How to create a new System.String type with other name?

Question

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:

1. Declare the type: Dim Str1 as String
2. 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:

1. Declare the type: Dim Str2 as UrlString
2. 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.

UPDATE:

Thanks Marc Gravell and GSerg. Here is the solution:

Class UrlString
    Private ReadOnly value As String

    Public Sub New(ByVal value As String)
        Me.value = value
    End Sub

    Public Shared Widening Operator CType(ByVal value As String) As UrlString
        Return New UrlString(value)
    End Operator

    Public Shared Widening Operator CType(ByVal u As UrlString) As String
        Return u.value
    End Operator

    Public Overrides Function GetHashCode() As Integer
        Return If(value Is Nothing, 0, value.GetHashCode())
    End Function

    Public Overrides Function Equals(ByVal obj As Object) As Boolean
        Return String.Equals(value, DirectCast(obj, String))
    End Function

    Public Overrides Function ToString() As String
        Return value
    End Function
End Class

Answer 1

You need to add an implicit conversion operator from string to UrlString for that to work. In C#:

class UrlString
{
    private readonly string value;
    public UrlString(string value) { this.value = value; }
    public static implicit operator UrlString(string value)
    {
        return new UrlString(value);
    }
    public override int GetHashCode()
    {
        return value == null ? 0 : value.GetHashCode();
    }
    public override bool Equals(object obj)
    {
        return string.Equals(value, (string)obj);
    }
    public override string ToString()
    {
        return value;
    }
}

Then:

UrlString foo = "abc";