Extension methods to enumerate files and directories, include or exclude by name or extension, and search file contents. Example is in LINQPad format.

LINQPadSearchFileContents.cs

void Main()
{
  string path = Path.GetFullPath(@"C:\Users\Username"); 
	System.IO.DirectoryInfo di = new DirectoryInfo(path);
	
	var query = di.GetFiles("*.*",SearchOption.TopDirectoryOnly).IncludeFileExtensions("*.asp").SearchFileContents("searchterm");
	query.Dump();
}

public static class FileIOExtensions
{
	//Search File Contents
	public static IEnumerable<KeyValuePair<string,string>> SearchFileContents(this FileInfo[] files, string text)
	{	
		return files.SelectMany(f => System.IO.File.ReadLines(f.FullName).Where (l => l.ToLower().Contains(text)).Select (l => new KeyValuePair<string, string>(f.FullName,l)));
	}
	
	#region File Extensions
	//Example: FileInfo[] pages = folderInfo.GetFiles("*.*").WithExtension(".aspx", ".aspx", ".htm", ".html");
	public static FileInfo[] ExcludeFileExtensions(this FileInfo[] files, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => files.Where(f => !a.ToLowerInvariant().Contains(f.Extension.ToLowerInvariant()))).ToArray();
	}
	
	public static FileInfo[] IncludeFileExtensions(this FileInfo[] files, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => files.Where(f => a.ToLowerInvariant().Contains(f.Extension.ToLowerInvariant()))).ToArray();
	}

	#endregion
	
	#region File Names
	//Example: FileInfo[] pages = folderInfo.GetFiles("*.*").IncludeFileNames("index.aspx", "default.aspx", "index.htm", "index.html");
	public static FileInfo[] ExcludeFileNames(this FileInfo[] files, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => files.Where(f => !a.ToLowerInvariant().Contains(f.Name.ToLowerInvariant()))).ToArray();
	}
	
	public static FileInfo[] IncludeFileNames(this FileInfo[] files, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => files.Where(f => a.ToLowerInvariant().Contains(f.Name.ToLowerInvariant()))).ToArray();
	}
	
	#endregion
	
	#region Directory Names
	//Example: foreach (DirectoryInfo folder in folders.ExcludeDirectoryNames("app_code", "app_data", "bin", "assets", "images"))
	public static DirectoryInfo[] ExcludeDirectoryNames(this DirectoryInfo[] dirs, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => dirs.Where(d => !a.ToLowerInvariant().Contains(d.Name.ToLowerInvariant()))).ToArray();
	}
	
	public static DirectoryInfo[] IncludeDirectoryNames(this DirectoryInfo[] dirs, params string[] args)
	{
		if (args == null)
			throw new ArgumentNullException("args");
		return args.SelectMany (a => dirs.Where(d => a.ToLowerInvariant().Contains(d.Name.ToLowerInvariant()))).ToArray();
	}
	
	#endregion
}

ExcludeDirectoryNames doesn't work for me.
Correct version would be the following:

public static IEnumerable<string> ExcludeDirectoryNames(this IEnumerable<string> dirs, params string[] args) {

        if (args == null)
            throw new ArgumentNullException("args");
        return dirs.Where(x => args.All(q => !x.Contains(q, StringComparison.InvariantCultureIgnoreCase)));
    }

@dmitriymarushko Thanks!