Recommend users to cache serializers created from FromTypes(Type[]) and FromMappings(XmlMapping[])

[jiayi11]

Fixes dotnet/corefx#33950
@Lxiamail @mconnew

[jiayi11]

Hi @mairaw, is there a way for me to view what pages looks like after the change?

[mairaw]

Hi @yujayee, on the status box, you can find a line for OpenPublishing.Build with a link to Details. On that, you can find links to the review site and then you can search for the API you're editing.

For example:
https://review.docs.microsoft.com/en-us/dotnet/api/system.xml.serialization.xmlserializer.frommappings?view=netframework-4.7.2&branch=pr-en-us-1964

[BillWagner]

These LGTM @yujayee
I'll now.