using System;

​

public class Program

{

    public static void Main()

    {

        string detail = "google:blablabla";

        string result = GetValue("google:",detail,"not google");

        Console.WriteLine(result);

        string result2 = GetValue("stackoverflow:",detail,"not stack overflow");

        Console.WriteLine(result2);     

    }

    public static string GetValue(string token, string text, string defaultValue=null){

        return text.StartsWith(token) ? text.Replace(token, string.Empty).Trim() : defaultValue;

    }

}