Define and use a “variable” in a Target - msbuild

Question

In a Target, I need to use %(Foo.Filename)%(Foo.Extension) several times.
Is there a way to define a “variable” (or something similar) that can be used instead of %(Foo.Filename)%(Foo.Extension)? Preferably inside the Target?
(For the sake of completeness: %(Foo.Identity) is something different.)

Answer 1

Depends on how the % is being used in that target, as it's not just a shorthand to reference metadata as if it's a class property but a method of batching item groups. Sans some edge cases, e.g. item group of 1, creating a property during execution won't work as it'll overwrite itself on loop; most common way would be to either do target batching, transforms or custom metadata.
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<ItemGroup>
<Foo Include="*.cs" />
</ItemGroup>
<Target Name="Foo">
<ItemGroup>
<Foo>
<Lorem>Ipsum.%(Filename)%(Extension)</Lorem>
</Foo>
</ItemGroup>
<Message Text="%(Foo.Lorem)" />
</Target>
<Target Name="Bar">
<ItemGroup>
<Lorem Include="Ipsum.%(Foo.Filename)%(Extension)" />
</ItemGroup>
<Message Text="#(Lorem)" />
<Message Text="#(Foo -> 'Ipsum.%(Filename)%(Extension)')" />
</Target>
<Target Name="Baz" Inputs="#(Foo)" Outputs="Ipsum.%(Filename)%(Extension)">
<PropertyGroup>
<Lorem>Ipsum.%(Foo.Filename)%(Extension)</Lorem>
</PropertyGroup>
<Message Text="$(Lorem)" />
</Target>
</Project>