PowerShell "Add-Type" compile error in CI

By Kaveh Alemi·Latchkey

Add-Type tried to compile inline C# (or VB) and failed. The compiler reported errors, a referenced assembly was missing, or the runtime lacked a usable compiler.

What this error means

Add-Type aborts with "Cannot add type. There were compilation errors." and lists C# error codes. Deterministic for a given source string and runtime.

powershell

Add-Type : Cannot add type. There were compilation errors.
At line:1 char:1
+ Add-Type -TypeDefinition $source
    + CategoryInfo          : InvalidData: (...) [Add-Type], InvalidOperationException
    + FullyQualifiedErrorId : COMPILER_ERRORS,Microsoft.PowerShell.Commands.AddTypeCommand
error CS0246: The type or namespace name 'JsonConvert' could not be found

Diagnose it: which runtime is actually on PATH?

Runners ship multiple versions of most runtimes and select one through a version manager. When a setup action and a version file disagree, the resulting error is about your code rather than the version.

Terminal

which -a <runtime>
<runtime> --version
echo "PATH=$PATH" | tr ":" "\n" | head -20

# does a version file in the repo disagree with the workflow?
cat .tool-versions .nvmrc .ruby-version .python-version 2>/dev/null

Common causes

A required assembly was not referenced

Inline C# that uses a type outside the default references fails to compile until you add the assembly via -ReferencedAssemblies.

API differs between Windows PowerShell and PowerShell 7

Code compiled fine under .NET Framework (PowerShell 5.1) may not compile under .NET (PowerShell 7) because the available APIs differ.

How to fix it

Reference the assemblies the code needs

Pass the required assemblies explicitly so the type resolves at compile time.

powershell

Add-Type -TypeDefinition $source `
  -ReferencedAssemblies 'System.Net.Http','System.Text.Json'

Pin the PowerShell edition for the step

Decide whether the C# targets .NET Framework (powershell: / 5.1) or .NET (pwsh: / 7).
Run the Add-Type step under that edition consistently so the available APIs match.
Surface the real compiler error with $error[0].Exception.Message for the exact CS code.

How to prevent it

Reference required assemblies explicitly and run Add-Type under a fixed PowerShell edition so the available .NET APIs are predictable.

Frequently asked questions

What causes PowerShell "Add-Type" compile error in CI?

There are 2 common causes: a required assembly was not referenced and api differs between windows powershell and powershell 7. Inline C# that uses a type outside the default references fails to compile until you add the assembly via -ReferencedAssemblies.

How do I fix PowerShell "Add-Type" compile error in CI?

There are 2 fixes depending on which cause you have: reference the assemblies the code needs and pin the powershell edition for the step. Work through them in order, since the first is the most common.

What does PowerShell "Add-Type" compile error in CI actually mean?

Add-Type aborts with "Cannot add type.

How do I stop PowerShell "Add-Type" compile error in CI happening again?

Reference required assemblies explicitly and run Add-Type under a fixed PowerShell edition so the available .NET APIs are predictable.

References

GitHub Actions documentation