How to use property functions inside transform modifiers in msbuild? - msbuild

Question

I have not found a correct syntax that would allow a transform modifiers to use property functions. For example, if we want to trim each filename in a list starting with the "lib" string:
<ListWithoutLib>#(MyOriginalList->%(Filename.TrimStart("lib"))</ListWithoutLib>
Is there any msbuild voodoo that could be written to accomplish this?

Answer 1

There is more than one issue in the question.
First, TrimStart doesn't have an overload that accepts a string and it doesn't remove a substring from the beginning of a string. Instead TrimStart accepts a set of chars and remove all instances of each char from the beginning of string.
For example in C#
"libFoo.dll".TrimStart('l', 'i', 'b')
will produce 'Foo.dll' and
"bilbobaggins.dll".TrimStart('l', 'i', 'b')
will produce 'obaggins.dll'.
But that issue is secondary to the question being asked.
You cannot use a property function inside of a metadata transform but you can apply a method of the string class as an item function.
<ItemGroup>
<MyOriginalList Include="libapple;libboat;cat;dog" />
</ItemGroup>
<PropertyGroup>
<ListWithoutLib>#(MyOriginalList->TrimStart('l'))</ListWithoutLib>
</PropertyGroup>
<Target Name="Default">
<Message Text="ListWithoutLib = $(ListWithoutLib)" />
</Target>
<!--
Output:
Default:
ListWithoutLib = ibapple;ibboat;cat;dog
-->
See String item functions.
This example shows how TrimStart can be called on an item collection but it doesn't do what you indicated that you need.
A solution to what you seem to need might be as follows:
<ItemGroup>
<SourceList Include="libapple;libboat;cat;dog"/>
</ItemGroup>
<PropertyGroup>
<libPrefix>lib</libPrefix>
</PropertyGroup>
<ItemGroup>
<MyOriginalList Include="#(SourceList)">
<IsStartsWithLibPrefix>$([System.String]::Copy('%(Filename)').StartsWith('$(libPrefix)'))</IsStartsWithLibPrefix>
<NameWithoutPrefix Condition="!'%(IsStartsWithLibPrefix)'">%(Filename)</NameWithoutPrefix>
<NameWithoutPrefix Condition="'%(IsStartsWithLibPrefix)'">$([System.String]::Copy('%(Filename)').Substring($(libPrefix.Length)))</NameWithoutPrefix>
</MyOriginalList>
</ItemGroup>
<Target Name="Default">
<Message Text="List of Filename:" />
<Message Text="#(MyOriginalList->' %(Filename)','%0d%0a')" />
<Message Text="List of NameWithoutPrefix:" />
<Message Text="#(MyOriginalList->' %(NameWithoutPrefix)','%0d%0a')" />
</Target>
<!--
Output:
Default:
List of Filename:
libapple
libboat
cat
dog
List of NameWithoutPrefix:
apple
boat
cat
dog
-->
There is a difference between item functions and property functions. Item functions are limited to being called on a collection reference, e.g. #(MyOriginalList). Item batching could be used to split the collection into references to the items with and without the lib prefix. Batching requires a target or task to batch over. The code that is shown forgoes using item functions and instead uses property functions. This allows all the 'work' to be done in the ItemGroup definition.
Assuming the inputs are not static, we need to determine which filenames have the prefix.
Although metadata values are strings, string methods can't be called on metadata values. A well known work-around is to create a new string object from the metadata value and then call a string method. [System.String]::Copy('%(Filename)') is creating a string object that is a copy of the value of the Filename metadata. The code $([System.String]::Copy('%(Filename)').StartsWith('$(libPrefix)')) is a property function call and the IsStartsWithLibPrefix metadata will be set to either True or False.
Because ItemGroups can be self-referential, we can use the IsStartsWithLibPrefix metadata to define the NameWithoutPrefix metadata. The NameWithoutPrefix definition has a condition. For a given item in the MyOriginalList ItemGroup only one variant of the NameWithoutPrefix definition will be used. For Items that have the prefix another property function call is used. Substring is called to create a new string instance that doesn't have the prefix.