Whats the difference between <RequiredTargetFramework> and <targetframeworkversion>?

We upgraded our .net 3.5 projects (c#) to .net 4.0. When you look at the project file there are t开发者_高级运维wo tags that I'm trying to make sense out of:

<RequiredTargetFramework>3.5</RequiredTargetFramework>
<TargetFrameworkVersion>4.0</TargetFrameworkVersion>

Why are there two seemingly similar tags with different values?

The <RequiredTargetFramework> element was already present in your 3.5 project. It's associated with the assembly <Reference> and only present on assemblies that are not available in .NET 2.0

I don't buy much stock in the single mention of it in MSDN, I don't see how batch building has anything to do assembly references. Nor is it used in any of the 3.5 MSBuild .target files. I think the IDE simply uses it to put the warning icon next to the reference in the References node when you change the Target Framework to a version less than what's needed to support the assembly.

There are other elements like this in a project file that don't affect MSBuild but have an effect in the IDE. Like <SubType> and <DependentUpon> in the <Compile> element.

Have you found this one link? link text. TargetFrameworkversion is easy, that's the one you can change in the project properties to say which framework to build against. The article says that RequiredTargetFramework is used to batch items (but it's still not clear on it' real purpose other than it's not used a lot)

batches the Reference items by their RequiredTargetFramework metadata. The output of the target looks like this:

Reference: 3.5;3.5

Reference: 4.0

Target batching is seldom used in real builds. Task batching is more common. For more information, see MSBuild Batching.