#11971 Enhance WinForms Application static class to get current application context

src/System.Windows.Forms/src/PublicAPI.Shipped.txt

@@ -2265,6 +2265,7 @@ static System.Windows.Forms.Application.ProductVersion.get -> string!
 static System.Windows.Forms.Application.RaiseIdle(System.EventArgs! e) -> void
 static System.Windows.Forms.Application.RegisterMessageLoop(System.Windows.Forms.Application.MessageLoopCallback? callback) -> void
 static System.Windows.Forms.Application.RemoveMessageFilter(System.Windows.Forms.IMessageFilter! value) -> void
+static System.Windows.Forms.Application.CurrentMainForm.get -> System.Windows.Forms.Form?
 static System.Windows.Forms.Application.RenderWithVisualStyles.get -> bool
 static System.Windows.Forms.Application.Restart() -> void
 static System.Windows.Forms.Application.Run() -> void

src/System.Windows.Forms/src/System/Windows/Forms/Application.cs

@@ -243,6 +243,12 @@ public static InputLanguage CurrentInputLanguage
         set => InputLanguage.CurrentInputLanguage = value;
     }
 
+    /// <summary>
+    /// Get current application context's main form
+    /// </summary>
+    /// <returns>Current application context's main form</returns>
+    public static Form? CurrentMainForm { get => ThreadContext.FromCurrent().ApplicationContext?.MainForm; }
+
     internal static bool CustomThreadExceptionHandlerAttached
         => ThreadContext.FromCurrent().CustomThreadExceptionHandlerAttached;
 

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ApplicationTests.cs

@@ -66,6 +66,50 @@ public void Application_CurrentCulture_Set_GetReturnsExpected()
         }).Dispose();
     }
 
+    [STAThread]
+    [WinFormsFact]
+    public void Application_CurrentMainForm_Get()
+    {
+        Application.EnableVisualStyles();
+        var form1 = new Form();
+        var form2 = new Form();
+        var event1 = new ManualResetEventSlim();
+        var event2 = new ManualResetEventSlim();
+        Form mainForm1 = default, mainForm2 = default, mainForm3 = default, mainForm4 = default;
+        form1.Load += (s, e) =>
+        {
+            mainForm1 = Application.CurrentMainForm;
+            form1.Close();
+            event1.Set();
+        };
+        form2.Load += (s, e) =>
+        {
+            mainForm2 = Application.CurrentMainForm;
+            form2.Close();
+            event2.Set();
+        };
+        var thread1 = new Thread(() => Application.Run(form1));
+        var thread2 = new Thread(() => Application.Run(form2));
+        var thread3 = new Thread(() => mainForm3 = Application.CurrentMainForm);
+        var thread4 = new Thread(() => mainForm4 = Application.CurrentMainForm);
+        thread1.SetApartmentState(ApartmentState.STA);
+        thread2.SetApartmentState(ApartmentState.STA);
+        thread3.SetApartmentState(ApartmentState.STA);
+        thread1.Start();
+        thread2.Start();
+        thread3.Start();
+        thread4.Start();
+        event1.Wait();
+        event2.Wait();
+        Assert.NotNull(mainForm1);
+        Assert.NotNull(mainForm2);
+        Assert.Null(mainForm3);
+        Assert.Null(mainForm4);
+        Assert.NotSame(mainForm1, mainForm2);
+        Assert.Same(form1, mainForm1);
+        Assert.Same(form2, mainForm2);
+    }
+
     [WinFormsFact]
     public void Application_CurrentCulture_SetNull_ThrowsArgumentNullException()
     {
@@ -327,9 +371,9 @@ public void Application_SetDefaultFont_NonSystemFont()
 
             // Retrieve the text scale factor, which is set via Settings > Display > Make Text Bigger.
             using RegistryKey key = Registry.CurrentUser.OpenSubKey("Software\\Microsoft\\Accessibility");
-            int textScale = (int)(key?.GetValue("TextScaleFactor", 100) ?? 100);            
+            int textScale = (int)(key?.GetValue("TextScaleFactor", 100) ?? 100);
             if (textScale == 100) // Application.DefaultFont must be the same
-            {                
+            {
                 font.Should().BeNull("Because TextScaleFactor == 100.");
                 Application.DefaultFont.Should().BeSameAs(customFont, "Because TextScaleFactor == 100.");
             }