MessageDigest equivalent to C#?

Question

What is the equivalent C# code of below Java code?

 

java.security.MessageDigest d = null;

d = java.security.MessageDigest.getInstance("SHA-1");

d.reset();

d.update(user_password.getBytes());

user_password = new String(d.digest());

user_password=user_password.replaceAll("\ufffd","?");

 

FYI... this code returns something likd ~£ž¹_WÁÞQAÐùß9Ã0Ï which I never done in C#.

 

Thanks in Advance !!!