (depending on what the type of the value supports).
Rust thread can return values, like tasks in C#, which becomes the return value of the join method.
It is possible to also pass data to the C# thread via a closure, like the Rust example, but the C# version does not need to worry about ownership since the memory behind the data will be reclaimed by the GC once no one is referencing it anymore.

Rust for C#/.NET Developers

Threading