How do I determine the true pixel size of my Monitor in .NET?

Question

I want to display an image at \'true size\' in my application. For that I need to know the pixel size of the display.

I know windows display resolution is nominally

Answer 1

TL;DR

WPF's True Size = Pixels * DPI Magnification

DPI Magnification:

Matrix dpiMagnification
= PresentationSource.FromVisual(MyUserControl).CompositionTarget.TransformToDevice;
double magnificationX = dpiMagnification.M11;
double magnificationY = dpiMagnification.M22;

Discussion

I had trouble solving this question still in 2020. Back when this question was asked/answered in 2009, .NET C# probably meant Windows Forms. But WPF is the de facto standard of the day...

By asking about "true size" you have probably already figured out that the operating system does some calculation with actual pixels (say 1366x768, which I understand is usual laptop resolutions) and the DPI (hard to find) in order to give a control's true size. And you are trying to make an app that scales to different monitors.

This DPI actual number seems to be hidden, but it has been normalized (converted to a percentage). Assume 100% = 96 DPI, just because the actual number does not matter anymore. People can easily increase the system-wide text size by going to Desktop on Windows 10 > right click > Display settings > section Scale and layout > change the percentage to magnify text and other elements.

You can find the pixels another way, and multiple/divide the pixel by the DPI percentage in order to get true size. For instance, I want to drag a UserControl around a canvas element of a WPF window with the mouse. The user control's pixel count and the mouse xy-coordinates were off by the normalized DPI. In order to keep the mouse moving at the same rate as the user control, I use:

double newXCoord = System.Windows.Forms.Cursor.Position.X;
double newYCoord = System.Windows.Forms.Cursor.Position.Y;
double deltaX = newXCoord - oldXCoord;
double deltaY = newYCoord - oldYCoord;

double magnificationX = 1;
double magnificationY = 1;
Matrix dpiMagnification
= PresentationSource.FromVisual(visual).CompositionTarget.TransformToDevice;
if (magnificationMatrix != null)
{
    magnificationX = dpiMagnification.M11;
    magnificationY = dpiMagnification.M22;
}
PixelsFromLeft += deltaX / m_magnificationX;
PixelsFromTop += deltaY / m_magnificationY;