I have had success using DIA from C# without having to manually build the type library or import library from source using Visual Studio 2019. If the COM Server(s) are registered, its possible to simply add a COM reference to the csproj, like this:
<ItemGroup>
<COMReference Include="Dia2Lib.dll">
<WrapperTool>tlbimp</WrapperTool>
<VersionMinor>0</VersionMinor>
<VersionMajor>2</VersionMajor>
<Guid>106173a0-0173-4e5c-84e7-e915422be997</Guid>
<Lcid>0</Lcid>
<Isolated>false</Isolated>
</COMReference>
</ItemGroup>
This works because the COM server dll actually has the type library embedded in it as a resource.
The reference can alternatively be added using the Visual Studio UI by opening your project, right clicking on Dependencies->Add COM Reference..., then search for "dia". An entry named "dia 2.0 Type Library" should be available if the COM server is registered.
Register the two required COM servers as follows, assuming your architecture is AMD64 and you have Visual Studio 2019 Enterprise installed. The path can be changed to match the your specific installation and architecture. This must be done from an elevated command prompt (right click and run as administrator).
regsvr32 /s "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\DIA SDK\bin\msdia140.dll"
regsvr32 /s "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\DIA SDK\bin\amd64\msdia140.dll"
The DIA SDK must of course be installed, which can be done by running Visual Studio Installer, selecting "Modify" for your installation, then choosing "Desktop development with C++" and installing.