How to create empty folder/object in Minio using C#

Question

I am doing a pet-project for cloud storage. I want to create a new empty folder in an existing bucket. How do i do this?

I have already tried doing it this way:

    public async Task CreateFolderAsync(string path, string folderName)
    {
        var args = new PutObjectArgs()
            .WithBucket(_configuration["MinIO:Bucket"])
            .WithObject(path + folderName + "/")
            .WithStreamData(new MemoryStream(new byte[] { }))
            .WithObjectSize(0);

        await _minioClient.PutObjectAsync(args).ConfigureAwait(false);
    }

but this code returns an error - System.InvalidOperationException: ObjectSize must be set

I haven't found any implementation of this one in c#, but I noticed that similar code works in java:

minioClient.putObject(
            PutObjectArgs.builder()
                    .bucket(bucket)
                    .stream(new ByteArrayInputStream(new byte[]{}), 0, -1)
                    .object("files/testFoler/")
                    .build());

What is the purpose of this? Folders aren't really a thing in object storage, since everything is based on keys. — ProgrammingLlama
– ProgrammingLlama, Commented Mar 5, 2024 at 15:47
@ProgrammingLlama I want to have nested directories, maybe I should just use a database with tables: "files" and "folders", and s3 just as file storage, but I'm not sure that this way I can create the concept of a root folder — x1m
– x1m, Commented Mar 5, 2024 at 17:29
You don't really need to create a folder. You can just put a file to a/b/c/d and another file to a/b/c/e, and it will be handled for you. Things like S3, GCS, etc. use the / to demark folders, but in reality there are no folders, and it's just how the UI presents them. — ProgrammingLlama
– ProgrammingLlama, Commented Mar 6, 2024 at 2:01

pomma89 · Accepted Answer · 2025-01-17 17:35:48Z

Today I faced the same problem and the workaround I used was to upload an empty file:

public async Task CreateFolderAsync(string path, string folderName)
{
    var args = new PutObjectArgs()
        .WithBucket(_configuration["MinIO:Bucket"])
        .WithObject(path + folderName + "/")
        .WithFileName("Empty.txt");

    await _minioClient.PutObjectAsync(args).ConfigureAwait(false);
}

In my case, Empty.txt is an empty file which the application creates before starting. I looked at the code of PutObjectArgs class and I think that this line is what blocks creating a folder starting from an empty stream:

if (ObjectStreamData is not null && ObjectSize == 0)
    throw new InvalidOperationException($"{nameof(ObjectSize)} must be set");

Specifying object size as zero, as shown in the question, does not work because size is compared to zero in order to check if it has been specified. I also tried setting a fake size, such as one, but that does not work either, because it is detected that stream length is not equal to object size.

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