What is the difference between AsXXX and ToXXX in MongoDb.Bson (official) internally

Question

I'm just wondering how the AsXXX and ToXXX operations work in MongoDb. For example, I have operations like the ones below

var person = new BsonDocument();
person.Add("age", 32);
BsonValue age= person["age"];

Console.WriteLine(age.AsBoolean); //throws an exception
Console.WriteLine(age.ToBoolean());  //True

The AsBoolean() throws an exception, specifically:

System.InvalidCastException : Unable to cast object of type 'MongoDB.Bson.BsonInt32' to type 'MongoDB.Bson.BsonBoolean'

Whereas ToBoolean() returns true.

So my question is when to use one than the other? How do they work internally?

Answer 1

The difference is in the type of conversion.

AsXXX (somewhat like as in C#) is simply a cast. It tries to cast the base BsonValue to the XXX Type and. If the instance is in fact of the right type the conversion works. If not, you'll get an InvalidCastException as you did.

ToXXX isn't a cast but a conversion. It contains some logic on how to convert from one BsonValue to the another and shouldn't throw an exception unless the conversion failed.

In your example the age is a BsonInt32 so AsBoolean fails as it tries to cast it to BsonBoolean but all ToBoolean does is return false if the value is 0 and true otherwise so it can't fail.