Why can't I do ??= in C#?

Question

I often find myself doing:

foo = foo ?? x;

Why can't I do:

foo ??= x;

Edit: I know it's not part of the language... My question is "why not"? I find the necessity to repeat "foo" to be unpleasing and potentially error-prone. It looks just as ugly as:

foo = foo + x;

Answer 1

The null-coalescing assignment operator ??= was introduced in C# 8.0

https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-8#null-coalescing-assignment