sting.Equals(string a,string b)
 
Explanation:
 Determines whether two specified system.string objects have
the same values,a parameter specifies the culture,case and
sort rules used in the comparison

Here is the code that solve ur problem:

       string str = "virtual";
        if (str.Equals("virtual"))
        {
          // your code here....
        }

string str1="Hello1";
string str2="Hello2";

String.Compare(str1,str2,true)

//true if you want compare two string without case 
sensative and false if you want to case sensative.

//It will return Zero if compare as equal and 1 if not.