How can I force IIS Express to run in 32 bit mode?

Add <Use64BitIISExpress>false</Use64BitIISExpress> under the <PropertyGroup> tag of your csproj project.

It's also available through the UI by clicking on project properties -> Web tab -> Bitness (change to x64).

In IIS Express property window of VS studio, change the first option: 'Managed Application Pool' from 'Integrated' to 'Classic'

For Visual Studio 2013 Visual Studio 2015 Visual Studio 2017 Visual Studio 2019:

 Tools
  -> Options
   -> Projects and Solutions
    -> Web Projects
     -> Uncheck "Use the 64 bit version of IIS Express for web sites and projects"`

According to this, IIS Express 7.5 comes only as a 32 bit application. IIS Express 8.0 however, comes with both 32 bit and 64 bit versions of the application. The 32 bit version can be found at c:\program files (x86)\IIS Express\iisexpress.exe and the 64 bit version can be found at c:\program files\IIS Express\iisexpress.exe.

IIS Express ignores the enable32BitAppOnWin64 attribute. To run the app pool as 32 bit, you must execute the 32 bit version of IIS Express.

You can determine which version is running in Task Manager by examining the process on the Processes tab. The 64 bit version will be displayed as iisexpress.exe and the 32 bit version will be displayed as iisexpress.exe *32.