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Unity Engine

Create or edit the assembly definitions

Configure assembly definitions so the Editor compiles package scripts as separate assemblies.
Read time 3 minutesLast updated 20 days ago
When you create a Unity Package Manager (UPM) package that contains code, you must include at least one assembly definition file (
.asmdef
).
The Unity Editor relies on the assembly definition file to compile the scripts in your package as a separate unit. Unlike scripts in your project's 
Assets
folder (which the Editor automatically compiles), the Editor ignores scripts inside the 
Packages
folder unless they're part of an assembly definition.
For more information about assembly definitions, refer to organizing scripts into assemblies.

Create an assembly definition

When you start developing your package, you need only one assembly definition per package folder that contains code (
Editor
, 
Runtime
, 
Tests/Editor
, 
Tests/Runtime
). When you create a package using the Package Manager window, Unity creates the assembly definitions in these folders for you. If you decide to organize your code into more complex folder structures, you might need more assembly definition files:
  • If you create folders at the same level as the four basic code folders, the additional folders require 
    .asmdef
    files.
  • If you create subfolders within the four basic code folders, you can optionally create additional 
    .asmdef
    files for the code if you want to put that code in separate assemblies.
If you create a more complex folder structure and need to create additional assembly definitions, refer to creating assembly assets.
Important
If the code in your 
Editor
folder depends on runtime code, then you need to add a reference from the 
Editor
assembly definition to the 
Runtime
assembly definition. For more information, refer to Adding a reference to another assembly. Runtime code cannot reference Editor code.

Edit an assembly definition

When you create a package using the Package Manager window, Unity populates some assembly definition fields for you. Use these steps to review the populated fields, add values to optional fields, and add references to other assemblies.
Note
Although you can edit 
.asmdef
files directly, the recommended best practice is to use the Unity Editor, which helps prevent errors and validates your changes.
To edit an assembly definition:
  1. Open the Project window and go to the folder for your package.
  2. Go to the 
    Editor
    , 
    Runtime
    , or 
    Test
    folder for the assembly definition you want to edit.
  3. Select the assembly definition file.
  4. Go to the
    Inspector

    ?

    window.
  5. Set the fields as necessary. Common fields that might need editing include:
    • Name: The name of the assembly. Other scripts can reference this name in their own 
      .asmdef
      files.
    • Assembly Definition References: If your assembly depends on other assemblies, you can add them here. Examples include 
      UnityEngine.UI
      or an 
      Editor
      assembly that depends on a custom 
      Runtime
      assembly.
    • Platforms: You can limit this assembly to specific platforms.
  6. Select Apply at the bottom of the Inspector window.

Additional resources