When working with Unreal Engine’s BuildGraph system, you may need to define custom preprocessor macros (defines) at build time. While Unreal Build Tool (UBT) allows defines to be injected via command-line arguments, the process differs slightly depending on whether you’re using a BuildCookRun or Compile node within BuildGraph.

Injecting Defines into Unreal Build Tool (UBT)

UBT provides a straightforward way to inject defines using command-line arguments:

• Global define: -Define:YOUR_DEFINE=WhatYouWant
• Project-level define: -ProjectDefine:YOUR_DEFINE=WhatYouWant

Setting a Define in BuildGraph

Using Defines in BuildCookRun

For BuildCookRun, you can pass the define to UBT using the -ubtargs parameter when invoking BuildCookRun:

<Command Name="BuildCookRun" Arguments="-ubtargs=&quot;-Define:YOUR_DEFINE=WhatYouWant&quot;"/>

This ensures that the define is passed correctly to UBT when running the BuildCookRun process.

Using Defines in Compile Nodes

For Compile nodes, you can directly inject the define using the Arguments attribute:

<Compile Arguments="-Define:YOUR_DEFINE=WhatYouWant" />

This approach simplifies the process, ensuring that the define is included when compiling the specified modules within BuildGraph.