FlaUI

using FlaUI.Core.AutomationElements;
using FlaUI.UIA3;

// Note: Works only pre-Windows 8 with the legacy calculator
var app = FlaUI.Core.Application.Launch("calc.exe");
using (var automation = new UIA3Automation())
{
	var window = app.GetMainWindow(automation);
	var button1 = window.FindFirstDescendant(cf => cf.ByText("1"))?.AsButton();
	button1?.Invoke();
	...
}