When using different contracts that broker interaction between two apps, it’s a natural question to wonder if your app is really going to work with all other arbitrary apps. This is the intent of contracts in the first place, but it’s good to put apps in the Store when you’re reasonably confident that they’ll work according to that intent.
With sharing data through the Share contract, in particular, it really boils down to finding target apps that can consume the data formats you provide. If you’re using only known formats (HTML, text, bitmap, files), then the Share target sample app in the Windows SDK provides a good generic target. When you invoke it through Share, it basically shows you all the data it finds in the shared package, which is very helpful for checking out that the package contains the data you think it should.
You’ll then likely want to test with some likely common targets. The built-in Mail app is a good candidate, as is the Twitter app that you can find in the Windows Store.
For custom formats, you probably don’t share such data without having some target app in mind, so you’ll know who to test with.
Beyond that, the APIs for sharing are actually designed so that you don’t need to worry too much about more extensive testing. The only way standard format data gets into a shared data package is through method calls like DataPackage.setBitmap and DataPackage.setUri. That is, rather than having the source app just drop a string or some other object into the package directly, these method calls can validate the data before making it available. This guarantees that the target app will have good data to consume, meaning that the target app can just examine the available formats and use them how it will.