在WPF中取得DPI的設定

要取得DPI的設定，需要一個針對一個視窗
在WPF中可以透過主視窗來取得形變的比率
再乘上預設的DPI(96)即可

void GetDpiSetting(
out double DpiX, out double DpiY)
{
const double DEFAULT_DPI = 96.0;

/// get transform matrix from current main window
Matrix m = PresentationSource
.FromVisual(Application.Current.MainWindow)
.CompositionTarget.TransformToDevice;

/// scale default dpi
DpiX = m.M11 * DEFAULT_DPI;
DpiY = m.M22 * DEFAULT_DPI;
}

另外習慣使用Win32的程式設計師
一樣可以使用GetDeviceCaps取得DPI的設定

const int LOGPIXELSX = 88;
const int LOGPIXELSY = 90;

[DllImport("gdi32.dll")]
static extern int GetDeviceCaps(IntPtr hdc, int Index);

[DllImport("user32.dll")]
static extern IntPtr GetDC(IntPtr Hwnd);

void GetDpiSetting(
out double DpiX, out double DpiY)
{
/// get desktop dc
IntPtr h = GetDC(IntPtr.Zero);

/// get dpi from dc
DpiX = GetDeviceCaps(h, LOGPIXELSX);
DpiY = GetDeviceCaps(h, LOGPIXELSY);
}

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參考資料
Getting system DPI in WPF app
GetDeviceCaps Function