Переопределение метода ToString для Double

X.G = 0
MsgBox(X.G)              'Возвращает 0  
MsgBox(X.G.ToString) 'Как сделать что бы возвращала 90°00'

Public Class DegreeFormatter
    Implements IFormatProvider, ICustomFormatter
    Public Function Format(frmt As String, arg As Object, formatProvider As IFormatProvider) As String Implements ICustomFormatter.Format
        If (Not (TypeOf arg Is Double)) AndAlso
            (Not (TypeOf arg Is Single)) AndAlso
            (Not (TypeOf arg Is Decimal)) Then
            Return arg.ToString()
        End If
        Return ToDegrees(arg)
    End Function
 
    Private Function ToDegrees(val As Double) As String
        Dim isNeg = Math.Sign(val) < 0
        Dim angle = Math.Abs(val)
        Dim degrees = CInt(Math.Floor(angle))
        Dim minutes = (angle - degrees) * 60
        Return String.Format("{2}{0}°{1:00}'", degrees, minutes, If(isNeg, "-", ""))
    End Function
 
    Public Function GetFormat(formatType As Type) As Object Implements IFormatProvider.GetFormat
        Return If(formatType Is GetType(ICustomFormatter), Me, Nothing)
    End Function
End Class

String.Format(New DegreeFormatter(), "{0}", 90.54)'90°32'

Imports System.Runtime.CompilerServices
 
Module Helpers
    <Extension>
    Function ToDegrees(val As Double) As String
        Return String.Format(New DegreeFormatter(), "{0}", val)
    End Function
End Module

90.54.ToDegrees()

    Private Function ToDegrees(ByVal val As Double) As String
        If Math.Abs(val) > 1 Then Return "Error"
        Dim GRD As Double = Math.Acos(val) * 180 / Math.PI
        Dim Min As Byte = CDbl(GRD - Math.Floor(GRD)) * 60
        Return String.Format("{0}°{1:00}'", CStr(Math.Floor(GRD)), CStr(Min))
    End Function