System.InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.

[AnthonyMastrean]

The EndOfStream call in the CSV input formatter is synchronous and causes the model binder to throw an exception due to this breaking change.

System.InvalidOperationException: Synchronous operations are disallowed. Call ReadAsync or set AllowSynchronousIO to true instead.

That loop condition can be changed to something like this:

string line;
while ((line = await reader.ReadLineAsync()) != null)