string method extension

/// <summary>
/// Summary description for StringExtensions
/// </summary>
public static class StringExtensions
{
    /// <summary>
    /// Will return false if str is null or empty.
    /// </summary>
    /// <param name="str"></param>
    /// <returns></returns>
    public static bool SmtContains(this string str1, string str)
    {
        return (string.IsNullOrEmpty(str)) ? false : str1.Contains(str);
    }

    /// <summary>
    /// Retrieves a substring from this instance. The substring starts at 0
    /// and has a specified length.       
    /// </summary>
    /// <param name="length">The number of characters in the substring.</param>
    /// <returns>A System.String equivalent to the substring of length length that begins
    ///     at startIndex in this instance.</returns>
    public static string FitSubstring(this string str, int length)
    {
        return (str.Length > length) ? str.Substring(0, length) : str;
    }

    /// <summary>
    /// Remove characters in string.    
    /// </summary>
    /// <param name="str">Like "-'|/?*"</param>
    /// <returns></returns>
    public static string RemoveChars(this string str1, string str)
    {
        string sRtn = str1;
        char[] aChar = str.ToCharArray();
        string sItem = string.Empty;
        foreach (char item in aChar)
        {
            sItem = item.ToString();
            sRtn = sRtn.Contains(sItem) ? sRtn.Replace(sItem, string.Empty) : sRtn;
        }
        return sRtn;
        //return Regex.Replace(str1, str, string.Empty, RegexOptions.Compiled);
    }

    /// <summary>
    /// add css for string pattern.
    /// </summary>
    /// <param name="sPattern"></param>
    /// <param name="sCss">like "color: red;"</param>
    /// <returns></returns>
    public static string ChangeStyle(this string str, string sPattern, string sCss)
    {
        if (!str.Contains(sPattern)) { return str; }
        string sTmp = string.Format("<span style='{1}'>{0}</span>", sPattern, sCss);
        return str.Replace(sPattern, sTmp);
    }

    ///
    /// Summary:
    ///     Indicates whether the specified System.String object is null or an System.String.Empty
    ///     string.     
    /// Returns:
    ///     true if the value parameter is null or an empty string (""); otherwise, false.
    public static bool IsNullOrEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }
}