Instead of comparing using "==" operator, use String.Equals method.
If you need a case-insensitive comparison just use System.StringComparison.OrdinalIgnoreCase
StringComparison enum.
Description from MSDN:
basic ordinal comparison (System.StringComparison.Ordinal) is case-sensitive, which means that the two strings must match character for character: "and" does not equal "And" or "AND". A frequently-used variation is System.StringComparison.OrdinalIgnoreCase, which will match "and", "And", and "AND". StringComparison.OrdinalIgnoreCase is often used to compare file names, path names, network paths, and any other string whose value does not change based on the locale of the user's computer. For more information
Further info can be found here
In your case I would use the following condition:
if (usertype.Equals("yahoo", StringComparison.OrdinalIgnoreCase))
//then do whatever you want...
Another option is to lowercase everything but the first option is preferred.
if (usertype.ToLower() == "yahoo")
//then do whatever you want...