title: How to get all C# types or namespaces that Roslyn couldn't find?
body: <p>I'm using Roslyn to parse a C# project. I have a <a href="https://docs.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.compilation?view=roslyn-dotnet" rel="nofollow noreferrer"><code>Microsoft.CodeAnalysis.Compilation</code></a> object that represents this project. However, this project might not have been compiled successfully; there may be several reasons for this, but I'm specifically interested in any references to a type or a namespace that couldn't be resolved. <strong>How can I use my <code>Compilation</code> object to retrieve all unknown namespaces or types as <a href="https://docs.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.ierrortypesymbol?view=roslyn-dotnet" rel="nofollow noreferrer"><code>IErrorTypeSymbol</code></a>s?</strong></p>