lesson_59.cs

class Program
{
    static void Main(string[] args)
    {
        string s = "Привет мир";
        char c = 'и';
        int i = s.CharCount(c);
        Console.WriteLine(i);

        Console.Read();
    }
}

public static class StringExtension
{
    public static int CharCount(this string str, char c)
    {
        int counter = 0;
        for (int i = 0; i < str.Length; i++)
        {
            if (str[i] == c)
                counter++;
        }
        return counter;
    }
}