C#

// Create a DirectoryInfo class and point it to the 
// directory you want to search in
DirectoryInfo dirInf = new DirectoryInfo(@"C:\");

// Create a FileInfo array and get the file information 
// of all the TXT files in the directory
FileInfo[] files = dirInf.GetFiles("*.txt");

// Loop though all the files found
foreach (FileInfo currentFile in files)
{
    // Create a files stream
    FileStream fs = new FileStream(currentFile.FullName, FileMode.Open);

    //create a stream reader
    StreamReader reader = new StreamReader(fs);

    //read the text in the file
    string fileContent = reader.ReadToEnd();

    if (fileContent.Contains("the text your searching for"))
    {
        MessageBox.Show("The File " + currentFile.FullName +
                        " Contains the text your searching for");
    }

    // flush and close the IO objects
    fs.Flush();
    reader.Close();
    fs.Close();
}