# Resolving package version conflicts in Dart

Source: https://iiro.dev/resolving-dart-package-version-conflicts/
Published: 2018-08-28

Learn how Dart pub resolves package version conflicts and how to work with dependency constraints when packages disagree.

---

**Disclaimer: This is one of those articles that you'll really want to read until to the very end.**

**Another disclaimer: This article was written a long time ago. Look at [pub outdated](https://dart.dev/tools/pub/cmd/pub-outdated) instead.**

The more you play around with Flutter and introduce new dependencies in your project, the more likely it is to run into a dependency conflict at some point. Especially around the time when [significant updates](https://medium.com/dartlang/dart-2-stable-and-the-dart-web-platform-3775d5f8eac7) happen. 

Chances are, sooner or later you'll encounter something like this:

<pre class="highlight" style="white-space: pre-line; color: #ff6a64;">
Because intl_translation 0.17.0 depends on petitparser ^1.1.3 and xml >=3.2.0 depends on petitparser ^2.0.0, intl_translation 0.17.0 is incompatible with xml >=3.2.0.

So, because my_project depends on both xml ^3.2.0 and intl_translation 0.17.0, version solving failed.
</pre>

From the error message, it's clear that these versions of the `xml` and `intl_translation` packages don't play together nicely. Since those packages depend on a different major version of `petitparser`, there's an unresolvable dependency version conflict here.

The pubspec file might look a little something like this:


**File:** `pubspec.yaml`

```yaml
dependencies:
  # ...
  xml: ^3.2.0
  intl_translation: ^0.17.0
```

Although _we could_ go to [pub.dartlang.org](https://pub.dartlang.org/) and find the compatible versions manually by trial and error, we don't have to. There's something that's _made exactly for that_. It's also built into Pub and way faster than we could ever be.

## The solution

The fastest way to resolve this problem is to set the versions of both of the conflicting dependencies to `any`.

**Hold on - I know what your thinking. We will NOT leave them as `any`.**


**File:** `pubspec.yaml`

```yaml
dependencies:
  # ...
  xml: any # <- don't leave me like this - read further!
  intl_translation: any # <- don't leave me like this either!
```

That _loosens_ the version constraints from major versions (`1.x.x - 2.x.x`) to, you know, _any_ version of the package that has ever existed. 

**This lets [pub's version constraint solver](https://medium.com/@nex3/pubgrub-2fb6470504f) do its magic** and figure out the compatible packages for you. After that change, refetch your dependencies by running `flutter packages get`. There's a good chance that the following happens:

```
Resolving dependencies... 
Got dependencies!
```

**We're not done yet!**

After getting the project to build, **you should tighten the dependency versions back** to use [semantic versioning](https://www.dartlang.org/tools/pub/versioning#semantic-versions) like they previously did. Open the generated `pubspec.lock` file and find the dependencies that were previously conflicting.


**File:** `pubspec.lock`

```yaml
# Generated by pub
# See https://www.dartlang.org/tools/pub/glossary#lockfile
packages:
  xml:
    # ...
    version: "3.0.1" # the version of "xml" package that worked fine
                     # with "intl_translation".
  intl_translation:
    # ...
    version: "0.17.0" # the version of "intl_translation" package
                      # that worked fine with "xml".
```

From that lockfile, we can see that the `xml` package version `3.0.1` and `intl_translation` package version `0.17.0`  play along together well. As the last step, replace `any` with the correct versions on your pubspec file:


**File:** `pubspec.yaml`

```yaml
dependencies:
  # ...
  xml: ^3.0.1
  intl_translation: ^0.17.0
```

Refetch your dependencies one last time by running `flutter packages get` to verify that this does indeed work and then you're good to go.

**Remember: you should never leave your versions as `any`** - that's simply just asking for trouble. Find the compatible versions from pubspec.lock and use those ones. Having nondeterministic dependencies is a surefire way of breaking your app in hard to debug ways in the future. Just don't do it.