C# language version history:

These are the versions of C# known about at the time of this writing:

• C# 1.0 released with .NET 1.0 and VS2002 (January 2002)
• C# 1.2 (bizarrely enough); released with .NET 1.1 and VS2003 (April 2003). First version to call Dispose on IEnumerators which implemented IDisposable. A few other small features.
• C# 2.0 released with .NET 2.0 and VS2005 (November 2005). Major new features: generics, anonymous methods, nullable types, and iterator blocks
• C# 3.0 released with .NET 3.5 and VS2008 (November 2007). Major new features: lambda expressions, extension methods, expression trees, anonymous types, implicit typing (var), and query expressions
• C# 4.0 released with .NET 4 and VS2010 (April 2010). Major new features: late binding (dynamic), delegate and interface generic variance, more COM support, named arguments, tuple data type and optional parameters
• C# 5.0 released with .NET 4.5 and VS2012 (August 2012). Major features: async programming, and caller info attributes. Breaking change: loop variable closure.
• C# 6.0 released with .NET 4.6 and VS2015 (July 2015). Implemented by Roslyn. Features: initializers for automatically implemented properties, using directives to import static members, exception filters, element initializers, await in catch and finally, extension Add methods in collection initializers.
• C# 7.0 released with .NET 4.7 and VS2017 (March 2017). Major new features: tuples, ref locals and ref return, pattern matching (including pattern-based switch statements), inline out parameter declarations, local functions, binary literals, digit separators, and arbitrary async returns.
• C# 7.1 released with VS2017 v15.3 (August 2017). New features: async main, tuple member name inference, default expression, and pattern matching with generics.
• C# 7.2 released with VS2017 v15.5 (November 2017). New features: private protected access modifier, Span<T>, aka interior pointer, aka stackonly struct, and everything else.
• C# 7.3 released with VS2017 v15.7 (May 2018). New features: enum, delegate and unmanaged generic type constraints. ref reassignment. Unsafe improvements: stackalloc initialization, unpinned indexed fixed buffers, custom fixed statements. Improved overloading resolution. Expression variables in initializers and queries. == and != defined for tuples. Auto-properties' backing fields can now be targeted by attributes.
• C# 8.0 released with .NET Core 3.0 and VS2019 v16.3 (September 2019). Major new features: nullable reference-types, asynchronous streams, indices and ranges, readonly members, using declarations, default interface methods, static local functions, and enhancement of interpolated verbatim strings.
• C# 9.0 released with .NET 5.0 and VS2019 v16.8 (November 2020). Major new features: init-only properties, records, with-expressions, data classes, positional records, top-level programs, improved pattern matching (simple type patterns, relational patterns, logical patterns), improved target typing (target-type new expressions, target typed ?? and ?), and covariant returns. Minor features: relax ordering of ref and partial modifiers, parameter null checking, lambda discard parameters, native ints, attributes on local functions, function pointers, static lambdas, extension GetEnumerator, module initializers, and extending partial.

In response to the OP's question:

What are the correct version numbers for C#? What came out when? Why can't I find any answers about C# 3.5?

There is no such thing as C# 3.5 - the cause of confusion here is that the C# 3.0 is present in .NET 3.5. The language and framework are versioned independently, however - as is the CLR, which is at version 2.0 for .NET 2.0 through 3.5, .NET 4 introducing CLR 4.0, service packs notwithstanding. The CLR in .NET 4.5 has various improvements, but the versioning is unclear: in some places it may be referred to as CLR 4.5 (this MSDN page used to refer to it that way, for example), but the Environment.Version property still reports 4.0.xxx.

As of May 3, 2017, the C# Language Team created a history of C# versions and features on their GitHub repository: Features Added in C# Language Versions. There is also a page that tracks upcoming and recently implemented language features.

The short answer to this question is that none of these values are a reliable indicator of how much memory an executable is actually using, and none of them are really appropriate for debugging a memory leak.

Private Bytes refer to the amount of memory that the process executable has asked for - not necessarily the amount it is actually using. They are "private" because they (usually) exclude memory-mapped files (i.e. shared DLLs). But - here's the catch - they don't necessarily exclude memory allocated by those files. There is no way to tell whether a change in private bytes was due to the executable itself, or due to a linked library. Private bytes are also not exclusively physical memory; they can be paged to disk or in the standby page list (i.e. no longer in use, but not paged yet either).

Working Set refers to the total physical memory (RAM) used by the process. However, unlike private bytes, this also includes memory-mapped files and various other resources, so it's an even less accurate measurement than the private bytes. This is the same value that gets reported in Task Manager's "Mem Usage" and has been the source of endless amounts of confusion in recent years. Memory in the Working Set is "physical" in the sense that it can be addressed without a page fault; however, the standby page list is also still physically in memory but not reported in the Working Set, and this is why you might see the "Mem Usage" suddenly drop when you minimize an application.

Virtual Bytes are the total virtual address space occupied by the entire process. This is like the working set, in the sense that it includes memory-mapped files (shared DLLs), but it also includes data in the standby list and data that has already been paged out and is sitting in a pagefile on disk somewhere. The total virtual bytes used by every process on a system under heavy load will add up to significantly more memory than the machine actually has.

So the relationships are:

• Private Bytes are what your app has actually allocated, but include pagefile usage;
• Working Set is the non-paged Private Bytes plus memory-mapped files;
• Virtual Bytes are the Working Set plus paged Private Bytes and standby list.

There's another problem here; just as shared libraries can allocate memory inside your application module, leading to potential false positives reported in your app's Private Bytes, your application may also end up allocating memory inside the shared modules, leading to false negatives. That means it's actually possible for your application to have a memory leak that never manifests itself in the Private Bytes at all. Unlikely, but possible.

Private Bytes are a reasonable approximation of the amount of memory your executable is using and can be used to help narrow down a list of potential candidates for a memory leak; if you see the number growing and growing constantly and endlessly, you would want to check that process for a leak. This cannot, however, prove that there is or is not a leak.

One of the most effective tools for detecting/correcting memory leaks in Windows is actually Visual Studio (link goes to page on using VS for memory leaks, not the product page). Rational Purify is another possibility. Microsoft also has a more general best practices document on this subject. There are more tools listed in this previous question.

I hope this clears a few things up! Tracking down memory leaks is one of the most difficult things to do in debugging. Good luck.