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To draw a rectangular outline on an existing screenshot in C#, load it into a Bitmap, create a Graphics object from that bitmap, and call DrawRectangle with a Pen and a Rectangle. The example below saves a new PNG and targets Windows: System.Drawing.Common is Windows-specific starting with .NET 6. If you need to capture the desktop as well as annotate it, use Graphics.CopyFromScreen first, then draw on the resulting bitmap.
Draw a rectangle on an existing screenshot
The following console program takes an input image, an output path, and a rectangle expressed in the image’s pixel coordinates. It draws a red outline and writes a PNG. Keeping the input and output paths separate avoids problems that can occur if you try to overwrite an image while it is still open.
Set up a Windows project
Use a stable Windows target framework, such as .NET 8. In the project file, enable the Windows desktop framework reference so the program can use System.Drawing:
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<PropertyGroup>
<OutputType>Exe</OutputType>
<TargetFramework>net8.0-windows</TargetFramework>
<UseWindowsForms>true</UseWindowsForms>
<ImplicitUsings>enable</ImplicitUsings>
<Nullable>enable</Nullable>
</PropertyGroup>
</Project>Save this as Program.cs. The program accepts six arguments: input image, output PNG, x, y, width, and height. For example, run dotnet run -- input.png marked.png 100 80 300 180.
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using System.Drawing;
using System.Drawing.Imaging;
if (args.Length != 6)
{
Console.Error.WriteLine("Usage: dotnet run -- <input-image> <output.png> <x> <y> <width> <height>");
return 2;
}
if (!int.TryParse(args[2], out int x) ||
!int.TryParse(args[3], out int y) ||
!int.TryParse(args[4], out int width) ||
!int.TryParse(args[5], out int height) ||
width <= 0 || height <= 0)
{
Console.Error.WriteLine("x and y must be integers; width and height must be positive integers.");
return 2;
}
try
{
using var screenshot = new Bitmap(args[0]);
// Check the rectangle against the bitmap dimensions before drawing.
if (x < 0 || y < 0 || x + (long)width > screenshot.Width || y + (long)height > screenshot.Height)
{
Console.Error.WriteLine($"Rectangle must fit within the {screenshot.Width}x{screenshot.Height} image.");
return 2;
}
using (Graphics graphics = Graphics.FromImage(screenshot))
using (var pen = new Pen(Color.Red, 4))
{
graphics.DrawRectangle(pen, new Rectangle(x, y, width, height));
}
screenshot.Save(args[1], ImageFormat.Png);
Console.WriteLine($"Saved annotated image to {args[1]}");
return 0;
}
catch (ArgumentException ex)
{
Console.Error.WriteLine($"Could not read the input image or draw the rectangle: {ex.Message}");
return 1;
}
catch (IOException ex)
{
Console.Error.WriteLine($"Could not read or write a file: {ex.Message}");
return 1;
}
Bitmap holds the pixels being edited. Graphics.FromImage(screenshot) creates a drawing surface for that bitmap; this is different from drawing in a Windows Forms paint event, where the control supplies a graphics object for painting the live interface. DrawRectangle draws an outline, and the Pen determines its color and stroke width. Dispose the graphics and pen when finished, as the code does, and do not use the bitmap after it has been disposed.
Choose the rectangle’s coordinates and appearance
Use the screenshot’s coordinate space
A Rectangle consists of an upper-left location and a width and height. Its coordinates should describe pixels in the bitmap, not positions in a window, browser, or preview unless those coordinate systems happen to match. By default, drawing units are pixels; graphics settings such as PageUnit and PageScale can change how coordinates are interpreted.
If a person selects a box on a preview displayed at a different size, convert the selection before calling DrawRectangle. For a preview scaled uniformly from bitmap dimensions imageWidth × imageHeight to previewWidth × previewHeight, a preview point (px, py) maps approximately to (px × imageWidth / previewWidth, py × imageHeight / previewHeight). Apply the same scale to the selected width and height, and account for any padding or letterboxing around the image. Round the resulting values consistently to integer pixels.
Account for the pen stroke
The rectangle describes a path, and the pen stroke is centered on that path. A four-pixel pen therefore extends on both sides of the nominal border. If the rectangle lies exactly on the image edge, part of a thick stroke can be clipped or fall outside the intended area. Leave room for the stroke by moving the rectangle inward, or use a thinner pen. Increase the width for a large or high-resolution screenshot, but check the saved result at its intended display size.
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Outline versus filled highlight
Use DrawRectangle(pen, rectangle) for a border. To cover the rectangle with a solid color, use FillRectangle(brush, rectangle) instead; a brush fills the area, while a pen traces the outline. A filled brush is opaque unless you deliberately configure a color with an alpha component and use an implementation that supports the intended compositing. If you need a translucent highlight, test it on the target framework and verify the saved output, rather than assuming that a normal fill will preserve visibility beneath it.
Capture the desktop before annotating
Capturing and annotating are separate operations. Graphics.CopyFromScreen transfers a specified rectangle of display pixels into a drawing surface. Its source point identifies where on the screen to copy from, its destination point identifies where in the bitmap to place those pixels, and its size determines the copied area.
This optional Windows example captures a region, draws an outline into the in-memory bitmap, and saves it as a PNG. It assumes the requested source rectangle is within the display area available to the process; it does not provide multi-monitor coordinate discovery.
using System.Drawing;
using System.Drawing.Imaging;
int screenX = 100;
int screenY = 100;
int captureWidth = 800;
int captureHeight = 600;
var captureSize = new Size(captureWidth, captureHeight);
var mark = new Rectangle(40, 50, 240, 120);
using var screenshot = new Bitmap(captureWidth, captureHeight);
using (Graphics graphics = Graphics.FromImage(screenshot))
{
graphics.CopyFromScreen(
new Point(screenX, screenY),
Point.Empty,
captureSize);
using var pen = new Pen(Color.Red, 4);
graphics.DrawRectangle(pen, mark);
}
screenshot.Save("desktop-marked.png", ImageFormat.Png);
The mark coordinates here are relative to the captured bitmap: its top-left is the destination origin. Adjust screenX, screenY, and the capture size to choose another source region. If the copy operation fails, the exception may be a Win32Exception; the exact behavior can depend on the Windows environment and how the application is run.
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Save the annotated image in the format you need
Save only after drawing is complete. In the main example, ImageFormat.Png explicitly writes PNG data even if the output filename does not end in .png. Use the matching ImageFormat when you want another supported output format. PNG is a practical choice for screenshots with text and sharp edges; JPEG uses lossy compression, which can soften fine interface details.
- Keep the original file if the annotation should be reversible; write to a distinct output path.
- Make sure the destination directory exists and that the process can write there.
- Dispose the bitmap after saving. If you need to keep editing, do not dispose it until all drawing and saves are finished.
- When processing many images, dispose each bitmap and its graphics resources before moving to the next file to release native resources promptly.
Platform and workflow choices
Windows applications using System.Drawing
This guide’s System.Drawing.Common approach is for Windows. Microsoft identifies the library as Windows-specific beginning with .NET 6. Do not treat older non-Windows workarounds or compatibility switches as general current support. If your application must run on Linux or macOS, choose a cross-platform graphics library that supports your target platforms and verify its maintenance, licensing, and supported frameworks before adopting it.
Manual capture and markup
If you only need to mark up one screenshot and do not need repeatable automation, Windows Snipping Tool can capture a rectangular region and offers drawing, shapes, erasing, cropping, and saving after capture. A C# program is more useful when the rectangle must be applied consistently, coordinates come from another process, or images need to be generated in a workflow.
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If the screenshot you need is of a webpage rather than the desktop, ScreenshotNeo can capture the page through one API request. It does not draw the rectangle for you: download the resulting image, then use the C# annotation code above to mark it. Cookie banners are accepted and removed before the shot, and known newsletter popups and chat widgets are removed; those cleanup steps can be turned off. Bot checks, blank pages, timeouts, failed loads, and cache hits are not billed, with page and billing outcomes indicated in response headers. ScreenshotNeo also offers an MCP server with screenshot tools for AI agents.
Here is the cURL request from the ScreenshotNeo API documentation:
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Troubleshooting
The project cannot find System.Drawing
Check that the project targets Windows, uses a Windows target framework such as net8.0-windows, and has UseWindowsForms enabled as shown above. A plain cross-platform target is not enough to make this Windows-specific drawing approach portable.
The rectangle is missing, shifted, or the wrong size
Confirm that x and y refer to the screenshot bitmap’s top-left origin and that width and height are positive. If coordinates came from a scaled preview or a UI control, map them into bitmap coordinates first. Check for preview margins, nonuniform scaling, or a changed PageUnit or PageScale.
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The border is partly cut off
The pen is centered on the rectangle path, so a stroke near an edge can extend beyond the image. Move the rectangle inward by roughly half the pen width or reduce the stroke width.
The image does not save
Check that the output directory exists, the output file is not locked by another process, and the current user has write access. Save to a new filename while testing rather than overwriting the input image. Confirm the selected image format matches the output you expect.
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Desktop capture throws an exception
Check that the source coordinates and requested dimensions describe a valid region of the active display and that the destination bitmap is large enough. CopyFromScreen can fail with a Win32Exception; inspect the exception details in the actual Windows environment rather than assuming all desktop sessions permit the same capture behavior.
Frequently asked questions
Can I draw a rectangle directly in a Windows Forms paint event?
Yes. A paint event draws into a control for display. To edit and save screenshot pixels, draw into a graphics object created from the screenshot bitmap, as in the examples above.
Does DrawRectangle fill the box?
No. It draws an outline. Use FillRectangle with a brush when the rectangle’s interior should be colored.
Can I use this approach on Linux or macOS?
Not as a supported general System.Drawing.Common solution starting with .NET 6. Use a graphics library that explicitly supports the platforms your application targets.
Can I annotate only one object in the screenshot?
Yes. Set the rectangle’s coordinates and dimensions to cover that object, or identify those values in your application before calling the drawing method. The drawing API paints the specified geometry; it does not detect objects by itself.
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