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Hi @katlv_zheng

Welcome to MongoDB forums!

You need to convert the _id field of your document to a bsoncxx::types::bson_value::view using the bsoncxx::types::bson_value::view_or_value constructor. I have shared sample code below.

// ---------------UPLOAD---------------//
    
     // "sample_gridfs_file" is the name of the GridFS file stored on the server.
    auto uploader = bucket.open_upload_stream("sample_gridfs_file");

    // ASCII for "HelloWorld"
    std::uint8_t bytes[10] = {72, 101, 108, 108, 111, 87, 111, 114, 108, 100};

    // Write 50 bytes to the file.
    for (auto i = 0; i < 5; ++i) {
        uploader.write(bytes, 10);
    }

    auto result = uploader.close();

    // // ---------------DELETE---------------//

    // Delete the uploaded file by using the id. 
    // "id" can be reused directly, but construct a document for the sake of example.
    bsoncxx::types::bson_value::view id = result.id();
    bsoncxx::document::value doc_value = bsoncxx::builder::basic::make_document(bsoncxx::builder::basic::kvp("_id", id));
    bsoncxx::document::view doc_view = doc_value.view();
    
    bsoncxx::types::bson_value::view_or_value id_value {doc_view["_id"].get_value()};
    
    // Pass the id to the delete_file
    bucket.delete_file(id_value);