I got the solution
if(tableOfContents.Count > 0)
{
WordInterop.Range range = tableOfContents[_index].Range;
if (range.Find.Execute(text))
result = true;
}
if (index.Count > 0)
{
WordInterop.Range range = index[_index].Range;
if (range.Find.Execute(text))
result = true;
}