How To Update MongoDB Documents in C# - What You Need To Know

Recently I've been publishing content on working with MongoDB from C#, and this article continues on that path. If you haven't already covered how you can perform filtering of documents from MongoDB in C#, I'd highly suggest you get a basic understanding before continuing. In this article, I'll cover how to update MongoDB documents in C# but the code examples will assume you know how to filter what you're interested in!

If you're looking to incorporate MongoDB into your C# application and want to understand how to update documents, read on!

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Basics of MongoDB in C#

To get started with MongoDB in C#, you'll need to install the MongoDB driver for C#. This driver provides a high-level API for interacting with MongoDB from your C# code, and it's as simple as getting the NuGet package installed. Once you have the driver installed, you can establish a connection to your MongoDB database and start working with documents.

If you're using something like MongoDB Atlas and the desktop tool Compass, these should walk you through how you can get your connection string sorted out. However, if you're using other hosting providers or hosting locally, you'll need to follow relevant instructions for how to get your connection string set up so that you can connect properly. Unfortunately, I can't document this for every possibility :)

In MongoDB, data is stored in collections, which are analogous to tables in a relational database. Maybe not structurally or how they're implemented, but conceptually this is the easiest way to think about them. Each document within a collection is a JSON-like object that can have different fields and values. In SQL databases, we're used to thinking about rows to update, and a single row spans multiple columns in a table. However, in a document database like MongoDB, the "row" is an entire document, which can be hierarchical data. To update a document in MongoDB, you need to specify the collection and the document you want to update.

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Updating Documents in MongoDB Using C#

Before getting ahead of ourselves here with updating, it's important to make sure we understand how filtering works. This is because to update the correct documents, we need to make sure that we can identify WHICH documents we need to update! If your filter is wrong, you'll be in for a world of pain -- or maybe not pain but a high amount of discomfort as you're debugging database issues.

If you need a primer on how to filter, you can read this article on filtering MongoDB records from C#. And if that's not straightforward or aligned with your learning style, you may find this video on using C# to filter MongoDB records of better value:

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UpdateOne and UpdateOneAsync in MongoDB

The UpdateOne method updates a single document that matches the specified filter criteria. You can use the $set operator to update specific fields within the document, or in C# specifically, by calling the Set() method on the update builder.

It's important to note here that there's nothing that forces you to write a filter that will match a single document. In fact, you can write a filter that's just empty to match EVERY document and call UpdateOne. By doing that, you're going to match every document but the method call will still only update one document. Is it the document you expect? Probably by definition, no, if you wrote a filter that matches multiple. Try to pay special attention to this!

Here's a simple example of calling the UpdateOne method, and an important callout that there is an async version as well:

var filter = Builders<MyDocument>.Filter.Eq("fieldName", "fieldValue");
var update = Builders<MyDocument>.Update.Set("fieldName", "newValue");

var result = collection.UpdateOne(filter, update);

// async version
// var result = await collection.UpdateOneAsync(filter, update);

For an acknowledged update, MatchedCount counts documents matched by the operation, while ModifiedCount counts documents actually changed. Setting a field to its existing value can produce a match without a modification. UpdateOne matches at most one document for this operation, so its count cannot establish how many documents a broader filter could match. Check IsAcknowledged before accessing counts and IsModifiedCountAvailable before accessing ModifiedCount, as defined by the driver's result contract.

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UpdateMany and UpdateManyAsync in MongoDB

Much like the previous example, UpdateMany and UpdateManyAsync have *almost* the same behavior -- except for one tiny detail which you probably figured out already. The many part.

UpdateMany will allow you to take that same approach with your filter and update all of the records that match the filter in the way that you've defined your update definition. If you truly expect your filter to be able to match multiple, I would advise using this -- otherwise, UpdateOne makes more sense.

The code example below shows the sync and async variations:

var filter = Builders<MyDocument>.Filter.Gte("fieldValue", minValue);
var update = Builders<MyDocument>.Update.Set("fieldName", "newValue");

var result = collection.UpdateMany(filter, update);

// async version
//var result = await collection.UpdateManyAsync(filter, update);

Like UpdateOne, an acknowledged result has MatchedCount and, when available, ModifiedCount. With UpdateMany, the matched count is not limited to one, but the modified count can still be lower when matching documents already contain the requested values.

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FindOneAndUpdate and FindOneAndUpdateAsync Methods

FindOneAndUpdate and the async variation are very much like UpdateOne method variations. These methods will perform an update on up to one document matching the filter, but the interesting difference is the return value. The return type provides us access to a snapshot of the document. By default, FindOneAndUpdateOptions<MyDocument> uses ReturnDocument.Before; pass options with ReturnDocument = ReturnDocument.After to obtain the updated snapshot instead. The official API reference defines both choices.

Here's a code example of FindOneAndUpdate:

var filter = Builders<MyDocument>.Filter.Gte("fieldValue", minValue);
var update = Builders<MyDocument>.Update.Set("fieldName", "newValue");

var result = collection.FindOneAndUpdate(filter, update);

// async version
//var result = await collection.FindOneAndUpdateAsync(filter, update);

With the default options shown above, a returned document retains the previous fieldName value. Returning the snapshot as part of the update can avoid a separate query for that document.

Check out this video on these MongoDB document update features in C#:

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Wrapping Up How To Update MongoDB Documents in C#

And now you have all of the basics covered on how to update MongoDB documents in C#! In this article, I covered several variations that are all very similar given that they require a proper filter and a proper update definition to be written:

• UpdateOne: updates up to one single document based on the filter
• UpdateMany: updates all of the matching documents based on the filter
• FindOneAndUpdate: updates up to one single document based on the filter and returns a document snapshot according to the return-document option

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Frequently Asked Questions: How To Update MongoDB Documents in C#

What is MongoDB?

Why is updating documents important in MongoDB?

What are the different ways to update documents in MongoDB using C#?

How does the UpdateOne method work in C# for updating MongoDB documents?

How is the UpdateMany method used for MongoDB in C#?