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Rust has been a hot topic for a while, and although I had been meaning to try it someday, I kept putting it off. Recently, I learned that the official Rust crate for Windows looks quite promising, so I decided to try creating and debugging programs that call various Windows APIs from Rust.

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Reference: microsoft/windows-rs: Rust for Windows

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The following is a crate that can call C-style Windows APIs.

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Reference: windows-sys - crates.io: Rust Package Registry

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This one also seems to be a crate that can call APIs such as COM.

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Reference: windows - crates.io: Rust Package Registry

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I would like to try various implementations using crates like these.

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Table of Contents

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Notes on Rust

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Before implementing the program, I will first briefly summarize what I looked up about what kind of language Rust is in the first place.

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Rust is generally said to have the following advantages.

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In particular, because Rust does not use garbage collection and is compiled directly into machine code, it is said to deliver performance second only to C/C++.

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Also, Rust’s focus on zero-cost abstractions, as in C++, is apparently one reason it can achieve high performance.

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Reference: The Rust Programming Language - The Rust Programming Language

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Installing Rust and Setting Up the Development Environment

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This time I want to implement a program in Rust that calls Windows APIs, so I will set up the development environment on Windows rather than Linux.

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Basically, I followed the steps in the official documentation below.

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Reference: Set up a development environment for Rust on Windows | Microsoft Learn

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To set up the Rust environment, I first installed Visual Studio so that I could install the build tools.

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At that time, I installed [.Net desktop development] and [Desktop development with C++] together.

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Next, I installed Rust on Windows using the installer downloaded from the following page.

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Reference: Install Rust - Rust Programming Language

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This completes the minimum required Rust setup, but because I wanted to use VSCode as the IDE, I also installed the rust-analyzer and CodeLLDB extensions.

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Reference: Install Visual Studio Code | Microsoft Learn

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Creating a Rust Project

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Once the basic setup is complete, you can create a HelloWorld project by running the following command.

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cargo new HelloWorld
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The created project includes main.rs, Cargo.toml, and .gitignore.

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Using the MessageBoxW API from Rust

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This time, I will try creating a program that calls the MessageBoxW API from Rust using the windows crate.

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Reference: Rust for Windows and the windows crate | Microsoft Learn

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By using the windows crate published on crates.io, you can easily use Windows APIs such as MessageBoxW from a Rust program.

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A crate is something like a library in other programming languages, and even with external crates published on crates.io, such as the windows crate, it seems that if you add their definitions to [dependencies] in the Cargo.toml file, the cargo command will automatically resolve the dependencies when building.

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Adding the windows Crate to Use the MessageBoxW API

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First, I added the following line to the Cargo.toml file to use the MessageBoxW API.

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In the following entry, I specify version 0.60.0 of the windows crate published on crates.io as a dependency, and I specify Win32_UI_WindowsAndMessaging with the features option.

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[dependencies]\nwindows = { version = \"0.60.0\", features = [\"Win32_UI_WindowsAndMessaging\"] }
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It seems that the features option can be used to specify that particular functionality should be enabled at compile time.

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In this case, it enables the Win32_UI_WindowsAndMessaging feature, which includes MessageBoxW.

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Reference: How to Use Feature Flags in Rust #Rust - Qiita

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Reference: windows::Win32::UI::WindowsAndMessaging - Rust

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Reference: MessageBoxExW in windows::Win32::UI::WindowsAndMessaging - Rust

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Creating MessageBoxW.rs and Loading the Module

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Next, I created a MessageBoxW.rs file directly under src/bin/ and wrote the following code.

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use windows::{\n    core::PCWSTR,\n    Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION},\n};\n\nfn to_wide_null(s: &str) -> Vec<u16> {\n    s.encode_utf16().chain(std::iter::once(0)).collect()\n}\n\nfn main() {\n    let caption = to_wide_null(\"Title\");\n    let message = to_wide_null(\"This is my first WinAPI with Rust.\");\n\n    unsafe {\n        MessageBoxW(\n            None,\n            PCWSTR(message.as_ptr()),\n            PCWSTR(caption.as_ptr()),\n            MB_OK | MB_ICONINFORMATION,\n        );\n    }\n}
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First, at the top of the file, use declares that the required components will be imported.

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In this example, it instructs Rust to import core::PCWSTR and Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION} from the windows crate.

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use windows::{\n    core::PCWSTR,\n    Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION},\n};
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core::PCWSTR is a pointer type that points to a null-terminated UTF-16 string in the Windows API.

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Also, Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION} imports three items from Win32::UI::WindowsAndMessaging: MessageBoxW, MB_OK, and MB_ICONINFORMATION.

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Because it is written compactly, it is actually a bit hard to read until you get used to it, but in practice I was able to run the program the same way even if I wrote it as follows.

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use windows::core::PCWSTR;\nuse windows::Win32::UI::WindowsAndMessaging::MessageBoxW; \nuse windows::Win32::UI::WindowsAndMessaging::MB_OK;\nuse windows::Win32::UI::WindowsAndMessaging::MB_ICONINFORMATION;
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Defining the Function

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In the next section, I define a function called to_wide_null.

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In Rust, functions are defined in the form fn <function_name>(). The notation after -> specifies the function’s return type.

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In other words, in the following code, Vec<u16> shows that this function returns a vector type.

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fn to_wide_null(s: &str) -> Vec<u16> {\n    s.encode_utf16().chain(std::iter::once(0)).collect()\n}
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More specifically, s.encode_utf16().chain(std::iter::once(0)).collect() inside the function UTF-16 encodes the string slice s of type &str, adds a null character (0) element at the end with .chain(std::iter::once(0)), and returns it as the Vec<u16> vector specified by .collect(). (Hard to follow!)

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Calling the MessageBoxW API

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Finally, the following main function calls the MessageBoxW API.

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fn main() {\n    let caption = to_wide_null(\"Title\");\n    let message = to_wide_null(\"This is my first WinAPI with Rust.\");\n\n    unsafe {\n        MessageBoxW(\n            None,\n            PCWSTR(message.as_ptr()),\n            PCWSTR(caption.as_ptr()),\n            MB_OK | MB_ICONINFORMATION,\n        );\n    }\n}
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Here, I first use the to_wide_null function to define caption and message as null-terminated wide strings.

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In VSCode, the types are displayed directly in the code, which makes this much easier to read.

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Next, I call MessageBoxW inside an unsafe block.

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unsafe seems to be syntax used when bypassing Rust’s compile-time-enforced memory safety and running code for which memory safety is not enforced at compile time.

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There seem to be several situations where unsafe is used, but in this case I need to use an unsafe block because I am calling MessageBoxW, an unsafe function exported via extern.

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Reference: Unsafe Rust - The Rust Programming Language

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Easily Converting String Literals to UTF-16 Wide Strings

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Up to this point in the code, I had been using the to_wide_null function to convert strings into the UTF-16 strings passed as arguments to MessageBoxW.

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However, someone more knowledgeable told me that using a macro called w! makes string conversion much easier, so I rewrote the code.

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Reference: w in windows_sys - Rust

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The rewritten code is shown below. It is much cleaner.

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The windows::core::w! macro converts the target string into a null-terminated UTF-16 wide string, so there is no longer any need to obtain a pointer to a vector containing the wide string via windows::core::PCWSTR.

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use windows::{\n    core::w,\n    Win32::UI::WindowsAndMessaging::{MessageBoxW, MB_OK, MB_ICONINFORMATION},\n};\n\n\nfn main() {\n    let caption = w!(\"Title\");\n    let message = w!(\"This is my first WinAPI with Rust.\");\n\n    unsafe {\n        MessageBoxW(\n            None,\n            message,\n            caption,\n            MB_OK | MB_ICONINFORMATION,\n        );\n    }\n}
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When I built and ran this code, I was able to use MessageBoxW just like with the original code.

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Running the Program

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At this point the code is complete, so you can call the MessageBoxW API from Rust by running cargo run --bin MessageBoxW.

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The windows crate is convenient, so invoking the API was easier than I expected.

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Summary

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From here on, I plan to try implementing programs in Rust that use various Windows APIs, so as a first step I summarized the procedure up to using MessageBoxW.

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