PowerShell environment variable in Jenkins Pipeline gives `org.jenkinsci.plugins.workflow.cps.EnvActionImpl@1b69f5bb:VARIABLE_NAME`

The syntax forms $env:BRANCH_NAME and the equivalent ${env:BRANCH_NAME} are PowerShell constructs, which means that in order to pass them through to PowerShell in an interpolating Groovy string ("..."), you must \-escape the $ character to prevent Groovy from interpreting the construct up front:

powershell "dotnet sonarscanner begin ... /d:sonar.branch.name=\$env:BRANCH_NAME"

That said, given that your command contains no Groovy variables that need to be interpolated (it's safe and more robust to let PowerShell reference environment variables), you can simply use a literal Groovy string, '...', where $ chars. destined for PowerShell need no escaping:

powershell 'dotnet sonarscanner begin ... /d:sonar.branch.name=$env:BRANCH_NAME'

As for what you tried:

"... $env:BRANCH_NAME" in an interpolating Groovy string causes Groovy to interpolate variable env (since it is preceded by $), and treat :BRANCH_NAME as a literal.

Since env refers to the object that contains all environment variables, what you're seeing is the (unhelpful) stringification of that object, which is the class name (org.jenkinsci.plugins.workflow.cps.EnvActionImpl) followed by an instance-specific hash code (@54ee3a8).

Using ${env.BRANCH_NAME} would have worked - given that the value of environment variable BRANCH_NAME can be accessed as a property of the env object - but note that this means that Groovy interpolates the value up front and that PowerShell then only sees the resulting value.

In simple cases (environment-variable values without whitespace or special characters), "${env.BRANCH_NAME}" (up-front interpolation by Groovy) and "\${env:BRANCH_NAME}" (later interpretation by PowerShell) are interchangeable, but only the latter approach works robustly with all values.

Turns out specifying the environment variable this way works:

powershell "dotnet sonarscanner begin /k:project-key /d:sonar.branch.name=${env:BRANCH_NAME}"

You can also use a period instead of a colon (${env.BRANCH_NAME}).