I was unable to find the proper way to call shell command from Mongo C# driver version 2.7.2
public async Task RsStatus()
{
var
db.adminCommand
function expects and object to be passed as a parameter (here) so you can take advantage of BsonDocumentCommand
generic type and also get a result as a BsonDocument, try:
var command = new BsonDocumentCommand<BsonDocument>(
new BsonDocument() { { "replSetGetStatus", 1 } });
var res = await _admin.RunCommandAsync<BsonDocument>(command);