HTML substring in C#.NET

Getting a substring of text containing HTML tags is more tricky than you think. Assume that you want the first 10 characters of the following:

"<p>this is paragraph 1<p><p>this is paragraph 2</p>"

The output would be:

"<p>this is"

As you can see, the returned text contains an unclosed P tag. If this is rendered to a page, subsequent content will be affected by the open P tag. Ideally, the preferred output would close any unclosed HTML tags in reverse of when they were opened:

"<p>this is</p>"

Here is a function that returns a subtring of HTML, making sure that no tags are left unclosed:

using System;
using System.Collections.Generic;
 
public static class StringUtility
{
    public static string HTMLSubstring( string html, int length )
    {
        if ( html == null )
        {
            throw new ArgumentNullException( "html" );
        }
 
        List<string> unclosedTags = new List<string>();
 
        bool isQuoted = false;
 
        if ( html.Length > length )
        {
            for ( int i = 0; i < html.Length; i++ )
            {
                char currentCharacter = html[i];
 
                char nextCharacter = ' ';
 
                if ( i < html.Length - 1 )
                {
                    nextCharacter = html[i + 1];
                }
 
                // Check if quotes are on.
                if ( !isQuoted )
                {
                    if ( currentCharacter == '<' && nextCharacter != ' ' && nextCharacter != '>' )
                    {
                        if ( nextCharacter != '/' ) // Open tag.
                        {
                            int startIndex = i + 1;
 
                            if ( startIndex < html.Length )
                            {
                                int finishIndex = html.IndexOf( ">", startIndex );
 
                                if ( finishIndex > 0 )
                                {
                                    if ( html[finishIndex - 1] != '/' )
                                    {
                                        string tag = html.Substring( startIndex, finishIndex - startIndex );
 
                                        if ( tag.Contains( " " ) )
                                        {
                                            int temporaryFinishIndex = html.IndexOf( " ", startIndex );
 
                                            tag = html.Substring( startIndex, temporaryFinishIndex - startIndex );
                                        }
 
                                        if ( !tag.Equals( "br", StringComparison.InvariantCultureIgnoreCase ) )
                                        {
                                            unclosedTags.Add( tag );
                                        }
                                    }
 
                                    int tagLength = finishIndex + 1 - i;
 
                                    length += tagLength;
 
                                    i = finishIndex;
                                }
                            }
                        }
                        else if ( nextCharacter == '/' ) // Close tag.
                        {
                            int startIndex = i + 2;
 
                            if ( startIndex < html.Length )
                            {
                                int finishIndex = html.IndexOf( ">", startIndex );
 
                                if ( finishIndex > 0 )
                                {
                                    string tag = html.Substring( startIndex, finishIndex - startIndex );
 
                                    // FILO.
                                    int index = unclosedTags.LastIndexOf( tag );
 
                                    if ( index >= 0 )
                                    {
                                        unclosedTags.RemoveAt( index );
 
                                        int tagLength = finishIndex + 1 - i;
 
                                        length += tagLength;
 
                                        i = finishIndex;
                                    }
                                }
                            }
                        }
                    }
                }
                else
                {
                    if ( currentCharacter == '"' )
                    {
                        isQuoted = false;
                    }
                }

                if ( i >= length )
                {
                    html = string.Format( "{0}...", html.Substring( 0, i ) );
 
                    unclosedTags.Reverse();
 
                    foreach ( string unclosedTag in unclosedTags )
                    {
                        html += string.Format( "</{0}>", unclosedTag );
                    }
                }
            }
        }
 
        return html;
    }
}