String compression by using repeated characters count
String as an Array

You got your string parameter:

string originalString


Then you turned it into a list:

OriginalList.AddRange(originalString);


Then you turned it into an array:

charArray = OriginalList.ToArray();


And finally used it as:

character = (charArray[i]);


Instead of all that, you could have just done this:

character = (originalString[i]);


Class Design

Instantiating a StringCompression class every time you wanna compress a string can be avoided by just declaring the function CompressionMethod as static since the class has no members anyway. So in Main, you only have to do:

StringCompression.CompressionMethod("String here");


Or better: make it an extension method.

String Builder

Instead of a list of strings in:

CompressedList = new List&lt;string&gt;();


You could instead use a String Builder to create the resulting string.