if ($Csc = gci "$env:windir\Microsoft.NET\Framework64\*\csc.exe" -ea silent | select -last 1) {
    Set-Alias -Name csc -Value $Csc
    Remove-Variable Csc
  }

Or even:

  gci "$env:windir\Microsoft.NET\Framework64\*\csc.exe" -ea silent | % {
    Set-Alias -Name csc -Value $_
  }