如何在 Windows 窗体中创建半透明或模糊的背景色

Question

I am creating a Speed Meter to measure my internet speed and stays at top of every app, then I how do I make Its background semi transparent or blurred something like this.

Answer 1

A WinForms Form with a Blur effect applied to it, using:
Windows 7 ⇒ DwmEnableBlurBehindWindow() DWM function.
Windows 10 ⇒ SetWindowCompositionAttribute() User32 function (undocumented)

How to apply the Blur Behind effect :
Windows 7 :
The Window needs to register a rendering policy with DwmSetWindowAttribute() , setting the DWMWINDOWATTRIBUTE enumerator to NCRenderingPolicy and enable it, setting the DWMNCRENDERINGPOLICY enumerator to Enabled .
You may also want to override a Form's class WndProc ; when a WM_DWMCOMPOSITIONCHANGED message is received (informing of the Aero composition state), to verify whether DWN Composition is enabled.

Before showing the Window interface, call DwmEnableBlurBehindWindow() , setting its DWM_BLURBEHIND structure relevant fields ( dwFlags and fEnable ) to 1 .
Here, I'm calling it from the Form's constructor.

Windows 10 :
This code is using the not yet documented SetWindowCompositionAttribute() function.
The User32 function only requires the use of an internal structure, here called WinCompositionAttrData , where the Attribute member is set to DWMWINDOWATTRIBUTE.AccentPolicy and the Data member is set to the pointer of an internal structure, here called AccentPolicy , where the AccentState member is set to DWMACCENTSTATE.ACCENT_ENABLE_BLURBEHIND

DWM Compostion is always enabled. No need to verify it or override WndProc to handle a DWM Composition change.

It's more clear in code:
(As a note, the BlurBehind effect does not work if the Form has a white background color)

See this other question about a DwmExtendFrameIntoClientArea() implementation.

public partial class frmBlurBehind : Form
{
    WinApi.Dwm.DWMNCRENDERINGPOLICY policy = WinApi.Dwm.DWMNCRENDERINGPOLICY.Enabled;
    WinApi.Dwm.WindowSetAttribute(this.Handle, WinApi.Dwm.DWMWINDOWATTRIBUTE.NCRenderingPolicy, (int)policy);
    Version osVersion = Environment.OSVersion.Version;
    if (DWNCompositionEnabled()) {
        if (osVersion.Major > 6)
        {
            WinApi.Dwm.Windows10EnableBlurBehind(this.Handle);
        }
        else if(osVersion.Major == 6 && osVersion.Minor == 1)
        {
            WinApi.Dwm.WindowEnableBlurBehind(this.Handle);
        }
    }

    private bool IsDWNCompositionEnabled() => (Environment.OSVersion.Version.Major >= 6)
                                           ? WinApi.Dwm.IsCompositionEnabled()
                                           : false;

    //Eventually
    protected override void WndProc(ref Message m)
    {
        switch (m.Msg)
        {
            case (int)WinApi.WinMessage.WM_DWMCOMPOSITIONCHANGED:
                if (IsDWNCompositionEnabled())
                {
                    WinApi.Dwm.DWMNCRENDERINGPOLICY policy = WinApi.Dwm.DWMNCRENDERINGPOLICY.Enabled;
                    WinApi.Dwm.WindowSetAttribute(this.Handle, WinApi.Dwm.DWMWINDOWATTRIBUTE.NCRenderingPolicy, (int)policy);
                    WinApi.Dwm.WindowBorderlessDropShadow(this.Handle, 1);
                    m.Result = (IntPtr)0;
                }
                break;
            default:
                break;
        }
        base.WndProc(ref m);
    }
}


public partial class WinApi
{
    public enum WinMessage : int
    {
        WM_DWMCOMPOSITIONCHANGED = 0x031E,          //The system will send a window the WM_DWMCOMPOSITIONCHANGED message to indicate that the availability of desktop composition has changed.
        WM_DWMNCRENDERINGCHANGED = 0x031F,          //WM_DWMNCRENDERINGCHANGED is called when the non-client area rendering status of a window has changed. Only windows that have set the flag DWM_BLURBEHIND.fTransitionOnMaximized to true will get this message.
        WM_DWMCOLORIZATIONCOLORCHANGED = 0x0320,    //Sent to all top-level windows when the colorization color has changed.
        WM_DWMWINDOWMAXIMIZEDCHANGE = 0x0321        //WM_DWMWINDOWMAXIMIZEDCHANGE will let you know when a DWM composed window is maximized. You also have to register for this message as well. You'd have other windowd go opaque when this message is sent.
    }

    public class Dwm

        public enum DWMWINDOWATTRIBUTE : uint
        {
            NCRenderingEnabled = 1,     //Get only atttribute
            NCRenderingPolicy,          //Enable or disable non-client rendering
            TransitionsForceDisabled,
            AllowNCPaint,
            CaptionButtonBounds,
            NonClientRtlLayout,
            ForceIconicRepresentation,
            Flip3DPolicy,
            ExtendedFrameBounds,
            HasIconicBitmap,
            DisallowPeek,
            ExcludedFromPeek,
            Cloak,
            Cloaked,
            FreezeRepresentation
            PlaceHolder1,
            PlaceHolder2,
            PlaceHolder3,
            AccentPolicy = 19
        }

        public enum DWMNCRENDERINGPOLICY : uint
        {
            UseWindowStyle, // Enable/disable non-client rendering based on window style
            Disabled,       // Disabled non-client rendering; window style is ignored
            Enabled,        // Enabled non-client rendering; window style is ignored
        };

        // Values designating how Flip3D treats a given window.
        enum DWMFLIP3DWINDOWPOLICY : uint
        {
            Default,        // Hide or include the window in Flip3D based on window style and visibility.
            ExcludeBelow,   // Display the window under Flip3D and disabled.
            ExcludeAbove,   // Display the window above Flip3D and enabled.
        };

        [StructLayout(LayoutKind.Sequential)]
        public struct DWM_BLURBEHIND
        {
            public DWM_BB dwFlags;
            public int fEnable;
            public IntPtr hRgnBlur;
            public int fTransitionOnMaximized;

            public DWM_BLURBEHIND(bool enabled)
            {
                dwFlags = DWM_BB.Enable;
                fEnable = (enabled) ? 1 : 0;
                hRgnBlur = IntPtr.Zero;
                fTransitionOnMaximized = 0;
            }

        [Flags]
        public enum DWM_BB
        {
            Enable = 1,
            BlurRegion = 2,
            TransitionOnMaximized = 4
        }

        public enum DWMACCENTSTATE
        {
            ACCENT_DISABLED = 0,
            ACCENT_ENABLE_GRADIENT = 1,
            ACCENT_ENABLE_TRANSPARENTGRADIENT = 2,
            ACCENT_ENABLE_BLURBEHIND = 3,
            ACCENT_INVALID_STATE = 4
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct AccentPolicy
        {
            public DWMACCENTSTATE AccentState;
            public int AccentFlags;
            public int GradientColor;
            public int AnimationId;

            public AccentPolicy(DWMACCENTSTATE accentState, int accentFlags, int gradientColor, int animationId)
            {
                AccentState = accentState;
                AccentFlags = accentFlags;
                GradientColor = gradientColor;
                AnimationId = animationId;
            }
        }

        [StructLayout(LayoutKind.Sequential)]
        public struct WinCompositionAttrData
        {
            public DWMWINDOWATTRIBUTE Attribute;
            public IntPtr Data;  //Will point to an AccentPolicy struct, where Attribute will be DWMWINDOWATTRIBUTE.AccentPolicy
            public int SizeOfData;

            public WinCompositionAttrData(DWMWINDOWATTRIBUTE attribute, IntPtr data, int sizeOfData)
            {
                Attribute = attribute;
                Data = data;
                SizeOfData = sizeOfData;
            }
        }

        public struct MARGINS
        {
            public int leftWidth;
            public int rightWidth;
            public int topHeight;
            public int bottomHeight;

            public MARGINS(int LeftWidth, int RightWidth, int TopHeight, int BottomHeight)
            {
                leftWidth = LeftWidth;
                rightWidth = RightWidth;
                topHeight = TopHeight;
                bottomHeight = BottomHeight;
            }

            public void NoMargins()
            {
                leftWidth = 0;
                rightWidth = 0;
                topHeight = 0;
                bottomHeight = 0;
            }

            public void SheetOfGlass()
            {
                leftWidth = -1;
                rightWidth = -1;
                topHeight = -1;
                bottomHeight = -1;
            }
        }


        [SuppressUnmanagedCodeSecurity]
        internal static class SafeNativeMethods
        {
            //https://msdn.microsoft.com/en-us/library/windows/desktop/aa969508(v=vs.85).aspx
            [DllImport("dwmapi.dll")]
            internal static extern int DwmEnableBlurBehindWindow(IntPtr hwnd, ref DWM_BLURBEHIND blurBehind);

            //https://msdn.microsoft.com/it-it/library/windows/desktop/aa969512(v=vs.85).aspx
            [DllImport("dwmapi.dll")]
            internal static extern int DwmExtendFrameIntoClientArea(IntPtr hWnd, ref MARGINS pMarInset);

            //https://msdn.microsoft.com/en-us/library/windows/desktop/aa969515(v=vs.85).aspx
            [DllImport("dwmapi.dll")]
            internal static extern int DwmGetWindowAttribute(IntPtr hwnd, DWMWINDOWATTRIBUTE attr, ref int attrValue, int attrSize);

            //https://msdn.microsoft.com/en-us/library/windows/desktop/aa969524(v=vs.85).aspx
            [DllImport("dwmapi.dll")]
            internal static extern int DwmSetWindowAttribute(IntPtr hwnd, DWMWINDOWATTRIBUTE attr, ref int attrValue, int attrSize);

            [DllImport("User32.dll", SetLastError = true)]
            internal static extern int SetWindowCompositionAttribute(IntPtr hwnd, ref WinCompositionAttrData data);

            [DllImport("dwmapi.dll")]
            internal static extern int DwmIsCompositionEnabled(ref int pfEnabled);
        }

        public static bool IsCompositionEnabled()
        {
            int pfEnabled = 0;
            int result = SafeNativeMethods.DwmIsCompositionEnabled(ref pfEnabled);
            return (pfEnabled == 1) ? true : false;
        }

        public static bool IsNonClientRenderingEnabled(IntPtr hWnd)
        {
            int gwaEnabled = 0;
            int result = SafeNativeMethods.DwmGetWindowAttribute(hWnd, DWMWINDOWATTRIBUTE.NCRenderingEnabled, ref gwaEnabled, sizeof(int));
            return (gwaEnabled == 1) ? true : false;
        }

        public static bool WindowSetAttribute(IntPtr hWnd, DWMWINDOWATTRIBUTE Attribute, int AttributeValue)
        {
            int result = SafeNativeMethods.DwmSetWindowAttribute(hWnd, Attribute, ref AttributeValue, sizeof(int));
            return (result == 0);
        }

        public static void Windows10EnableBlurBehind(IntPtr hWnd)
        {
            AccentPolicy accPolicy = new AccentPolicy() {
                AccentState = DWMACCENTSTATE.ACCENT_ENABLE_BLURBEHIND,
            };

            int accentSize = Marshal.SizeOf(accPolicy);
            IntPtr accentPtr = Marshal.AllocHGlobal(accentSize);
            Marshal.StructureToPtr(accPolicy, accentPtr, false);
            var data = new WinCompositionAttrData(DWMWINDOWATTRIBUTE.AccentPolicy, accentPtr, accentSize);

            SafeNativeMethods.SetWindowCompositionAttribute(hWnd, ref data);
            Marshal.FreeHGlobal(accentPtr);
        }

        public static bool WindowEnableBlurBehind(IntPtr hWnd)
        {
            //Create and populate the Blur Behind structure
            DWM_BLURBEHIND Dwm_BB = new DWM_BLURBEHIND(true);

            int result = SafeNativeMethods.DwmEnableBlurBehindWindow(hWnd, ref Dwm_BB);
            return (result == 0);
        }

        public static bool WindowExtendIntoClientArea(IntPtr hWnd, WinApi.Dwm.MARGINS Margins)
        {
            // Extend frame on the bottom of client area
            int result = SafeNativeMethods.DwmExtendFrameIntoClientArea(hWnd, ref Margins);
            return (result == 0);
        }

        public static bool WindowBorderlessDropShadow(IntPtr hWnd, int ShadowSize)
        {
            MARGINS Margins = new MARGINS(0, ShadowSize, 0, ShadowSize);
            int result = SafeNativeMethods.DwmExtendFrameIntoClientArea(hWnd, ref Margins);
            return (result == 0);
        }

        public static bool WindowSheetOfGlass(IntPtr hWnd)
        {
            MARGINS Margins = new MARGINS();
            Margins.SheetOfGlass();

            //Margins set to All:-1 - Sheet Of Glass effect
            int result = SafeNativeMethods.DwmExtendFrameIntoClientArea(hWnd, ref Margins);
            return (result == 0);
        }

        public static bool WindowDisableRendering(IntPtr hWnd)
        {
            DWMNCRENDERINGPOLICY NCRP = DWMNCRENDERINGPOLICY.Disabled;
            int ncrp = (int)NCRP;
            // Disable non-client area rendering on the window.
            int result = SafeNativeMethods.DwmSetWindowAttribute(hWnd, DWMWINDOWATTRIBUTE.NCRenderingPolicy, ref ncrp, sizeof(int));
            return (result == 0);
        }
    }
}

This is a sample result with a Form.BackColor set to MidnightBlue :

                Window Selected                              Windows Unselected

With some controls on it: