How to read the assemblyversion from assemblyInfo.cs?

You can read lines from files, get the string using regex and change it if you need. And if you're using MSBuild 4.0 you can use Property Functions, which give you an access to .NET API. This example should give you first three numbers of the AssemblyVersion.

<Target Name="ReadAssemblyVersion">

    <ReadLinesFromFile File="$(VersionFile)">
        <Output TaskParameter="Lines"
                ItemName="ItemsFromFile"/>
    </ReadLinesFromFile>

    <PropertyGroup>
        <Pattern>\[assembly: AssemblyVersion\(.(\d+)\.(\d+)\.(\d+)</Pattern>
        <In>@(ItemsFromFile)</In>
        <Out>$([System.Text.RegularExpressions.Regex]::Match($(In), $(Pattern)))</Out>
    </PropertyGroup>

    <Message Text="Output : $(Out.Remove(0, 28))"/>

</Target>

http://blogs.msdn.com/b/visualstudio/archive/2010/04/02/msbuild-property-functions.aspx

Awesome thread, building upon the work of Alex and RobPol, I was able to define extended msbuild properties that is inspired by semver.org (Major, Minor, Patch, PreRelease). I chose to parse the AssemblyInformalVersion since that is the only attribute compatible with SemVer. Here is My example:

<PropertyGroup>
    <In>$([System.IO.File]::ReadAllText('$(MSBuildProjectDirectory)\Properties\AssemblyInfo.cs'))</In>
    <Pattern>\[assembly: AssemblyInformationalVersion\("(?&lt;Major&gt;\d+)\.(?&lt;Minor&gt;\d+)\.(?&lt;Patch&gt;[\d]+)(?&lt;PreReleaseInfo&gt;[0-9A-Za-z-.]+)?</Pattern>
    <AssemblyVersionMajor>$([System.Text.RegularExpressions.Regex]::Match($(In), $(Pattern), System.Text.RegularExpressions.RegexOptions.Multiline).Groups["Major"].Value)</AssemblyVersionMajor>
    <AssemblyVersionMinor>$([System.Text.RegularExpressions.Regex]::Match($(In), $(Pattern), System.Text.RegularExpressions.RegexOptions.Multiline).Groups["Minor"].Value)</AssemblyVersionMinor>
    <AssemblyVersionPatch>$([System.Text.RegularExpressions.Regex]::Match($(In), $(Pattern), System.Text.RegularExpressions.RegexOptions.Multiline).Groups["Patch"].Value)</AssemblyVersionPatch>
    <AssemblyVersionPreRelease>$([System.Text.RegularExpressions.Regex]::Match($(In), $(Pattern), System.Text.RegularExpressions.RegexOptions.Multiline).Groups["PreReleaseInfo"].Value)</AssemblyVersionPreRelease>
</PropertyGroup>

You can test the output of this operation by adding the following to your .csproj:

  <Target Name="AfterBuild">
    <Message Text="$(AssemblyVersionMajor)"></Message>
    <Message Text="$(AssemblyVersionMinor)"></Message>
    <Message Text="$(AssemblyVersionPatch)"></Message>
    <Message Text="$(AssemblyVersionPreRelease)"></Message>
</Target>

Ex: Snippet from my AssemblyInfo.cs:

[assembly: AssemblyInformationalVersion("0.9.1-beta")]

Will output: Major: '0', Minor: '9', Patch: '1', PreRelease: '-beta'