List Files (edit)

 string[] allFilePaths = Directory.GetFiles(@"D:\6.5.22\", "*.cs", SearchOption.AllDirectories);

 List<string> filePaths = new List<string>();

 for (int i = 0; i < allFilePaths.Length; i++)
 {
    string filePath = allFilePaths[i];
    string filePathLower = filePath.ToLower();
    string contents = File.ReadAllText(filePath);
    if (!filePathLower.Contains(@"\obj\") && !filePathLower.Contains(@"\bin\")
        && (contents.Contains(".Send") || contents.Contains(".SendAsync"))
        && contents.Contains("using MySolution.Framework"))
    {
        filePaths.Add(filePath);
    }
 }

 var nppExePath = @"C:\Program Files\Notepad++\notepad++.exe";
 var sb = new StringBuilder();
 for (int i = 0; i < filePaths.Count; i++)
 {
    var filePath = filePaths[i];
    Process.Start(nppExePath, string.Format("\"{0}\"", filePath));
 }