.net - Create A New System.String Type With Other Name?

I try to describe my problem step by step because I do not know how to say it in correct programming terms.

When I use a System.String type, I do the following:

Declare the type: Dim Str1 as String
Assign its value: Str1 = "This is a string"

I want to create a new type that just like the System.String type but in different name. For example, I want to create a UrlString type for string like this:

Declare the type: Dim Str2 as UrlString
Assign its value: Str2 = "http://www.example.com"

My question is: How do I create the UrlString type?

The reason: I want to create the UrlString type to help me to identify the value of the content. For example, UrlString type means the string is in url format, PhoneString means the string is in phone format, CreditCardString type means the string is in credit card format and so on.

Class UrlString
Private ReadOnly value As String
Public Sub New(ByVal value As String)

[Code].....