stringReversal.cs

public static string reverseString(string str)
{
    return !string.IsNullOrEmpty(str) ? reverse(str) : string.Empty;
}

private static string reverse(string str)
{
    var stk = new Stack();
    for (int i = 0; i < str.Length; i++)
    {
        stk.push(str[i]);
    }

    int k = 0;
    while(stk.count > 0)
    {
        str[k++] = stk.top();
        stk.pop();
    }
    return str;
}