Click-through in C# Form

You can do this with SetWindowLong:

int initialStyle = GetWindowLong(this.Handle, -20);
SetWindowLong(this.Handle, -20, initialStyle | 0x80000 | 0x20);

There are a few magic numbers in here:

  • -20GWL_EXSTYLE

    Retrieves the extended window styles.

  • 0x80000WS_EX_LAYERED

    Creates a layered window.

  • 0x20WS_EX_TRANSPARENT

    Specifies that a window created with this style should not be painted until siblings beneath the window (that were created by the same thread) have been painted. The window appears transparent because the bits of underlying sibling windows have already been painted.

There are numerous articles all over the web on how to do this, such as this one.

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