Question

How can I count number of sentences in a given string?

Answer 1

If you already have Word installed, you can use Word interop to get the sentence count, as well as other statistics. This also has the benefit of potentially working with other languages besides English.

object oMissing = System.Reflection.Missing.Value;

var oWord = new Microsoft.Office.Interop.Word.Application();
oWord.Visible = false;
var oDoc = oWord.Documents.Add(ref oMissing, ref oMissing, ref oMissing, ref oMissing);

oDoc.Content.Text = inputTextBox.Text;

//get just sentence count
sentenceCountLabel.Text = oDoc.Sentences.Count.ToString();

//get all statistics
foreach (Microsoft.Office.Interop.Word.ReadabilityStatistic stat in oDoc.ReadabilityStatistics)
{
    Console.WriteLine("{0}: {1}", stat.Name, stat.Value);
}

object oFalse = false;

oDoc.Close(ref oFalse, ref oMissing, ref oMissing);

This will output:

 Words: 283 
 Characters: 1271 
 Paragraphs: 3 
 Sentences: 6 
 Sentences per Paragraph: 2 
 Words per Sentence: 47.1 
 Characters per Word: 4.3 
 Passive Sentences: 0 
 Flesch Reading Ease: 55.2 
 Flesch-Kincaid Grade Level: 12.5

This may not be the most efficient, but it only requires a few lines of code and may be suitable depending on your needs.