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Capture the screen into a bitmap, encode it as an image, then send the encoded bytes over TCP with a length prefix so the receiver knows exactly where the screenshot ends. The example below uses Graphics.CopyFromScreen on Windows and a four-byte big-endian length header; the receiver validates that length and reads the entire payload before saving it.
Keep capture, encoding, transport, and decoding or storage as separate steps. That makes it easier to change the image format or transport without entangling the code, and it gives the receiver a clear point to reject oversized or incomplete messages.
What the socket protocol needs to do
TCP delivers an ordered stream of bytes, not separate image messages. A call that writes one image does not guarantee that the peer will receive it in one matching read. Define a framing rule: this example sends a four-byte unsigned payload length in network byte order (big-endian), followed by exactly that many image bytes.
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Capture and send a screenshot from Windows
Graphics.CopyFromScreen copies pixels from a selected screen rectangle into a drawing surface. The code below captures the primary display, encodes the bitmap as PNG in memory, and sends it to a receiver listening on port 5000. The capture API is Windows-specific; it is not a portable way to capture screens on macOS or Linux.
Create a Windows console project targeting Windows, for example dotnet new console -n ScreenshotSender -f net8.0-windows, and enable Windows Forms in the project file so the Windows desktop drawing APIs are available:
<Project Sdk="Microsoft.NET.Sdk">
<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net8.0-windows</TargetFramework>
<UseWindowsForms>true</UseWindowsForms>
<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>
</PropertyGroup>
</Project>Replace Program.cs with the following. Set the receiver address to a machine reachable from the sender. The receiver example later in this article listens on all interfaces; restrict that exposure with your firewall and use TLS and authentication outside a trusted, controlled network.
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using System.Drawing;
using System.Drawing.Imaging;
using System.Net.Sockets;
using System.Buffers.Binary;
const string host = "127.0.0.1";
const int port = 5000;
// Capture the primary display. For a specific region, substitute its
// screen-coordinate origin and width/height here.
var bounds = System.Windows.Forms.Screen.PrimaryScreen?.Bounds
?? throw new InvalidOperationException("No primary display was found.");
byte[] imageBytes;
using (var bitmap = new Bitmap(bounds.Width, bounds.Height))
{
using (var graphics = Graphics.FromImage(bitmap))
{
graphics.CopyFromScreen(bounds.Left, bounds.Top, 0, 0, bounds.Size);
}
using var imageStream = new MemoryStream();
bitmap.Save(imageStream, ImageFormat.Png);
imageBytes = imageStream.ToArray();
}
if (imageBytes.Length == 0)
throw new InvalidOperationException("The encoded screenshot is empty.");
using var client = new TcpClient();
await client.ConnectAsync(host, port);
await using NetworkStream stream = client.GetStream();
byte[] header = new byte[4];
BinaryPrimitives.WriteUInt32BigEndian(header, checked((uint)imageBytes.Length));
await stream.WriteAsync(header);
await stream.WriteAsync(imageBytes);
await stream.FlushAsync();
Console.WriteLine($"Sent {imageBytes.Length} bytes of PNG data to {host}:{port}.");The bitmap and graphics objects are disposed as soon as capture and encoding finish. Image.Save(Stream, ImageFormat) writes the encoded image to a memory stream; ToArray() provides the bytes that are sent. PNG is a useful default for screenshots containing text and interface edges because it is lossless. JPEG can reduce payload size for photographic content, but it is lossy; choose based on the image and the acceptable trade-off, not an assumed universal winner.
Receive the framed image and save it
This cross-platform console receiver listens on port 5000, accepts one connection, reads the header and complete payload, then writes the bytes to received.png. It caps the accepted payload at 25 MiB; adjust that limit to the use case, but keep a deliberate bound rather than trusting a sender-provided length.
Create it with dotnet new console -n ScreenshotReceiver and replace Program.cs:
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using System.Buffers.Binary;
using System.Net;
using System.Net.Sockets;
const int port = 5000;
const uint maxPayloadBytes = 25 * 1024 * 1024;
var listener = new TcpListener(IPAddress.Any, port);
listener.Start();
Console.WriteLine($"Listening on port {port}...");
try
{
using TcpClient client = await listener.AcceptTcpClientAsync();
await using NetworkStream stream = client.GetStream();
byte[] header = new byte[4];
await ReadExactlyAsync(stream, header);
uint payloadLength = BinaryPrimitives.ReadUInt32BigEndian(header);
if (payloadLength == 0 || payloadLength > maxPayloadBytes)
throw new InvalidDataException($"Rejected payload length: {payloadLength} bytes.");
byte[] imageBytes = new byte[checked((int)payloadLength)];
await ReadExactlyAsync(stream, imageBytes);
// This sample stores the received bytes. Validate/decode the image before
// using it in an application that treats senders as untrusted.
await File.WriteAllBytesAsync("received.png", imageBytes);
Console.WriteLine($"Saved {imageBytes.Length} bytes to received.png.");
}
catch (EndOfStreamException ex)
{
Console.Error.WriteLine($"Connection ended before a complete message arrived: {ex.Message}");
}
finally
{
listener.Stop();
}
static async Task ReadExactlyAsync(Stream stream, Memory<byte> buffer)
{
int totalRead = 0;
while (totalRead < buffer.Length)
{
int read = await stream.ReadAsync(buffer[totalRead..]);
if (read == 0)
throw new EndOfStreamException("The peer closed the connection.");
totalRead += read;
}
}Run the receiver first, then run the sender. TcpClient.GetStream() returns the NetworkStream used to exchange data. The exact-read loop matters: a stream read may return fewer bytes than requested, including for the small header. The receiver’s one-client, one-message shape is suitable for demonstrating framing, not a complete multi-client service. A production listener should accept clients repeatedly, isolate each connection, apply timeouts or cancellation, and define how it reports failures.
Adapt the capture, format, and message envelope
Capture a region or another display
For a rectangle, create the bitmap with the rectangle’s width and height, then pass its screen-coordinate origin to CopyFromScreen. The destination origin is normally 0, 0 because the new bitmap starts at its own top-left corner. For a different monitor, select its bounds instead of PrimaryScreen.Bounds. Check that the selected display exists and that dimensions are positive before allocating a bitmap; very large dimensions can consume substantial memory.
Choose an image format
Use an explicit image format instead of relying on a filename extension. PNG retains sharp text and is broadly suitable for UI captures. JPEG is lossy and may be smaller for photographic material. The transmitted header in this example carries only the length, so both peers must agree on the image format. If peers may use different formats, add a small, versioned envelope containing a format identifier and validate it before handling the payload.
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Extend framing deliberately
A four-byte length prefix is enough for one bounded image per connection. If the application needs multiple messages per connection, add a protocol version, message type, request identifier, or timestamp as needed, and specify each field’s width and byte order. Validate metadata and lengths before allocating buffers. For sustained traffic or large captures, consider streaming a bounded payload to storage rather than allocating a second full-size array; the example favors clarity over minimizing copies.
Protect screenshots when the network is not trusted
Plain TCP does not encrypt screenshots or prove the identity of the peer. On a network beyond a controlled trusted boundary, wrap the network stream in SslStream, authenticate the server and, where appropriate, the client, and send the framed message only after authentication succeeds. TLS reads are not available until authentication completes. Use certificate validation rather than accepting any certificate, and retain application-level authorization, payload limits, and careful handling of screenshot data in memory and on disk. Encryption does not decide who is allowed to capture or view a screen.
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- Payload size: full-display captures create larger payloads than cropped regions. Capture only the area the receiver needs, and select PNG or JPEG based on fidelity requirements.
- Responsiveness: the examples use asynchronous network operations so a UI or service can avoid blocking while connecting and transferring. Capture and encoding still consume CPU and memory; run them off a UI thread in a desktop application.
- Connection failures: a refused connection usually means no listener is available at the target host and port; a timeout or unreachable host points to routing or firewall configuration. Retry only with a policy that prevents duplicate processing or uncontrolled retry loops.
- Partial or truncated data: the receiver must complete both reads and treat early closure as failure. Do not save or process an incomplete payload as if it were valid.
- Resource limits: validate the length before allocating, and consider limits on concurrent clients, capture dimensions, and total processing time. A valid but very large image can still exhaust memory.
The code performs no compression beyond the selected image encoding and makes no claim about throughput; actual transfer time depends on image content, capture dimensions, network conditions, and receiver work.
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Troubleshooting common failures
- “No primary display was found” or a capture exception: run in an interactive Windows desktop session with an available display. The shown API is not a headless cross-platform capture solution.
- Connection refused: start the receiver before the sender, confirm both use the same port and correct host, and check that local or network firewall rules allow the connection.
- Sender connects but receiver hangs: confirm both sides use the same framing: exactly four big-endian length bytes followed by that many payload bytes. A disagreement about byte order or message format can make the receiver wait for an implausible amount of data.
- Receiver rejects the length: ensure the header is not being sent as text or in little-endian order, and verify that the sender’s payload is within the receiver’s configured maximum.
- Received file will not open: confirm the sender encoded PNG and that the receiver received the entire payload. Do not use the filename extension as proof that the bytes are actually PNG.
- Works locally but not between machines: replace
127.0.0.1with the receiver machine’s reachable address, bind and firewall the listener appropriately, and verify network routing. Do not expose an unauthenticated screenshot receiver to an untrusted network.
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If what you need is a screenshot of a web page rather than pixels from a local Windows desktop, ScreenshotNeo can return a webpage screenshot through one API request. It is a different workflow from sending a locally captured desktop image over your own socket: use the socket example when your application needs to capture and deliver its own screen data.
For API parameters and response details, see the ScreenshotNeo documentation. This cURL request saves a Stripe page capture as WebP:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpScreenshotNeo accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each of those steps can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and response headers report page verdict and billing status. Its MCP server provides take_screenshot, get_page_info, and capture_pdf tools for Claude, Cursor, and other MCP clients. The Free plan includes 1,000 shots a month with no card; paid plans start at $5 for 3,000 shots, and every feature is on every plan.
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Frequently Asked Questions
Can I use the same length-prefix protocol for JPEG instead of PNG?
Yes. Encode as JPEG, keep the same framing, and make sure the receiver knows the agreed format or add a validated format identifier to the message.
Does the socket example capture a web page?
No. It captures pixels from a Windows display. For a webpage screenshot returned by an API rather than a local screen capture, ScreenshotNeo is a separate option.
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