How to draw a string over other full screen apps ?

First of all, create a new class library in the Visual Studio. This is a .net remoting example.

using System;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.IO;

namespace MyRemotingLibrary
{
    public interface IHttpRemoting
    {
        string SendPressedKey(string key);
    }
    
    public class MyHttpRemoting : MarshalByRefObject, IHttpRemoting
    {
        public string SendPressedKey(string key)
        {
            string path = @"c:\KeyTable.txt";
            StreamWriter writer = new StreamWriter(path);
            writer.WriteLine(key);
            writer.Close();
            return key;
        }
    }
}

And then, create a new server application for sending pressed keys from the client to the server application. (Client may be a classic .net application, a notepad or internet browser running on your machine.)

Create a new Windows Form application and add a Timer component and a TextBox control on your design form. ( And don't forget to reference System.Runtime.Remoting and our created remoting library to your project.)

using System.IO
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Http;
using MyRemotingLibrary;

namespace ServerApplication
{
    public partial class Form1 : Form
    {
        private void Form1_Load(object sender, EventArgs e)
        {
            textBox1.Multiline = true;
            /* ... */
            HttpChannel channelHttp = new HttpChannel(8000);
            ChannelServices.RegisterChannel(channelHttp);
            RemotingConfiguration.RegisterWellKnownServiceType(typeof(MyHttpRemoting), "MyKeyPressedWebService", WellKnownObjectMode.SingleCall);
            /* ... */
            timer1.Interval = 10; // Ten milliseconds.
            timer1.Start();
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            string path = @"c:\KeyTable.txt";
            try
            {
                FileInfo fInfo = new FileInfo(path);
                if(fInfo.Exists)
                {
                    StreamReader reader = new StreamReader(path)
                    string newLineString = reader.ReadLine();
                    textBox1.Text += newLineString;
                    reader.Close();
                    /* ... */
                    fInfo.Delete();
                }
            }
            catch {;}
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            string path = @"c:\KeyTable.txt";
            FileInfo fInfo = new FileInfo(path);
            if(fInfo.Exists)
            {
                fInfo.Delete(); // Delete the created file when we're closing the server application.
            }
        }
    }
}

And finally create a new client application for handling pressed keys. After than, this application sends pressed keys to our side. (This is a main application for us.)

1. Create a new Windows Form application.
2. Reference to System.Runtime.Remoting and our created library to our client project.
3. Add a new TextBox and two timer component to your design form.

using System;
using System.Windows.Forms;
using System.Runtime.Remoting;
using System.Runtime.Remoting.Channels;
using System.Runtime.Remoting.Channels.Tcp;
using System.Runtime.InteropServices;
using System.IO;
using MyRemotingLibrary;

namespace ClientApplication
{
    public partial class Form1 : Form
    {
        /* API Declarations */
        [DllImport("User32")]
        private static extern short GetAsyncKeyState(Int32 value);
        
        [DllImport("User32")]
        private static extern short GetAsyncKeyState(System.Windows.Forms.Keys virtualKeys);
        
        string strOne, strTwo;

        public delegate string Trigger(string key);
        TcpChannel channelTcp;
        Trigger InvokeMethod;

        private void Form1_Load(object sender, EventArgs e)
        {
            timer1.Interval = 1;
            timer2.Interval = 2;
            timer1.Enabled = true;
            timer2.Enabled = true;
            textBox1.MaxLength = 1; // Maximum one character.
            strOne = String.Empty;
            /* ... */
            channelTcp = new TcpChannel();
            ChannelServices.RegisterChannel(channelTcp);
            IHttpRemoting remoteService = (IHttpRemoting)Activator.GetObject(typeof(IHttpRemoting), "http://localhost:8000/MyKeyPressedWebService");
            /* ... */
            InvokeMethod = new Trigger(remoteService.SendPressedKey);
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            foreach(int current in Enum.GetValues(typeof(Keys)))
            {
                if(GetAsyncKeyState(current) == -32767)
                {
                    strOne += Enum.GetName(typeof(Keys), current) + " ";
                }
            }

            strTwo = null; // Clear it.
        }

        private void timer2_Tick(object sender, EventArgs e)
        {
            try
            {
                strTwo = strOne;
                if(strTwo.Length <= 3)
                {
                    textBox1.Text = strOne;
                }
                strOne = null;
            }
            catch {;}
        }
        
        /* TextChanged event for TextBox control. */
        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            string key = textBox1.Text;
            InvokeMethod(key);
        }
    }
}

Test it, in your client application or write some text in a editor(Notepad like), or enter a text in the google search bar and then look at your server application something that happen.