MongoDB in .NET part 10: other file operations


In the previous post on MongoDb we talked about inserting files to GridFS and linking them to a Car object. In this post we’ll look at how to read, delete and update a file.

We’ll build on the CarRental demo project we’ve been working on, so have it ready in Visual Studio.

Reading a file

The primary way of reading a file with the C# driver of MongoDb is the Download method of MongoGridFS and its numerous overloads. With Download you can extract the contents of a file to a stream or to the local file system. With the MongoGridFS.Open method you can put the contents of a file to a Stream but you can only locate the file by file name. There are also the MongoGridFS.Find methods – Find, FindOne, FindAll, FindOneById which allows you to find metadata on a file and indirectly open the contents of the file through the MongoGridFSFileInfo object that the Find methods return. MongoGridFSFileInfo has methods to open a file: Open, OpenRead and OpenText. The Find method lets you search for a specific file using an IMongoQuery object. These Find methods are very much the same as what we saw in the post on querying documents.

Deleting and updating a file

Deletes and updates are handled under the same section as there’s no separate update method. An update means first removing a file and then inserting a new one instead.


Let’s show the image associated with the Car object on Image.cshtml if one exists. We’ll need a helper method on the CarViewModel object to determine whether it has an image. Add the following property to CarViewModel.cs:

public bool HasImage
		return !string.IsNullOrEmpty(ImageId);

We’ll retrieve the Image from a controller action. Add a new Controller to the Controllers folder called ImagesController. Select the Empty MVC Controller template type. Make it derive from BaseController and insert an Image action method:

public class ImagesController : BaseController
        public ActionResult Image(string imageId)
		MongoGridFSFileInfo imageFileInfo = CarRentalContext.CarRentalDatabase.GridFS.FindOneById(new ObjectId(imageId));
		return File(imageFileInfo.OpenRead(), imageFileInfo.ContentType);

The last missing piece is to extend Image.cshtml to show the image. Add the following markup just below the closing brace of the Html.BeginForm statement:

@if (Model.HasImage)
	<img src="@Url.Action("Image", "Images", new { imageId = @Model.ImageId})" />

Run the application, navigate to /cars and click on the Image link of a Car which has a valid image file. If everything’s gone fine then you should see the associated image:

Show car image from GridFS

In case you’re wondering: that’s right, I uploaded the Windows logo from the background of my computer as the car image. It doesn’t make any difference what image you’ve associated with the Car, the main thing is that it’s shown correctly.

Deleting and updating a file

As hinted above, there’s no separate update function for files. Therefore files are updated in two steps: delete the existing one and insert a new one instead. We’ve already seen how to insert a file and link it to an object, so we only need to consider deletions in this section.

Deletions are performed with the Delete function and its overloads: delete by a query, delete by file name and delete by id. You can also delete a file by its MongoGridFSFileInfo wrapper, an instance of which we’ve seen above.

If you use the Delete(IMongoQuery) overload then all files will be deleted one by one that match the search criteria, i.e. the matching files are not deleted in an atomic operation.

We’ll extend our demo app as follows: when an image is uploaded then we check if the Car already has one. If so, then the image is replaced. Otherwise we’ll upload the file like we saw in the previous post.

Insert the following method to CarsController:

private void DeleteCarImage(Car car)
	car.ImageId = string.Empty;

We first delete the image by its ID. Then we need to set the ImageID property of the car to an empty string as there’s no automatic mechanism that can determine that the image of a Car has been deleted and update the ImageId property. Lastly we save the Car object.

We can call this function in an extended version of POST Image:

public ActionResult Image(string id, HttpPostedFileBase file)
	Car car = CarRentalContext.Cars.FindOneById(new ObjectId(id));
	if (!string.IsNullOrEmpty(car.ImageId))
	AttachImageToCar(file, car);
	return RedirectToAction("Index");

That’s it. Run the app as usual and try to replace an image attached to a Car, it should go fine.

Other interesting operations

The MongoGridFS and MongoGridFSFileInfo objects have a MoveTo function. They don’t actually move anything, they set the file name metadata field to a different value. If you want to duplicate a file, then call the CopyTo method that’s available on each object.

You can also modify the metadata of a file through the SetMetada method of MongoGridFS.

The end

This was the last post in the series on MongoDb in .NET. There’s of course loads more to look at but we’d need a book to cover all aspects. However, this should be enough for you to start coding and explore MongoDb in further detail on your own.

View the posts related to data storage here.


About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

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