Powershell: resolve path that might not exist?

You want:

c:\path\exists\> $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath(".\nonexist\foo.txt")

returns:

c:\path\exists\nonexists\foo.txt

This has the advantage of working with PSPaths, not native filesystem paths. A PSPath may not map 1-1 to a filesystem path, for example if you mount a psdrive with a multi-letter drive name.

What's a pspath?

ps c:\> new-psdrive temp filesystem c:\temp
...
ps c:\> cd temp:
ps temp:\> 

temp:\ is a drive-qualified pspath that maps to a win32 (native) path of c:\temp.

-Oisin

When Resolve-Path fails due to the file not existing, the fully resolved path is accessible from the thrown error object.

You can use a function like the following to fix Resolve-Path and make it work like you expect.

function Force-Resolve-Path {
    <#
    .SYNOPSIS
        Calls Resolve-Path but works for files that don't exist.
    .REMARKS
        From http://devhawk.net/blog/2010/1/22/fixing-powershells-busted-resolve-path-cmdlet
    #>
    param (
        [string] $FileName
    )

    $FileName = Resolve-Path $FileName -ErrorAction SilentlyContinue `
                                       -ErrorVariable _frperror
    if (-not($FileName)) {
        $FileName = $_frperror[0].TargetObject
    }

    return $FileName
}

I think you're on the right path. Just use [Environment]::CurrentDirectory to set .NET's notion of the process's current dir e.g.:

[Environment]::CurrentDirectory = $pwd
[IO.Path]::GetFullPath(".\xyz")

Join-Path (Resolve-Path .) newdir\newfile.txt

This has the advantage of not having to set the CLR Environment's current directory:

[IO.Path]::Combine($pwd,"non\existing\path")

NOTE

This is not functionally equivalent to x0n's answer. System.IO.Path.Combine only combines string path segments. Its main utility is keeping the developer from having to worry about slashes. GetUnresolvedProviderPathFromPSPath will traverse the input path relative to the present working directory, according to the .'s and ..'s.