The different ways you can append and get message arguments can be a bit bewildering. I've iterated a few times on the design and didn't want to lose backwards compatibility.

This guide is to help you on your way. In addition, many of the examples in the examples directory append and read arguments. There is also a reference at the end of this document.

First - if you can get D-Bus introspection data, you can use the the dbus-codegen tool to generate some boilerplate code for you. E g, if you want to talk to NetworkManager:

cargo install dbus-codegen
dbus-codegen-rust -s -g -m None -d org.freedesktop.NetworkManager -p /org/freedesktop/NetworkManager > networkmanager.rs

You would then use this code like:

// main.rs
mod networkmanager;

/* ... */

// Start a connection to the system bus.
let c = Connection::new_system()?;

// Make a "ConnPath" struct that just contains a Connection, a destination and a path.
let p = c.with_proxy("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager", Duration::new(5, 0));

// Bring our generated code into scope.
use networkmanager::OrgFreedesktopNetworkManager;

// Now we can call methods on our connpath from the "org.freedesktop.NetworkManager" interface.
let devices = p.get_all_devices()?;

There is also pre-generated code for standard D-Bus interfaces in the stdintf module. A similar example:

let c = Connection::new_session()?;

// Make a "ConnPath" struct that just contains a Connection, a destination and a path.
let p = c.with_path("org.mpris.MediaPlayer2.rhythmbox", "/org/mpris/MediaPlayer2", Duration::new(5, 0));

// The ConnPath struct implements many traits, e g `org.freedesktop.DBus.Properties`. Bring the trait into scope.
use stdintf::org_freedesktop_dbus::Properties;

// Now we can call org.freedesktop.DBus.Properties.Get just like an ordinary method and get the result back.
let metadata = p.get("org.mpris.MediaPlayer2.Player", "Metadata")?;

For more details, see dbus-codegen-rust --help and the README.md in the dbus-codegen directory.

Now, if you want to make a service yourself, the generated code is more complex. And for some use cases, codegen isn't really an option, so let's move on:

If you just want to get/append simple types, just use append1 / append2 / append3, and read1 / read2 / read3. The imaginary method below takes one byte parameter and one string parameter, and returns one string parameter and one int parameter.

let m = Message::new_method_call(dest, path, intf, member)?.append2(5u8, "Foo");
let r = c.send_with_reply_and_block(m, 2000)?;
let (data1, data2): (&str, i32) = c.read2()?;

D-Bus arrays and dictionaries usually correspond to Vec and HashMap. You can just append and get them like basic types:

let v = vec![3i32, 4i32, 5i32];
let mut map = HashMap::new();
map.insert("Funghi", 5u16);
map.insert("Mold", 8u16);

let m = Message::new_method_call(dest, path, intf, member)?.append2(v, map);
let r = c.send_with_reply_and_block(m, 2000)?;
let (data1, data2): (Vec<i32>, HashMap<&str, u16>) = r.read2()?;

Or combine them as you wish, e g, use a Vec<Vec<u8>>, a HashMap<u64, Vec<String>> or HashMap<String, HashMap<String, i32>> to construct more difficult types.

Slices can sometimes be used as arrays - e g, &[&str] can be appended, but only very simple types can be used with get and read, e g &[u8].

This is the easiest way to get started, but in case you want to avoid the overhead of creating Vec or HashMaps, the "Array and Dict types" and "Iter / IterAppend" sections offer useful alternatives.

Things are getting slightly more complex with Variants, because they are not strongly typed and thus not fit as well into Rust's strongly typed as arrays and dicts.

If you know the type beforehand, it's still easy:

let v = Variant("This is a variant containing a &str");
let m = Message::new_method_call(dest, path, intf, member)?.append1(v);
let r = c.send_with_reply_and_block(m, 2000)?;
let z: Variant<i32> = r.read1()?;
println!("Method returned {}", z.0);

The Variant struct is just a wrapper with a public interior, so you can easily both read from it and write to it with the .0 accessor.

Sometimes you don't know the type beforehand. We can solve this in two ways (choose whichever is more appropriate for your use case), either through the trait object Box<dyn RefArg> or through Iter / IterAppend (see later sections).

Through trait objects:

let x = Box::new(5000i32) as Box<dyn RefArg>;
let m = Message::new_method_call(dest, path, intf, member)?.append1(Variant(x));
let r = c.send_with_reply_and_block(m, 2000)?;
let z: Variant<Box<dyn RefArg>> = r.read1()?;

Ok, so we retrieved our Box<dyn RefArg>. We now need to use the RefArg methods to probe it, to see what's inside. You can use as_i64, as_u64, as_f64 or as_str if you want to test for number or string types. Second easiest is to use arg::cast to downcast to the specific type inside (see reference section below for what type you need to cast to).

This works for most types, but in some advanced cases, this is not possible, either because the RefArg is not 'static (work around this with the box_clone method if necessary), or because the internal representation is not specified. In this case, try to use as_static_inner or as_iter to iterate through the interior of the complex type.

let z: Variant<Box<dyn RefArg + 'static>> = r.read1()?;
let value = &z.0;

if let Some(s) = value.as_str() { println!("It's a string: {}", s); }
else if let Some(i) = value.as_i64() { println!("It's an integer: {}", i); }
else if let Some(f) = arg::cast::<f64>(value) { println!("It's a float: {}", f); }
else { println!("Don't know how to handle a {:?}", value.arg_type()) }

Dicts and variants are sometimes combined, e g, you might need to read a D-Bus dictionary of String to Variants. You can read these as PropMap (which is a type alias for HashMap<String, Variant<Box<dyn RefArg>>>) and use prop_cast to retrieve a value.

D-Bus structs are implemented as Rust tuples. You can append and get tuples like you do with other types of arguments. You can also use VecDeque<Box<dyn RefArg>> for when the types of the struct cannot be statically typed.

For an example of the latter, the Desktop Notifications Specification specifies that the notify() method sends a dictionary of optional hints, one of which is "image-data" hint, which contains a struct. Here's how you might retrieve that "image-data" hint and cast it into a rust struct.

Implementing the notify() method in Rust -- trait generated by dbus-codegen-rust:

// impl OrgFreedesktopNotifications {
fn notify(
    &mut self,
    app_name: String,
    replaces_id: u32,
    app_icon: String,
    summary: String,
    body: String,
    actions: Vec<String>,
    hints: arg::PropMap,
    expire_timeout: i32
) -> Result<u32, dbus::MethodErr> {
    use dbus::arg::prop_cast;

    // Retrieving a property from a PropMap of type byte (`u8`).
    let urgency: Option<&u8> = prop_cast(&hints, "urgency");

    // Retrieving a property from a PropMap of type struct (`Image`, in this case).
    struct Image {
        width: i32,
        height: i32,
        rowstride: i32,
        one_point_two_bit_alpha: bool,
        bits_per_sample: i32,
        channels: i32,
        data: Vec<u8>,
    }

    // Structs are represented internally as `VecDeque<Box<RefArg>>`.
    let image_data: Option<&VecDeque<Box<dyn RefArg>>> = prop_cast(&hints, "image-data");
    let image = if let Some(img) = image_data {
        use dbus::arg::cast;
        // NOTE: It's possible that someone sent the "image-data" hint but didn't fill it correctly, which will
        // panic with this code!  You should make sure that the length of the `Vec` is correct and handle the
        // `cast` `Option` instead of just unwrapping.
        let width = *cast::<i32>(&img[0]).unwrap();
        let height = *cast::<i32>(&img[1]).unwrap();
        let rowstride = *cast::<i32>(&img[2]).unwrap();
        let one_point_two_bit_alpha = *cast::<bool>(&img[3]).unwrap();
        let bits_per_sample = *cast::<i32>(&img[4]).unwrap();
        let channels = *cast::<i32>(&img[5]).unwrap();
        let data = cast::<Vec<u8>>(&img[6]).unwrap().clone();
        Some(Image { width, height, rowstride, one_point_two_bit_alpha, bits_per_sample, channels, data })
    } else {
        None
    }

    // Do something with image / urgency.

    Ok(0)
}

TODO: tuple example

For dbus-crossroads, the method argument types are automatically deduced from your method closure, but the names have to repeated. Method arguments - both in and out - are always tuples, so if you have a single argument, it needs to be wrapped into a "one element tuple" (so a variable or type x becomes (x,)). Like this:

b.method("Hello", ("request",), ("reply",), |_, _, (request,): (HashMap<i32, Vec<(i32, bool, String)>>,)| {
  // Returns a Result<String>
});

For dbus-tree, you want to declare what input and output arguments your method expects - so that correct D-Bus introspection data can be generated. You'll use the same types as you learned earlier in this guide:

factory.method( /* ... */ )
.inarg::<HashMap<i32, Vec<(i32, bool, String)>>,_>("request")
.outarg::<&str,_>("reply")

The types are just for generating a correct signature, they are never instantiated. Many different types can generate the same signature - e g, Array<u8, _>, Vec<u8> and &[u8] will all generate the same signature. Variant will generate the same type signature regardless of what's inside, so just write Variant<()> for simplicity.

Iter and IterAppend are more low-level, direct methods to get and append arguments. They can, e g, come handy if you have more than five arguments to read.

E g, for appending a variant with IterAppend you can use IterAppend::new(&msg).append_variant(|i| i.append(5000i32)) to append what you need to your variant inside the closure. To read a variant you can use let i = msg.read1::<Variant<Iter>>::()? and then examine the methods on i.0 to probe the variant.

These provide slightly better flexibility than using Vec and HashMap by instead integrating with Iterator. Here's an example where you can append and get a dictionary without having to create a HashMap:

let x = &[("Hello", true), ("World", false)];
let m = Message::new_method_call(dest, path, intf, member)?.append1(Dict::new(x));
let r = c.send_with_reply_and_block(m, 2000)?;
let z: Dict<i32, &str, _> = r.read1()?;
for (key, value) in z { /* do something */ }

An edge case where this is necessary is having floating point keys in a dictionary - this is supported in D-Bus but not in Rust's HashMap. I have never seen this in practice, though.

The types Path and Signature are not often used, but they can be appended and read as other argument types. Path and Signature will return strings with a borrowed lifetime - use .into_static() if you want to untie that lifetime.

You can also append and get a std::fs::File, this will send or receive a file descriptor. OwnedFd is an earlier design which was used for the same thing.

MessageItem was the first design - an enum representing a D-Bus argument. It still works, but I doubt you'll ever need to use it. Newer methods provide better type safety, speed, and ergonomics.

This is a translation table between D-Bus types and Rust types. Both the RefArg type and the other types can be used in most cases, e g append, get and/or read, as well as function arguments for dbus-crossroads. In addition, &T, Box<T>, Rc<T> and Arc<T> can (in general) be used instead of T, where T is a Rust type.

If you want to to use cast or prop_cast on a &RefArg however, you need to use the RefArg type. If the RefArg type is listed as N/A, it means that there is no stable RefArg type and casting is not supported.