First, when you create the swapchain you need to set the DXGI_USAGE_SHADER_INPUT flag.

Second: when do you want to read from it? If it's after you've called Present(), then you need to make sure you use the DXGI_SWAP_EFFECT_SEQUENTIAL or DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL effects, and that the buffer you get is (BufferCount - 1). It it's before you call Present(), then none of that matters and you can just make the view of buffer 0.

matters and you can just make the view of buffer 0.

I need to read it . The first(initial) render target. The current back buffer.

PSSetShaderResources(slot, 1, 0); ????

I need to read it.

Do you?

What are you actually trying to accomplish with this?

It's usually a far better idea to render into an off-screen buffer and use _that_ as your shader input, using the back buffer for only the very final image.

If you're trying to grab a screen cap or something, consider using CopyResource to make staging textures instead of trying to read or manipulate the back buffer directly.

The windowing system is generally setup to treat the Swap Chain buffers very specially. You don't want to read out of them if you can at all avoid doing so.

Thank you , I take it.