How does one use the Try-Catch-Finally block in a class method?

Question

I get contradictory error messages when trying to set up a try-catch-finally block in a class method which returns a string.

My code is:

class exampleClass {    
    [string]Create() {
        try {
            $rsa_Base64 = "string"            
            return $rsa_Base64
        }
        catch {
            {...}
        }
        finally {
            Remove-Item -Path $env:TEMP\test.txt
        }        
    }
}

In this situation I get the error: Not all code path returns value within method.

If I move the return statement to the finally block I get the error: Flow of control cannot leave a Finally block.

I want to add the finally block in order to ensure the removal of created files in the try statement.

mklement0 · Accepted Answer · 2020-12-18T23:00:30.000

You can avoid the problem by using the return statement unconditionally, outside the try / catch/ finally statement:

class exampleClass {    
    [string]Create() {
        $rsa_Base64 = ''  # must initialize the variable
        try {
            $rsa_Base64 = "string"            
        }
        catch {
            {...}
        }
        finally {
            Remove-Item -Path $env:TEMP\test.txt
        }
        return $rsa_Base64
    }
}

^{Note the use of '' to initialize the variable; using $null would ultimately not be effective, because the method is [string]-typed, and PowerShell doesn't support $null in [string] values and converts them to '' (the empty string) - see this answer for more information.}

More properly, however, you should ensure that your catch branch contains a scope-exiting flow-control statement as well^[1], namely return or throw (while exit technically works too, it should only be used in script files, as it exits the session as a whole):

class exampleClass {    
    [string]Create() {
        try {
            $rsa_Base64 = "string"            
            return $rsa_Base64
        }
        catch {
            {...}
            return ''  # must exit the method from this block too 
        }
        finally {
            Remove-Item -Path $env:TEMP\test.txt
        }        
    }
}

^{[1] By contrast, the finally block not only doesn't need a scope-exiting statement, by design it doesn't support any. The reason is that this block is run outside the normal flow of control and is intended solely for cleanup.}

How does one use the Try-Catch-Finally block in a class method?

1 Answers1