Is there a .Net library similar to GNU readline?

You may want to checkout Miguel de Icaza's getline.cs (the link in the blog post is broken, the code can now be found here). Depending on what features of readline you actually need, it might be enough for your purposes.

The nice thing is, that it is all contained in a single (hence getline.cs) file and MIT X11 licensed.

Using it is pretty easy.

If you want to give it try, just download the file and compile it:

C:\> csc.exe /d:DEMO getline.cs 
C:\> getline.exe
shell>

The #ifdef DEMO part also shows the basic REPL:

var le = new LineEditor("whatever");
string s;

while ((s = le.Edit("my prompt> ", "")) != null)
{
    // User input from command line / prompt now in "s".
}

The only thing I know of is Mono-Readline.

It provides a .NET interface to the GNU Readline library - it's a bit raw though, only version 0.0.1, and I've only ever seen it run on the Mono runtime.

You should be careful with licensing too ... AFAIK anything that links the GNU Readline libraries is required to be released under the GPL.

a project that aims to emulate most of the functions of RedLine is Deveel ReadLine, but I have to say that it isn't maintained. Last time I used it it worked very well on both .NET and Mono, although there's a small bug when canceling a line that was automatically folded.