unit-threaded 0.7.3

Advanced multi-threaded unit testing framework with minimal to no boilerplate using built-in unittest blocks

To use this package, run the following command in your project's root directory:

Manual usage
Put the following dependency into your project's dependences section:


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My DConf2016 Lightning talk demonstrating unit-threaded.

Multi-threaded advanced unit test framework for the D programming language.


"But doesn't D have built-in unittest blocks"? Yes, and they're massively useful. Even short scripts can benefit from them with 0 effort and setup. In fact, I use them to test this library. However, for larger projects it lacks some functionality:

  1. If all tests pass, great. If one fails, it's hard to know why.
  2. The only tool is assert, and you have to write your own assert messages (no assertEqual, assertNull, etc.)
  3. No possibility to run just one particular test
  4. Only runs in one thread.

So I wrote this library in and for a language with built-in support for unit tests. Its goals are:

  1. To run in parallel by default
  2. Support for built-in unittest blocks - no need to reinvent the wheel
  3. To be able to run specific tests or group of tests via the command-line
  4. No test registration. Tests are discovered with D's compile-time reflection
  5. Suppress tested code stdio and stderr output by default (important when running in multiple threads).
  6. Have a special mode that only works when using a single thread under which tested code output is turned back on, as well as special writelnUt debug messages.
  7. Ability to temporarily hide tests from being run by default whilst still being able to run them

Quick start with dub

dub runs tests with dub test. Unfortunately, due to the nature of D's compile-time reflection, to use this library a test runner file listing all modules to reflect on must exist. Since this is a tedious task and easily automated, unit-threaded has a dub configuration called gen_ut_main to do just that. To use unit-threaded with a dub project, you can use a unittest configuration as exemplified in this dub.json:

    "name": "myproject",
    "targetType": "executable",
    "targetPath": "bin",
    "configurations": [
        { "name": "executable" },
            "name": "unittest",
            "targetType": "executable",
            "preBuildCommands": ["dub run unit-threaded -c gen_ut_main -- -f bin/ut.d"],
            "mainSourceFile": "bin/ut.d",
            "excludedSourceFiles": ["src/main.d"],
            "dependencies": {
                "unit-threaded": "~>0.6.0"

excludedSourceFiles is there to not compile the file containing the main function to avoid linker errors. As an alternative to using excludedSourceFiles, the "real" main can be versioned out:

version(unittest) {}
else {
    void main() {

Your unittest blocks will now be run in threads and can be run individually. To name each unittest, simply attach a string UDA to it:

@("Test that 2 + 3 is 5")
unittest {
    assert(2 + 3 == 5);

You can also have multiple configurations for running unit tests, e.g. one that uses the standard D runtime unittest runner and one that uses unit-threaded:

"configurations": [
    {"name": "ut_default"},
      "name": "unittest",
      "preBuildCommands: ["dub run unit-threaded -c gen_ut_main -- -f bin/ut.d"],
      "mainSourceFile": "bin/ut.d",

In this example, dub test -c ut_default runs as usual if you don't use this library, and dub test runs with the unit-threaded test runner.

To use unit-threaded's assertions or UDA-based features, you must import the library:

version(unittest) { import unit_threaded; }
else              { enum ShouldFail; } // so production builds compile

int adder(int i, int j) { return i + j; }

@("Test adder") unittest {
    adder(2, 3).shouldEqual(5);

@("Test adder fails", ShouldFail) unittest {
    adder(2, 3).shouldEqual(7);

If using a custom dub configuration for unit-threaded as shown above, a version block can be used on Have_unit_threaded (this is added by dub to the build).

Advanced Usage: Attributes

@ShouldFail is used to decorate a test that is expected to fail, and can be passed a string to explain why. @ShouldFail should be preferred to @HiddenTest. If the relevant bug is fixed or not-yet-implemented functionality is done, the test will then fail, which makes them harder to sweep under the carpet and forget about.

Since code under test might not be thread-safe, the @Serial attribute can be used on a test. This causes all tests in the same module that have this attribute to be executed sequentially so they don't interleave with one another.

The @UnitTest and @DontTest attributes are explained below.

There is support for parameterized tests. This means running the test code multiple times, either with different values or different types. At the moment this feature cannot be used with the built-in unittest blocks.

For values and built-in unit tests, use the @Values UDA to supply test values and getValue with the appropriate type to retrive them:

@Values(2, 4, 6)
unittest {
    assert(getValue!int % 0 == 2);

This will run the test 3 times, and the reporting will consider it to be 3 separate tests.

If more than one @Values UDA is used, then the test gets instantiated with the cartesian product of values, e.g.

@Values(1, 2)
@Values("foo", "bar")
unittest {
    getValue!(int, 0); // gets the integer value (1 or 2)
    getValue!(string, 1); // gets the string value ("foo" or "bar")

The test above is instantiated 4 times for each one of the possible combinations. This helps to reduce boilerplate and repeated tests.

You can also declare a test function that takes parameters of the appropriate types and add UDAs with the values desired, e.g.

@(2, 4, 6)
void testEven(int i) {
    (i % 0 == 2).shouldBeTrue;

For a cartesian product, simply declare more parameters and add UDAs as appropriate.

For types, use the @Types UDA on a template function with exactly one compile-time parameter:

@Types!(int, byte)
void testInit(T)() {

The @Name UDA can be used instead of a plain string in order to name a unittest block.

unit-threaded uses D's package and module system to make it possible to select a subset of tests to run. Sometimes however, tests in different modules address cross-cutting concerns and it may be desirable to indicate this grouping in order to select only those tests. The @Tags UDA can be used to do that. Any number of tags can be applied to a test:

@Tags("foo", "tagged")
unittest { ... }

The strings a test is tagged with can be used by the test runner binary to constrain which tests to run either by selecting tests with or without tags:

./ut @foo ~@bar

That will run all tests that have the "foo" tag that also don't have the "bar" tag.

If using value or type parameterized tests, the @AutoTags UDA will give each sub-test a tag corresponding to their parameter:

@Values("foo", "bar")
@AutoTags // equivalent to writing @Tags("foo", "bar")
unittest {
   // ...

The @Setup and @Shutdown UDAs can be attached to a free function in a module. If they are, they will be run before/after each unittest block in a composite (usually a module). This feature currently only works for unittest blocks, not free functions. Classes could override setup and shutdown already.

Property-based testing

There is preliminary and experimental support for property-based testing. The current types supported are all primitive types, all 3 string types, and arrays of these types. To check a property use the check function from unit_threaded.property with a function returning bool:

check!((int a) => a % 2 == 0);

The above example will obviously fail. By default check runs the property function with 100 random values, pass it a different runtime parameter to change that:

check!((int a) => a % 2 == 0)(10_000); // will still fail

If using compile-time delegates as above, the types of the input parameters must be explicitly stated. Multiple parameters can be used as long as each one is of one of the currently supported types.


Classes and interfaces can be mocked like so:

interface Foo { int foo(int, string); }
int fun(Foo f, int i, string s) { return f.foo(i * 2, s ~ "stuff"); }

auto m = mock!Foo;
fun(m, 3, "bar");
m.verify; // throws if not called

To check the values passed in, pass them to expect:

m.expect!"foo"(6, "barstuff");
fun(m , 3, "bar");

Either call expect then verify or call expectCalled at the end:

fun(m, 3, "bar");
m.expectCalled!"foo"(6, "barstuff");

The return value is T.init unless returnValue is called (it's variadic):

m.returnValue!"foo"(2, 3, 4);
assert(fun(m, 3, "bar") == 2);
assert(fun(m, 3, "bar") == 3);
assert(fun(m, 3, "bar") == 4);
assert(fun(m, 3, "bar") == 0);

Structs can also be mocked:

int fun(T)(T f, int i, string s) { return f.foo(i * 2, s ~ "stuff"); }
auto m = mockStruct(2, 3, 4); // the ints are return values
assert(fun(m, 3, "bar") == 2);
m.expectCalled!"foo"(6, "barstuff");

Command-line Parameters

There is support for debug prints in the tests with the -d switch. This is only supported in single-threaded mode (-s). Setting -d without -s will trigger a warning followed by the forceful use of -s. TestCases and test functions can print debug output with the function writelnUt available here.

Tests can be run in random order instead of in threads. To do so, use the -r option. A seed will be printed so that the same run can be repeated by using the --seed option. This implies running in a single thread.

Integration tests and a sandbox environment

If you want to write tests that read from and write to the file system, you can use the Sandbox struct from unit_threaded.integration like so:

with(immutable Sandbox()) {
    writeFile("foo.txt", "foobarbaz\ntoto"); // can also pass string[] for lines
    shouldEqualLines("foo.txt", ["foobarbaz", "toto"]);

By default the sandbox main path is tmp/unit-threaded but you can change that by calling Sandbox.setPath

Test Registration and Test Runner

There are two example programs in the example folder, one with passing unit tests and the other failing, to show what the output looks like in each case. Because of the way D packages work, they must be run from the top-level directory of the repository.

The built-in D unittest blocks are included automatically, as seen in the output of both example programs (example.tests.pass_tests.unittest and its homologue in example_fail). A name will be automatically generated for them. The user can specify a name by decorating them with a string UDA or the included @Name UDA.

The easiest way to run tests is by doing what the example code does: calling runTests() in runner.d with the modules containing the tests as compile-time arguments. This can be done as symbols or strings, and the two approaches are shown in the examples.

There is no need to register tests. The registration is implicit and happens with:

  • D's unittest` blocks
  • Functions with a camelCase name beginning with test (e.g. testFoo())
  • Classes that derive from TestCase and override test()

The modules to be reflected on must be specified when calling runTests, but that's usually done as shown in the dub configuration above. Private functions are skipped. TestCase also has support for setup() and shutdown(), child classes need only override the appropriate functions(s).

Don't like the algorithm for registering tests? Not a problem. The attributes @UnitTest and @DontTest can be used to opt-in or opt-out. These are used in the examples. Tests can also be hidden with the @HiddenTest attribute. This means that particular test doesn't get run by default but can still be run by passing its name as a command-line argument. HiddenTest takes a compile-time string to list the reason why the test is hidden. This would usually be a bug id but can be anything the user wants.

Since D packages are just directories and there the compiler can't read the filesystem at compile-time, there is no way to automatically add all tests in a package. To mitigate this and avoid having to manually write the name of all the modules containing tests, a dub configuration called gen_ut_main runs unit-threaded as a command-line utility to write the file for you.

  • dunit: xUnit Testing Framework for D
  • DMocks-revived: a mock-object framework that allows to mock interfaces or classes
  • deject: automatic dependency injection
  • specd: a unit testing framework inspired by specs2 and ScalaTest
  • DUnit: a toolkit of test assertions and a template mixin to enable mocking
  • Atila Neves
2.2.0 2024-May-28
2.1.9 2024-Jan-23
2.1.8 2023-Nov-02
2.1.7 2023-Jul-31
2.1.6 2023-Apr-25
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