Introduction

In .NET and MSBuild-based projects, the obj folder holds intermediate build artifacts that help the tooling restore packages, generate code, and perform incremental compilation. It is not the final output directory, and understanding that distinction explains why obj appears automatically and why it often should not be committed to source control.

obj Is for Intermediate Build Output

When you build a project, the compiler and build system do not jump directly from source files to the final executable or library. They create and cache intermediate files along the way.

That is what the obj folder is for.

Typical responsibilities include:

• storing generated source or metadata
• holding restore results and dependency graphs
• caching files used for incremental builds
• separating per-configuration intermediates such as Debug and Release

The final binaries usually go to bin, while intermediate state goes to obj.

What You Often Find Inside It

The exact contents vary by project type, but common examples include:

• generated assembly info files
• NuGet restore artifacts such as project.assets.json
• temporary compiled resources
• generated code from source generators or Razor tooling
• configuration-specific subfolders such as obj/Debug/net8.0

In SDK-style .NET projects, one very common file is project.assets.json, which describes the resolved package graph after restore.

Why the Build System Needs It

The obj folder exists mainly for correctness and speed.

Speed matters because the build system can reuse information from a previous build instead of recomputing everything from scratch. If only one source file changed, MSBuild can often skip work that still has valid intermediate results.

Correctness matters because generated artifacts need a place to live before they are packaged into the final assembly. For example, assembly metadata or generated C# files may be created during the build and then compiled as if they were part of the project.

A Typical Layout

A project might look like this:

1MyApp/
2  MyApp.csproj
3  Program.cs
4  bin/
5    Debug/
6      net8.0/
7  obj/
8    Debug/
9      net8.0/
10        MyApp.AssemblyInfo.cs
11        project.assets.json
12        MyApp.dll

The exact filenames differ, but the pattern is consistent: bin is what you run or publish, while obj is what the toolchain uses during the build.

Can You Delete It?

Usually yes. The obj folder is generated output, so deleting it is a normal cleanup step when a build gets into a bad state.

Common cleanup commands:

dotnet clean

Or, if you want to force a fresh restore and rebuild:

rm -rf bin obj
dotnet restore
dotnet build

After deletion, the next build recreates the folder automatically.

This is why obj is usually ignored in source control with a .gitignore rule.

Do Not Confuse obj with bin

Developers new to .NET often ask why the project has both folders.

The short answer:

• 'obj contains temporary, intermediate build state'
• 'bin contains the final compiled output you execute, test, or publish'

If your application runs, you normally care about bin. If your build is behaving strangely, obj is one of the first places to suspect because stale intermediate state can cause confusing errors.

You Can Configure the Intermediate Path

MSBuild lets you change where intermediate files go.

1<Project Sdk="Microsoft.NET.Sdk">
2  <PropertyGroup>
3    <TargetFramework>net8.0</TargetFramework>
4    <BaseIntermediateOutputPath>artifacts/obj/</BaseIntermediateOutputPath>
5  </PropertyGroup>
6</Project>

That is useful in custom build layouts, monorepos, or CI setups where you want tighter control over generated artifacts.

Common Pitfalls

The biggest mistake is checking obj into version control. Those files are machine-generated, noisy, and often differ across environments, which creates useless churn in commits.

Another common issue is assuming obj contains only temporary junk with no build value. In reality, the build system relies on it heavily for restore metadata and incremental build tracking.

People also delete bin but forget obj when troubleshooting. If stale intermediate state is the problem, removing only the final output may not help.

Finally, do not treat every file inside obj as something you should reference directly from application code. Those files are for the build pipeline, not for runtime usage.

Summary

• The obj folder stores intermediate build artifacts used by MSBuild and related tooling.
• It is different from bin, which holds the final build output.
• Files in obj help with restore, code generation, and incremental builds.
• Deleting obj is usually safe because the build recreates it.
• In most projects, obj should be ignored by source control.