MongoDB C# - Getting BsonDocument for an Element that doesn't exist

问题

So I have a BsonDocument b (let's say it has FirstName, LastName, Age), which you could access as b["FirstName"], etc...

If I try to do b["asdfasdf"] (which doesn't exist of course), instead of returning null, it errors out the app. What's the correct way to check? Do I really have to do a try/catch?

回答1:

There is also an overload that lets you provide a default value:

BsonDocument document;
var firstName = (string) document["FirstName", null];
// or
var firstName = (string) document["FirstName", "N/A"];

which is slightly more convenient that using Contains when all you want to do is replace a missing value with a default value.

Edit: since the 2.0.1 version, it has been deprecated in favor of GetValue:

var firstName = document.GetValue("FirstName", new BsonString(string.Empty)).AsString;

回答2:

Try the Contains method:

var b = new BsonDocument();
var exists = b.Contains("asdfasdf");

回答3:

An update to Robert's answer, the correct syntax using the C# 2.0 driver is:

var firstName = report.GetValue("FirstName", null);

回答4:

You can use:

var GoodItems = Query.Exists("FirstName");

and than query

People.Find(GoodItems);

That way you'll get only the items that has "FirstName" defined.

回答5:

With the C# driver version 2, it might not be enough to check the existence of the field. The line:

var firstName = report.GetValue("FirstName", null);

will return a BsonNull object if FirstName is actually null in the db, when in fact you'd like to get a string. One way to take this into consideration in a one line code is:

BsonDocument document;
string firstName = ((Func<BsonDocument, string>)(d => { var v = d.GetValue("FirstName", null); return v.IsBsonNull ? null : v.AsString; }))(document);

来源：https://stackoverflow.com/questions/6628794/mongodb-c-sharp-getting-bsondocument-for-an-element-that-doesnt-exist

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