Reflection code gets error in .NetStandard or .NetFramework

I have forked HtmlAgilityPack to my github and clone it to my visual studio.

The HtmlAgilityPack targets .NetFramework 2.0 and HtmlAgilityPack.NetStandard targets .NetStandard 1.6

I added some reflection codes to it , for example :

public static PropertyInfo GetFirstPropertyInfo(Type t)
{       
     return t.GetProperties().First();
}

It gets me this error :

'Type' does not contain a definition for 'GetProperties' and no extension method 'GetProperties' accepting a first argument of type 'Type' could be found (are you missing a using directive or an assembly reference?) HtmlAgilityPack.NETStandard

After some StackOverflowing I found out I should use this code instead :

public static PropertyInfo GetFirstPropertyInfo(Type t)
{       
     return t.GetTypeInfo().GetProperties().First();
}

but this code gets error in .NetFramework !

'Type' does not contain a definition for 'GetTypeInfo' and no extension method 'GetTypeInfo' accepting a first argument of type 'Type' could be found (are you missing a using directive or an assembly reference?) HtmlAgilityPack

What should I do ???

Thanks.