Which c# project files should I version control?

Also exclude:

.suo
app.config (you should commit something like app-dev.config instead)

Re: app.config:

app.config usually contains machine specific info (like database connection strings, default settings, or paths to resources) and you don't want this clobbered everytime you check out from source control. These files should be created/copied with deploy scripts.

When there are multiple developers on a project each with their own database server, this is especially annoying. The reverse can be even more dangerous - if you deploy code to production by checking out from revision control, you may inadvertently set production to use a development database, which could be disastrous.

Update 2019-03

I have started to rely on the .gitignore suggestions here, as it seems to work perfectly regardless of the type of project I am working on: https://github.com/github/gitignore/blob/master/VisualStudio.gitignore

As of 2019-03-01 it looks like this:

## Ignore Visual Studio temporary files, build results, and
## files generated by popular Visual Studio add-ons.
##
## Get latest from https://github.com/github/gitignore/blob/master/VisualStudio.gitignore

# User-specific files
*.rsuser
*.suo
*.user
*.userosscache
*.sln.docstates

# User-specific files (MonoDevelop/Xamarin Studio)
*.userprefs

# Build results
[Dd]ebug/
[Dd]ebugPublic/
[Rr]elease/
[Rr]eleases/
x64/
x86/
[Aa][Rr][Mm]/
[Aa][Rr][Mm]64/
bld/
[Bb]in/
[Oo]bj/
[Ll]og/

# Visual Studio 2015/2017 cache/options directory
.vs/
# Uncomment if you have tasks that create the project's static files in wwwroot
#wwwroot/

# Visual Studio 2017 auto generated files
Generated\ Files/

# MSTest test Results
[Tt]est[Rr]esult*/
[Bb]uild[Ll]og.*

# NUNIT
*.VisualState.xml
TestResult.xml

# Build Results of an ATL Project
[Dd]ebugPS/
[Rr]eleasePS/
dlldata.c

# Benchmark Results
BenchmarkDotNet.Artifacts/

# .NET Core
project.lock.json
project.fragment.lock.json
artifacts/

# StyleCop
StyleCopReport.xml

# Files built by Visual Studio
*_i.c
*_p.c
*_h.h
*.ilk
*.meta
*.obj
*.iobj
*.pch
*.pdb
*.ipdb
*.pgc
*.pgd
*.rsp
*.sbr
*.tlb
*.tli
*.tlh
*.tmp
*.tmp_proj
*_wpftmp.csproj
*.log
*.vspscc
*.vssscc
.builds
*.pidb
*.svclog
*.scc

# Chutzpah Test files
_Chutzpah*

# Visual C++ cache files
ipch/
*.aps
*.ncb
*.opendb
*.opensdf
*.sdf
*.cachefile
*.VC.db
*.VC.VC.opendb

# Visual Studio profiler
*.psess
*.vsp
*.vspx
*.sap

# Visual Studio Trace Files
*.e2e

# TFS 2012 Local Workspace
$tf/

# Guidance Automation Toolkit
*.gpState

# ReSharper is a .NET coding add-in
_ReSharper*/
*.[Rr]e[Ss]harper
*.DotSettings.user

# JustCode is a .NET coding add-in
.JustCode

# TeamCity is a build add-in
_TeamCity*

# DotCover is a Code Coverage Tool
*.dotCover

# AxoCover is a Code Coverage Tool
.axoCover/*
!.axoCover/settings.json

# Visual Studio code coverage results
*.coverage
*.coveragexml

# NCrunch
_NCrunch_*
.*crunch*.local.xml
nCrunchTemp_*

# MightyMoose
*.mm.*
AutoTest.Net/

# Web workbench (sass)
.sass-cache/

# Installshield output folder
[Ee]xpress/

# DocProject is a documentation generator add-in
DocProject/buildhelp/
DocProject/Help/*.HxT
DocProject/Help/*.HxC
DocProject/Help/*.hhc
DocProject/Help/*.hhk
DocProject/Help/*.hhp
DocProject/Help/Html2
DocProject/Help/html

# Click-Once directory
publish/

# Publish Web Output
*.[Pp]ublish.xml
*.azurePubxml
# Note: Comment the next line if you want to checkin your web deploy settings,
# but database connection strings (with potential passwords) will be unencrypted
*.pubxml
*.publishproj

# Microsoft Azure Web App publish settings. Comment the next line if you want to
# checkin your Azure Web App publish settings, but sensitive information contained
# in these scripts will be unencrypted
PublishScripts/

# NuGet Packages
*.nupkg
# The packages folder can be ignored because of Package Restore
**/[Pp]ackages/*
# except build/, which is used as an MSBuild target.
!**/[Pp]ackages/build/
# Uncomment if necessary however generally it will be regenerated when needed
#!**/[Pp]ackages/repositories.config
# NuGet v3's project.json files produces more ignorable files
*.nuget.props
*.nuget.targets

# Microsoft Azure Build Output
csx/
*.build.csdef

# Microsoft Azure Emulator
ecf/
rcf/

# Windows Store app package directories and files
AppPackages/
BundleArtifacts/
Package.StoreAssociation.xml
_pkginfo.txt
*.appx

# Visual Studio cache files
# files ending in .cache can be ignored
*.[Cc]ache
# but keep track of directories ending in .cache
!?*.[Cc]ache/

# Others
ClientBin/
~$*
*~
*.dbmdl
*.dbproj.schemaview
*.jfm
*.pfx
*.publishsettings
orleans.codegen.cs

# Including strong name files can present a security risk
# (https://github.com/github/gitignore/pull/2483#issue-259490424)
#*.snk

# Since there are multiple workflows, uncomment next line to ignore bower_components
# (https://github.com/github/gitignore/pull/1529#issuecomment-104372622)
#bower_components/
# ASP.NET Core default setup: bower directory is configured as wwwroot/lib/ and bower restore is true
**/wwwroot/lib/

# RIA/Silverlight projects
Generated_Code/

# Backup & report files from converting an old project file
# to a newer Visual Studio version. Backup files are not needed,
# because we have git ;-)
_UpgradeReport_Files/
Backup*/
UpgradeLog*.XML
UpgradeLog*.htm
ServiceFabricBackup/
*.rptproj.bak

# SQL Server files
*.mdf
*.ldf
*.ndf

# Business Intelligence projects
*.rdl.data
*.bim.layout
*.bim_*.settings
*.rptproj.rsuser
*- Backup*.rdl

# Microsoft Fakes
FakesAssemblies/

# GhostDoc plugin setting file
*.GhostDoc.xml

# Node.js Tools for Visual Studio
.ntvs_analysis.dat
node_modules/

# Visual Studio 6 build log
*.plg

# Visual Studio 6 workspace options file
*.opt

# Visual Studio 6 auto-generated workspace file (contains which files were open etc.)
*.vbw

# Visual Studio LightSwitch build output
**/*.HTMLClient/GeneratedArtifacts
**/*.DesktopClient/GeneratedArtifacts
**/*.DesktopClient/ModelManifest.xml
**/*.Server/GeneratedArtifacts
**/*.Server/ModelManifest.xml
_Pvt_Extensions

# Paket dependency manager
.paket/paket.exe
paket-files/

# FAKE - F# Make
.fake/

# JetBrains Rider
.idea/
*.sln.iml

# CodeRush personal settings
.cr/personal

# Python Tools for Visual Studio (PTVS)
__pycache__/
*.pyc

# Cake - Uncomment if you are using it
# tools/**
# !tools/packages.config

# Tabs Studio
*.tss

# Telerik's JustMock configuration file
*.jmconfig

# BizTalk build output
*.btp.cs
*.btm.cs
*.odx.cs
*.xsd.cs

# OpenCover UI analysis results
OpenCover/

# Azure Stream Analytics local run output
ASALocalRun/

# MSBuild Binary and Structured Log
*.binlog

# NVidia Nsight GPU debugger configuration file
*.nvuser

# MFractors (Xamarin productivity tool) working folder
.mfractor/

# Local History for Visual Studio
.localhistory/

# BeatPulse healthcheck temp database
healthchecksdb

# Backup folder for Package Reference Convert tool in Visual Studio 2017
MigrationBackup/

You should include the Properties folder; it contains AssemblyInfo.cs (with all of the assembly attributes) and the project's default Resources and Settings files, if any.

You should also include the .sln file, if any.

Don't forget to add any images, icons etc. you might be using in your application. It's easy to forget these.

Also if you have any documentation associated with the project - specifications, designs, help files etc. Put these into separate folders under the project and then include them too.