<Window x:Class="SignInWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="サインイン" Height="450" Width="560">
    <Grid>
        <WebBrowser x:Name="SignInWebBrowser" />
    </Grid>
</Window>

Private ClientId As String
Private Scopes As New List(Of String)

Public Sub New(clientId As String, scope As IEnumerable(Of String))
    Me.ClientId = clientId
    Me.Scopes.AddRange(scope)

    InitializeComponent()
End Sub

Private Const AuthorizeUriFormat As String = "https://oauth.live.com/authorize?client_id={0}&scope={1}&response_type=token&locale=ja&display=popup&redirect_uri=https://oauth.live.com/desktop"

Private Sub SignInWindow_Loaded(sender As Object, e As System.Windows.RoutedEventArgs) Handles Me.Loaded
    SignInWebBrowser.Navigate(
        String.Format(AuthorizeUriFormat, Me.ClientId, String.Join("+", Me.Scopes)))
End Sub

Private _AccessToken As String
ReadOnly Property AccessToken As String
    Get
        Return _AccessToken
    End Get
End Property

Private Sub SignInWebBrowser_Navigated(sender As System.Object, e As System.Windows.Navigation.NavigationEventArgs) Handles SignInWebBrowser.Navigated

    ' 表示しているページの URL が https://oauth.live.com/desktop でなければ処理を抜ける
    Dim redirectUri = New Uri("https://oauth.live.com/desktop")
    If e.Uri.Host <> redirectUri.Host OrElse
       e.Uri.AbsolutePath <> redirectUri.AbsolutePath Then
        Exit Sub
    End If

    ' URL のフラグメント部分から access_token の値を取得する
    If e.Uri.Fragment.Contains("access_token") Then
        Dim items = New Dictionary(Of String, String)
        Dim param = e.Uri.Fragment.Substring(1).Split({"&"c, "="c})

        For i = 0 To param.Length - 1 Step 2
            items.Add(param(i), param(i + 1))
        Next

        If items.ContainsKey("access_token") Then
            ' URL に access_token が含まれていた場合

            _AccessToken = items("access_token")
            Me.DialogResult = True
        End If
    End If

    ' サインインウィンドウを閉じる
    Me.Close()
End Sub