It can be a real downer when a project won't load in Monodevelop. And when you get the error that Monodevelop "can't load a project with an unknown item type," it's even more frustrating. Fortunately, there are a few things you can do to get your project up and running again.
Step 1: Check if the item type is defined
The first step is to make sure that the item type of your project is actually defined. In the .csproj file for your project, look for the <ItemGroup> element and check the ItemType value for each item.
<ItemGroup>
<Compile Include="MyAlertScreen.cs" ItemType="Compile" />
<OtherObject Include="MyOtherFile.cs" ItemType="Code" />
<!-- ... -->
</ItemGroup>
If you find any ItemTypes that are undefined, you can add them just like the example above.
Step 2: Inspect the ExternalProject references
Next, make sure the ExternalProject references in your .csproj file have the right version information. Some versions are incompatible with Monodevelop, so if the version in the reference differs from the version you are using, you may need to remove the reference and add it again.
<ItemGroup>
<!-- Reference from a specific version or branch of a repository. -->
<ExternalProject Include="MyMonoExternalProject" Repository="git://somewhere.example.com/some-repository.git#1.2.3">
<ReferenceOutputAssembly>True</ReferenceOutputAssembly>
</ExternalProject>
</ItemGroup>
Step 3: Try recalling what you changed recently
Even if the item type and external project references appear to be in order, you can still get the "unknown type" error. This could be due to recent changes you have made. Try to recall what you have done recently and if it could be related to the error, undo it and try again.
Step 4: Reinstall Monodevelop
If none of the above have worked, it's time to try reinstalling Monodevelop. Make sure you have backed up your project before doing so. You can then reinstall it either through the command line:
sudo apt-get install monodevelop
or by downloading it from the Monodevelop downloads page.
Once it is reinstalled, try loading the project again. Hopefully, this will fix the issue.
Summary
If Monodevelop is giving you an "unknown item type" error when trying to load a project, don't panic. First, check if the item type is defined, then inspect the ExternalProject references to make sure they have the right version information. Next, try recalling what changes you have recently made, and if last resort, try reinstalling Monodevelop.
FAQ
Q. What is the usual cause for the unknown item type error?
A. The most common cause behind the unknown item type error is an undefined item type in the .csproj for the project. Check for ItemType elements in the <ItemGroup> element and make sure they are correctly defined.
Q. Do I need to reinstall Monodevelop if I see this error?
A. If none of the other steps have worked then reinstalling Monodevelop may be the last resort to fix the unknown item type error. Make sure you have backed up your project before doing this.
Q. What is an example of an undefined item type in the project's csproj file?
A. A undefined item type in the .csproj may look like this:
<ItemGroup>
<Compile Include="MyAlertScreen.cs" ItemType="Compile" />
<!-- ... -->
</ItemGroup>
Where the item type for the MyAlertScreen.cs is defined, but there is no definition for the MyOtherFile.cs.
Q. Is it possible to manually add the item type in the .csproj file?
A. It is possible to manually add the item type in the .csproj file. Just look for the <ItemGroup> element and add an ItemType value for each item. An example of this is given in the "Step 1: Check if the item type is defined" section of this article.