There was a similar question about database schemes: Copying a competitor's database schema? I answered there that copying the whole schema would probably be a copyright infringement, depending on the scale of the copying, appreciated by a judge.
Even if your question is similar, I will answer quite the opposite: you can copy method signatures. Why?
In fact, method signatures come from common sense.
Firstly, if we take an extreme example, Microsoft cannot copyright the name
Random and sue everyone who will use the word
Random in any application.
Secondly, what are you trying to do exactly? Reimplementing .NET Framework? Why? There is no need to reinvent the wheel. If you know how to make it better, more intuitive, etc., chances are you will come with better names of classes and methods, a better organization, etc. If you try to copy the structure of .NET Framework to port it to another language, then you're not a direct competitor of Microsoft, so they have no serious reasons to sue you (this being said, they will sue you if you copy the source code itself). Actually, they will even benefit from you: copying such structure to other frameworks will not only show the success of .NET Framework, but will also make it easier for .NET developers to work with other languages, and the developers from other languages to learn .NET.
Thirdly, they have more serious things to do than to sue everyone who will copy the names of the methods and classes of .NET Framework.
Now, if your intent is to copy the whole .NET Framework and to make a product which will be used by lots of developers, consult a lawyer before taking risks.
I was totally unhappy with the shaming 259 characters limitation in file paths in .NET Framework and the inability to use code transactions on file level. So I implemented my own
File class with the methods which would work as expected for any path, not just the tiny short ones, and implement transactions. In the first version, I decided to clone the names of methods in
Directory classes. Would Microsoft sue me for doing that? I doubt it.
In all cases, in the second version I came with a new syntax,
new File(string fileName), which I find much more intuitive for me. Cloning names of methods is sometimes useful, but you have to have a good reason to do it.