How to highlight word in word document using C#

[C# Programming]

i got a routine which said that it will highlight word in word document but it is giving error "Bad variable type. (Exception from HRESULT: 0x80020008 (DISP_E_BADVARTYPE))"

when this below line execute then exception occur and error as above occur.
range.find.highlight()
here is my full code

private static void HighlightText() { object fileName = "C:\\1.doc"; object textToFind = "test1"; object readOnly = true; Word.Application word = new Word.Application(); Word.Document doc = new Word.Document(); object missing = Type.Missing; try { doc = word.Documents.Open(ref fileName, ref missing, ref readOnly, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing); doc.Activate(); object matchPhrase = false; object matchCase = false; object matchPrefix = false; object matchSuffix = false; object matchWholeWord = false; object matchWildcards = false; object matchSoundsLike = false; object matchAllWordForms = false; object matchByte = false; object ignoreSpace = false; object ignorePunct = false; object highlightedColor = Word.WdColor.wdColorGreen; object textColor = Word.WdColor.wdColorLightOrange; Word.Range range = doc.Range(); bool highlighted = range.Find.HitHighlight(textToFind, highlightedColor, textColor, matchCase, matchWholeWord, matchPrefix, matchSuffix, matchPhrase, matchWildcards, matchSoundsLike, matchAllWordForms, matchByte, false, false, false, false, false, ignoreSpace, ignorePunct, false); System.Diagnostics.Process.Start(fileName.ToString()); } catch (Exception ex) { Console.WriteLine("Error : " + ex.Message); Console.ReadKey(true); } }tbhattacharjee