what I wish to do here's to show the ID on the textbox upon signing in however experienced this error

Not able to cast object of type 'SingleResult`1[Library_Catalog_System.SelectLoginIDResult]' to type 'System.String'.

Exist other means to fix this?

Private login As New LoginDataContext
 Private Sub frmLibrary_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    Dim log = login.SelectLoginID(frmLogin.txtusername.Text, frmLogin.txtPassword.Text)

    LoginIDTextBox.Text = log 
End Sub

@username NVARCHAR(20),
@password NVARCHAR(20)

AS

SELECT        LoginID
FROM            Login
WHERE        (password = @password) AND (username = @username)

Return
Dim log = CStr(login.SelectLoginID(frmLogin.txtusername.Text, _
          frmLogin.txtPassword.Text).SingleOrDefault())