Probably you should try string.IsNullOrEmpty. Need actual code to say more

Could you describe how it works?

valid point. I don't know what I was thinking.

Why from "zqdrhpviqslik","xffrkbdyjveb" the most similar to "rkacypviuburk" is the first one while the second has the same number of min changes (9)? (This is the last test case here.)

Just ignore these unit tests and submit? (They use AreEqual which is not right for floating point numbers comparision.)

Nice reducing!

static char[] symbols = new char[]{'0','1','2','3','4','5','6','7','8','9','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

Why not static string symboles = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ" ?

Why?

For those curious, check "C# 6.0 in a Nutshell" by Albahari br. at Ch.4 Advanced C# - Dynamic Binding:

A dynamic object binds at runtime based on its runtime type, not its compile-time type.
...
At runtime, if a dynamic object implements IDynamicMetaObjectProvider, that interface
is used to perform the binding. If not, binding occurs in almost the same way as
it would have had the compiler known the dynamic object’s runtime type.