Why is there no DirectX API for Linux?

Why not just use OpenGL? It supports all the functionality that DirectX does. Do you have a specific reason to use DirectX? As for a reason, Microsoft made DirectX and I guess they didn't see any need to allow it to run on Linux.

You can run DirectX games in Linux using Wine. You can program XNA on Linux using MonoGame. But all of these use OpenGL to provide hardware-accelerated rendering. AFAIK, OpenGL has been (and probably will be) the only option on Linux for the foreseeable future.

Is it possible to write your own DirectX implementation? Sure - but it involves writing drivers, proprietary knowledge and probably too much cost for anyone to REALLY benefit from it.

EDIT: These days, DXVK is a very real and performant way to run DirectX applications on Linux.

DirectX is a suit of API's:

• Direct3D (drawing 3D graphics)
• DirectX Graphics Infrastructure (enumerating adapters and monitors and managing swap chains)
• Direct2D (drawing 2D graphics)
• DirectWrite (fonts)
• DirectCompute (GPU Computing)
• DirectSound3D (playback of 3D sounds)
• DirectX Media (DirectAnimation for 2D/3D web animation, DirectShow for multimedia playback and streaming media, DirectX Transform for web interactivity, and Direct3D Retained Mode for higher level 3D graphics)
• DirectX Diagnostics (tool for diagnosing and generating reports on components related to DirectX, such as audio, video, and input drivers)
• DirectX Media Objects (support for streaming objects such as encoders, decoders, and effects)
• DirectSetup (installation of DirectX components, and the detection of the current DirectX version)

DirectX components deprecated, but still supported

• DirectDraw
• DirectInput
• DirectPlay
• DirectSound
• DirectMusic

As you can see many parts of the DirectX suit of API's would need to be rewritten with functionality for Drivers written for Linux. Also some parts of the DirectX suit are likely going to make calls to the Windows OS and that code would have to rewritten for a Linux machine without infringement of the copy-written parts of either DirectX or the Windows OS.

Yes you could write a open source API suit which does the same kind of thing as DirectX for Linux, however without a great knowledge of sound and graphic drivers/support by their manufactures for Linux, it would be a very hard task to do.

However if you question was more of a "Can I get/write something to be able to run games and programs made with DirectX to run on Linux?" in it's meaning, in short no due to more likely copyright infringement.

AFAIK, the DirectX contains 3 parts

• Vendor driver (provide low level API defined by DX)
• DirectX library (use low level API defined by DX to access hardware, provide DirectX API)
• Software (use DirectX API, ex. game)

There are no driver providing the low level API defined by DX on linux, so even it is possible to provide the DirectX library, but it won't work without proper vendor driver, and I can't see if there is any vendor is going to create one for their hardware platform.

Your question is wrong in 2019. DXVK is an implementation of D3D11 and D3D10 implementation for Linux using Vulkan and Wine

Also mesa has native Direct3D 9 implementation but in my knowledge, no one uses it except wine