Where is the IIS Express configuration / metabase file found?

Where can the IIS Express configuration / metabase file be found?

The configuration file is called applicationhost.config. It's stored here:

My Documents > IIS Express > config

usually, but not always, one of these paths will work

%userprofile%\documents\iisexpress\config\applicationhost.config
%userprofile%\my documents\iisexpress\config\applicationhost.config

Update for VS2019
If you're using Visual Studio 2019+ check this path:

$(solutionDir)\.vs\{projectName}\config\applicationhost.config

Update for VS2015 (credit: @Talon)
If you're using Visual Studio 2015-2017 check this path:

$(solutionDir)\.vs\config\applicationhost.config

In Visual Studio 2015+ you can also configure which applicationhost.config file is used by altering the <UseGlobalApplicationHostFile>true|false</UseGlobalApplicationHostFile> setting in the project file (eg: MyProject.csproj). (source: MSDN forum)

For VS 2015 & VS 2017: Right-click the IIS Express system tray icon (when running the application), and select "Show all applications":

Then, select the relevant application and click the applicationhost.config file path:

Since the introduction of Visual Studio 2015, this location has changed and is added into your solution root under the following location:

C:\<Path\To\Solution>\.vs\config\applicationhost.config

I hope this saves you some time!

To come full circle and include all versions of Visual Studio, @Myster originally stated that;

Pre Visual Studio 2015 the paths to applicationhost.config were:

%userprofile%\documents\iisexpress\config\applicationhost.config
%userprofile%\my documents\iisexpress\config\applicationhost.config

Visual Studio 2015/2017 path can be found at: (credit: @Talon)

$(solutionDir)\.vs\config\applicationhost.config

Visual Studio 2019 path can be found at: (credit: @Talon)

$(solutionDir)\.vs\config\$(ProjectName)\applicationhost.config

But the part that might get some people is that the project settings in the .sln file can repopulate the applicationhost.config for Visual Studio 2015+. (credit: @Lex Li)

So, if you make a change in the applicationhost.config you also have to make sure your changes match here:

$(solutionDir)\ProjectName.sln

The two important settings should look like:

Project("{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}") = "ProjectName", "ProjectPath\", "{XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX}"

and

VWDPort = "Port#"

What is important here is that the two settings in the .sln must match the name and bindingInformation respectively in the applicationhost.config file if you plan on making changes. There may be more places that link these two files and I will update as I find more links either by comments or more experience.