I found this example in VBA to find highlighted text but is it possible to be done in C# without using Microsoft.office.interop.word ?
C#:
Sub TestFind()
Dim myRange As Range
Set myRange = ActiveDocument.Content ' search entire document
With myRange.Find
.Highlight = True
Do While .Execute = True ' loop while highlighted text is found
Debug.Print myRange.Text ' myRange is changed to contain the found text
Loop
End With
End Sub