If you have developed any software in Java the chances are that you have come across Maven and the Standard Directory Layout. Some folks were not too keen on the layout, but Maven stuck, and now you can find the code within any Java based project using Maven.
Microsoft Visual Studio
Let's take a look at the projects created by Visual Studio. The solution is created in the root of the selected directory. Next the project itself is created in a sub-directory and added to the solution. I think this is done to keep up the file structure similar to that of the Solution Explorer in the IDE. If you add another project then a new directory is created and the project is added to the solution. This makes sense in the IDE focused development environment that is Visual Studio.
JetBrains Rider
The JetBrains Rider product follows the Visual Studio layout pattern of a solution file in the root with the projects underneath. This makes sense in that their product is considered a Visual Studio IDE replacement. The layout will be familiar to developers picking up the IDE. Even on Linux they keep up appearances and continue with the traditional project structure.
.NET Runtime
If you look at Microsoft's projects in the .NET universe you get a different
picture. The .NET Runtime
project contains a src directory which in turn contains a further
nested directory structure for projects. There is no solution file present in
the root. However, you will find 250+ more solution files embedded in the
directory tree. It's a big project. But the files' presence means they are using
Visual Studio (as expected). If you look into a project directory you will find
the solution file. However, this is not your ordinary Visual Studio directory
structure because there is another src and a tests
directory. The src directory contains the solution file. This
structure looks similar to
~/runtime/src/libraries/Common/Microsoft.CSharp/src. That is hardly
similar to the Visual Studio created default project layout. I am sure they have
their reasons.
David Fowler, a prominent
developer within .NET space, recommends following the solution in the root of
the repository and a src directory for projects. Please see his
Gist .NET
project structure. This makes sense in that he is at Microsoft and either
influenced or was influenced by their decisions for their public repositories.
Visual Studio Code
My point is that there is no standard. This comes to light if you happen to be developing .NET code on a non-Windows platform or if you are developing within Visual Studio Code. There the project structure is entirely up to you. That particular editor, and its usage of the command line tooling, enforces nothing. There is no directory standard layout. There is not even a suggested directory layout other than what is generated by Visual Studio.
Why do I care?
I can already hear you complaining: why you should care about this at all? The reality is that you should only care when working on a project with more than yourself. If you are working a personal project or temporary repository exploring some technology, please, do whatever you want. However, if you are working on a team (which is defined as you + 1) configuring the repositories should be a team effort. Without a Directory Standard Layout guideline for .NET development you can find yourself having multiple projects with differing layouts. I will grant you that is the worst case. The most likely case is that you are using the de facto standard Visual Studio layout. If Visual Studio Code is being used then I would suggest sticking with the Visual studio layout.
If you are working on a Microsoft project then the du jure standard appears to be something entirely different. Then I wish you Godspeed.