C# Equivalent for C++ std::variant (sum type)

In C++ we have std::variant for creating a sum-types.
For example, the following will allow v to hold either a std::string or an int:

#include <variant>
#include <string>

//...

std::variant< std::string, int> v;
v = "aaa";  // now v holds a std::string
v = 5;      // now v holds an int

In addition – the compiler will enforce that you assign v only with values convertible to std::string or int.

I am looking for a similar construct in C#.
Had a look at this post: Variant Type in C#,
but it didn’t offer the proper equivalent I am looking for.

Is there one in C#?

Edit:
The SO post Discriminated union in C# is related but does not exactly answer my question because I am looking for a general language construct and not for a solution for a specific case.
However one of the answers mentioned the OneOf library, which is also one of the solutions in the accepted answer here.

>Solution :

You can use Either monad from library language-ext. Install LanguageExt.Core NuGet package.

using LanguageExt;

//...

Either<string, int> v;
v = "aaa";
v = 5;

Or you could use OneOf library. Install OneOf NuGet package.

using OneOf;

//...

OneOf<string, int> v;
v = "aaa";
v = 5;