leetcode is using C# language version 7.2.\n\nVersion 7.3 of C# adds [ref reassignment](https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/ref-returns#ref-locals) - a useful feature for some solutions.\n\nFor example, the C# solution to [Swap Nodes in Pairs](https://leetcode.com/problems/swap-nodes-in-pairs/) can look very much like a good C++ solution if you can reassign ref locals.

leetcode is using C# language version 7.2.\n\nVersion 7.3 of C# adds ref reassignment - a useful feature for some solutions.\n\nFor example, the C# solution to 