StringExtension.cs

public static class StringExtension
{
	public static bool IsBase64(this string value)
  	{
		string pattern = @"^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)?$";
		Regex rg = new Regex(pattern);
		return rg.IsMatch(value);
	}
	
	public static string Base64Decode(this string value, bool returnSameValueIsNotBase64 = false)
	{
		if (string.IsNullOrWhiteSpace(value))
			return null;
		
		if ( (returnSameValueIsNotBase64) && !(value.IsBase64()) )
			return value;
		
		var base64EncodedBytes = System.Convert.FromBase64String(value);
		return Encoding.UTF8.GetString(base64EncodedBytes);
	}
	
	public static bool IsNumeric(this string value)
	{
		if (string.IsNullOrWhiteSpace(value))
			return false;
	
		return int.TryParse(value, out _);
	}

	public static string TryGetSubstring(this string value, int startIndex, int length)
	{
		return TryGetSubstring(value, startIndex, length, false);
	}
	
	public static string TryGetSubstring(this string value, int startIndex, int length, bool returnOriginalValue)
	{
		if (string.IsNullOrWhiteSpace(value) || value.Length < startIndex + length)
			return returnOriginalValue ? value : string.Empty;
	
		return value.Substring(startIndex, length);
	}
}