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Fixing the “Could not load file or assembly … or one of its dependencies” error

This post describes how to fix the “Could not load file or assembly ‘<assemblyname>’ or one of its dependencies. An attempt was made to load a program with an incorrect format.” error. 

Problem

Especially while installing a new dev machine, and building your project for the first time, you may end up getting the following exception:

Could not load file or assembly '<assemblyname>' or one of its dependencies. An attempt was made to load a program with an incorrect format.

No fear, though, as this is usually easily fixed. In quite a few cases, it’s simply a mismatch between architectures and easily changed.

Reason

The latter part of the error message should point you to the right direction. “An attempt was made to load a program with an incorrect format.” That means that the assembly, which was to be loaded, was in unexpected format. The format, in this case, refers most likely to 64-bit build of an application being deployed to IIS, which is being run in 32-bits.

By default, Visual Studio uses 32-bit version of IIS Express for your deployments. In case you know what you’re doing, it’s safe and easy to change this, though. If you’re building your solution in x64, running it in 32-bit IIS Express won’t work.

In my case, I was building a 64-bit application. Therefore I also had to change the IIS to be ran in 64-bit version. See below, how to fix this!

Solution

  • You’ll need to enable 64-bit IIS Express from VS settings:
    • Visual Studio – Tools – Options – Projects and Solutions – Web Projects – “User the 64 bit version of IIS Express for web sites and projects”, screenshot below:
      IIS Express 64-bit version - how to fix the "Could not load file or assembly..." error
      IIS Express 64-bit version – how to fix the “Could not load file or assembly…” error

 

And you should be golden!

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Antti K. Koskela

Solutions Architect / Escalations Engineer at Koskila / Norppandalotti Software / Valo Solutions
Antti Koskela is a proud digital native nomadic millenial full stack developer (is that enough funny buzzwords? That's definitely enough funny buzzwords!), who works as a Solutions Architect for Valo Intranet, the product that will make you fall in love with your intranet. Working with the global partner network, he's responsible for the success of Valo deployments happening all around the world. He's been a developer from 2004 (starting with PHP and Java), and he's been bending and twisting SharePoint into different shapes since MOSS. Nowadays he's not only working on SharePoint, but also on .NET projects, Azure, Office 365 and a lot of other stuff. This is his personal professional (e.g. professional, but definitely personal) blog.

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