Why can the 'as' operator not be used to parse non-nullable value types?

问题

Every Developer has his/her own standards. Some developers like to <type>.TryParse(), some developers like to cast using (type)object;, and some developers love using the keywords instead.

I noticed a hiccup with the 'as' operator - you cannot use it to perform conversions between non-nullable value types. I read the documentation on MSDN on the as Keyword and they also explain it as "You can use the as operator to perform certain types of conversions between compatible reference types or nullable types."

I tested this with the following:

                    int i = 0;
                    var k = i as int; //Breaks

                    int i = 0;
                    var k = i as int?; //Works

What were the reasons decided for the as keyword to perform in this way?

回答1:

as operator would return null if parsing fails. Since int is a non nullable value type, you get the error, whereas int? or Nullable<int> can hold null value, that is why your second code snippet works.

See: as (C# Reference)

You can use the as operator to perform certain types of conversions between compatible reference types or nullable types

also from the same doc link

The as operator is like a cast operation. However, if the conversion isn't possible, as returns null instead of raising an exception.

来源：https://stackoverflow.com/questions/24265139/why-can-the-as-operator-not-be-used-to-parse-non-nullable-value-types