What is Microsoft.csharp.dll in .NET 4.0

Question

It is used when/if you use the dynamic keyword in your project. The assembly contains the C# runtime binder.

The C# compiler has essentially been extracted out into a library so that it can emit, compile and run code needed to support the dynamic keyword. The first time you use dynamic in your code, this assembly (as well as System.dll, System.Core.dll and System.Dynamic.dll) will get loaded into your AppDomain.

Always removed it so far. No issues yet.