tl;dr
It is Azure that expands $(System.DefaultWorkingDirectory) before PowerShell sees the resulting commands; if the expanded $(...) value is to be seen as a string by PowerShell, it must be enclosed in quotes ('$(...)'):
Using $(...) (Azure macro syntax) embeds the Azure variable's verbatim value in the command text that PowerShell ends up interpreting.
- Note: Azure's macro syntax - which is evaluated before PowerShell sees the resulting command text - is not to be confused with PowerShell's own subexpression operator,
$(...).
For string values this means that you situationally have to surround the macro with quotes in order to make it work syntactically in PowerShell code, for which '...'-quoting (single-quoting) is best: '$(System.DefaultWorkingDirectory)'
Shayki Abramczyk's answer provides an effective solution, but let me provide some background information:
The variable expansion (substitution) that Azure performs via macro syntax ($(...)) functions like a preprocessor: it replaces the referenced variable with its verbatim value.
You need to make sure that this verbatim value works syntactically in the context of the target command.
As currently written:
$SourceDirectoryPath = $(System.DefaultWorkingDirectory) + "/solution/project/bin/Debug"
turns into the following command seen by PowerShell, assuming that the value of Azure property System.DefaultWorkingDirectory is d:\a\r1\a:
$SourceDirectoryPath = d:\a\r1\a + "/solution/project/bin/Debug"
This is a broken PowerShell command, because d:\a\r1\a - due to lack of quoting - is interpreted as a command name or path; that is, an attempt is made to execute putative executable d:\a\r1\a - see about_Parsing.
Therefore, in order for PowerShell to recognize the Azure-expanded value d:\a\r1\a as a string, you need to quote it - see about_Quoting_Rules.
Since the expanded-by-Azure value needs no further interpolation, single quotes are the best choice (for both operands, actually):
$SourceDirectoryPath = '$(System.DefaultWorkingDirectory)' + '/solution/project/bin/Debug'
In fact, you don't need string concatenation (+) at all in your case:
$SourceDirectoryPath = '$(System.DefaultWorkingDirectory)/solution/project/bin/Debug'
You could even combine that with expandable PowerShell strings ("..."), as long as the Azure-expanded value doesn't contain $-prefixed tokens that PowerShell could end up interpreting (unless that is your (unusual) intent).
One caveat re something like "$(System.DefaultWorkingDirectory)/$projectRoot/bin/Debug" (mixing an Azure-expanded value with a PowerShell variable reference) is that Azure's macro syntax ($(...)) looks the same as PowerShell's own subexpression operator, which is typically - but not exclusively - used in order to embed expressions in expandable strings (e.g., in pure PowerShell code, "1 + 1 equals $(1 + 1)").
As of this writing, the Define variables Azure help topic doesn't spell it out, but based on the official comment in a GitHub docs issue, ambiguity is avoided as follows:
There is no escape mechanism; instead, $(...) constructs that do not refer to Azure variables are left unchanged and therefore passed through to PowerShell.
In the typical case, PowerShell expressions will not look like an Azure variable reference (e.g, $($foo.bar) rather than $(foo.bar)), though hypothetically there can be ambiguity: $(hostname), which is a valid PowerShell subexpression, could be preempted by Azure if a hostname Azure variable were defined.
- In such a corner case, the solution is to avoid use of an inline script and instead place the code in an external script file.
