Changes version information in AssemblyInfo.cs files recursively from a given path. I use this with TeamCity to control the version number as the built in Build Feature runs before my psake script can obtain the version (clearing out the version!).

SetAssemblyInfoToVersion.ps1

# SetAssemblyVersion.ps1
#
# Sets the AssemblyVersion and AssemblyFile version attributes in assemblyinfo.cs or assemblyinfo.vb files
# for the specified path.
#
# from: https://gist.github.com/derekgates/4678882
#
# http://www.luisrocha.net/2009/11/setting-assembly-version-with-windows.html
# http://blogs.msdn.com/b/dotnetinterop/archive/2008/04/21/powershell-script-to-batch-update-assemblyinfo-cs-with-new-version.aspx
# http://jake.murzy.com/post/3099699807/how-to-update-assembly-version-numbers-with-teamcity
# https://github.com/ferventcoder/this.Log/blob/master/build.ps1#L6-L19
#

Param(
    [switch]$h=$false,
    [string]$path=$pwd,
    [ValidateScript({
                     If ($_ -notmatch "[0-9]+(\.([0-9]+|\*)){1,3}") {
                        Throw "$_ is not a valid version number."
                     } Else {$True}
                    })]
    [string]$version,
    [ValidateScript({
                     If ($_ -notmatch "[0-9]+(\.([0-9]+|\*)){1,3}") {
                        Throw "$_ is not a valid version number."
                     } Else {$True}
                    })]
    [string]$fileversion=""
)

function Help {
    "Sets the AssemblyVersion and AssemblyFileVersion of AssemblyInfo.cs files`n"
    " > .\SetAssemblyVersion.ps1 -version [VersionNumber] -fileversion [FileVersionNumber] -path [SearchPath]"
    " "
    "    [VersionNumber]         The AssemblyVerion number to set, for example: 1.1.9301.0"
    "    [FileVersionNumber]     The AssemblyFileVersion number to set."
    "    [SearchPath]            The path to search for AssemblyInfo files."
    " "
    "Example > SetAssemblyVersion.ps1 -version 1.1.42 -fileversion 1.1.42.* -path ./ "
    " "
}

function Update-SourceVersion
{
    Param ([string]$Version)

    $assemblyVersionPattern = 'AssemblyVersion\("[0-9]+(\.([0-9]+|\*)){1,3}"\)'
    $fileVersionPattern = 'AssemblyFileVersion\("[0-9]+(\.([0-9]+|\*)){1,3}"\)'
    $assemblyVersionValue = 'AssemblyVersion("' + $Version + '")'
    $fileversion = if($fileversion) { $fileversion } else { $version } 
    $fileVersionValue = 'AssemblyFileVersion("' + $fileversion + '")'
    
    foreach ($o in $input) 
    {
        Write-Host "Updating  '$($o.FullName)' "
        
        Set-ItemProperty $o.FullName -name IsReadOnly -value $false
        
        (Get-Content $o.FullName) | ForEach-Object  {            
           % {$_ -replace $assemblyVersionPattern, $assemblyVersionValue } |
           % {$_ -replace $fileVersionPattern, $fileVersionValue }
        } | Out-File $o.FullName -encoding UTF8 -force
    }
}

function Update-AllAssemblyInfoFiles ( $version )
{
    Write-Host "Searching '$path'"
   foreach ($file in "AssemblyInfo.cs", "AssemblyInfo.vb" ) 
   {
        get-childitem $path -recurse |? {$_.Name -eq $file} | Update-SourceVersion $version ;
   }
   Write-Host "done."
}

# ---------------------------------------------------
if($h)
{
    Help
    exit 1
}

Write-Host "Updating:  path= '$path'  version= '$version'  fileversion= '$fileversion'"

Update-AllAssemblyInfoFiles $version
exit $LASTEXITCODE