C# string “likeness” comparison test

Are there any libraries out there that make it easy to compare the "likeness" of 2 strings in C#? What I am thinking is if you have the strings,

"This is a test" // 100%
"THIS IS A TEST" // 95%
"    This is a test" // 98%
"This is" // 60%
"Foobar" // 0%
"this test is a" // 70%

etc. They would each be given a score compared to a base string.