best way to compare two strings and their upper and lower case signs

best way to compare two strings and their upper and lower case signs

Let's say I have 2 strings.First string is x = "abc" and the second one is
y = "ABC".When I write in c# following code:
if(x == y)
or
if(x.Equals(y))
return value is true.How can I check their upper and lower case?