If you are checking for proper errors, use the `ShouldBeErrorId` helper defined in HelpersCommon.psm1 module which is in your path if you import `build.psm1`.
Checking against `FullyQualifiedErrorId` is recommended because it does not change based on culture as an error message might.
Note that if get-item were to succeed, a different FullyQualifiedErrorId would be thrown and the test will fail.
This is the suggested path because Pester wants to check the error message, which will likely not work here because of localized builds, but the FullyQualifiedErrorId is constant regardless of the locale.
However, if you need to check the `InnerException` or other members of the ErrorRecord, the recommended pattern to use is:
For creation of PowerShell tests, the Describe block is the level of granularity suggested and one of three tags should be used: "CI", "Feature", or "Scenario". If the tag is not provided, tests in that describe block will be run any time tests are executed.
Creates a logical group of tests. All Mocks and TestDrive contents defined within a Describe block are scoped to that Describe; they will no longer be present when the Describe block exits. A `Describe block may contain any number of Context and It blocks.
Provides logical grouping of It blocks within a single Describe block. Any Mocks defined inside a Context are removed at the end of the Context scope, as are any files or folders added to the TestDrive during the Context block's execution. Any BeforeEach or AfterEach blocks defined inside a Context also only apply to tests within that Context .
#### It
The It block is intended to be used inside of a Describe or Context Block. If you are familiar with the AAA pattern (Arrange-Act-Assert), the body of the It block is the appropriate location for an assert. The convention is to assert a single expectation for each It block. The code inside of the It block should throw a terminating error if the expectation of the test is not met and thus cause the test to fail. The name of the It block should expressively state the expectation of the test.
A PSDrive is available for file activity during a tests and this drive is limited to the scope of a single Describe block. The contents of the drive are cleared when a context block is exited.
A test may need to work with file operations and validate certain types of file activities. It is usually desirable not to perform file activity tests that will produce side effects outside of an individual test. Pester creates a PSDrive inside the user's temporary drive that is accessible via a names PSDrive TestDrive:. Pester will remove this drive after the test completes. You may use this drive to isolate the file operations of your test to a temporary store.
Mocks the behavior of an existing command with an alternate implementation. This creates new behavior for any existing command within the scope of a Describe or Context block. The function allows you to specify a script block that will become the command's new behavior.
Code execution in Pester can be very subtle and can cause issues when executing test code. The execution of code which lays outside of the usual code blocks may not happen as you expect. Consider the following:
The DESCRIBE BeforeAll block is executed before any other code even though it was at the bottom of the Describe block, so if state is set elsewhere in the describe BLOCK, that state will not be visible (as the code will not yet been run). Notice, too, that the BEFOREALL block in Context is executed before any other code in that block.
Generally, you should have code reside in one of the code block elements of `[Before|After][All|Each]`, especially if those block rely on state set by free code elsewhere in the block.
Sometimes it is beneficial to skip all the tests in a particular `Describe` block. For example, tests which are not applicable to a platform could be skipped, and they would be reported as skipped. The following is an example of how this may be done:
```powershell
Describe "Should not run these tests on non-Windows platforms" {
Here is the output when run on a Linux distribution:
```
Describing Should not run these tests on non-Windows platforms
Context Block 1
[!] This block 1 test 1 691ms
[!] This is block 1 test 2 114ms
Context Block 2
[!] This block 2 test 1 73ms
[!] This is block 2 test 2 6ms
```
and here is the output when run on a Windows distribution:
```
Describing Should not run these tests on non-Windows platforms
Context Block 1
[+] This block 1 test 1 86ms
[+] This is block 1 test 2 33ms
Context Block 2
[-] This block 2 test 1 52ms
Expected: {1}
But was: {2}
22: 2 | should be 1
at <ScriptBlock>,<Nofile>:line22
[-] This is block 2 test 2 77ms
Expected: {1}
But was: {2}
25: 2 | should be 1
at <ScriptBlock>,<Nofile>:line25
```
this technique uses the `$PSDefaultParameterValues` feature of PowerShell to temporarily set the It block parameter `-skip` to true (or in the case of Windows, it is not set at all)
3. Be sure to tag your `Describe` blocks based on their purpose
1. Tag `CI` indicates that it will be run as part of the continuous integration process. These should be unit test like, and generally take less than a second.
2. Tag `Feature` indicates a higher level feature test (we will run these on a regular basis), for example, tests which go to remote resources, or test broader functionality
3. Tag `Scenario` indicates tests of integration with other features (these will be run on a less regular basis and test even broader functionality than feature tests.
4. Make sure that `Describe`/`Context`/`It` descriptions are useful
1. The error message should not be the place where you describe the test
5. Use `Context` to group tests
1. Multiple `Context` blocks can help you group your test suite into logical sections
6. Use `BeforeAll`/`AfterAll`/`BeforeEach`/`AfterEach` instead of custom initiators
7. Prefer Try-Catch for expected errors and check $_.fullyQualifiedErrorId (don't use `should throw`)
8. Use `-testcases` when iterating over multiple `It` blocks
9. Use code coverage functionality where appropriate
10. Use `Mock` functionality when you don't have your entire environment
11. Avoid free code in a `Describe` block
1. Use `[Before|After][Each|All]` see [Free Code in a Describe block](WritingPesterTests.md#free-code-in-a-describe-block)
12. Avoid creating or using test files outside of TESTDRIVE:
1. TESTDRIVE: has automatic clean-up
13. Keep in mind that we are creating cross platform tests
1. Avoid using the registry
2. Avoid using COM
14. Avoid being too specific about the _count_ of a resource as these can change platform to platform
1. ex: checking for the count of loaded format files, check rather for format data for a specific type