C# MongoDB mapping class

Question

Hi I'm really struggling with classes and mapping a MongoDB document, I will admit I am struggling to come to understand the whole OOP thing and my coding so far is limited to just functional coding at the moment. My document has many fields but I am only interested in a few of them for the time being. I wrote my program in Python without classes but I want to recreate it in C#.

My Python code that returns the two fields.

 stock_details = collection.find_one({'Stock Number' : stock_number})
        #print(stock_details.keys()) 
        val1 = stock_details['Stock Number']
        val2 = stock_details['Qty In Stock']

        return val1, val2,

My C# code that I'm struggling with

 public static StockItem GetStockItem(string StockNumber)
    {
        var client = new MongoClient()
        var db = client.GetDatabase("storesdb");
        var collection = db.GetCollection<BsonDocument>("storeslist");
        var filter = Builders<StockItem>.Filter.Eq("Stock Number", StockNumber);
        var findfilter = collection.Find(filter).FirstOrDefault();
        var returnValue = findfilter;
        return returnValue;
    }


    [BsonIgnoreExtraElements]
    public class StockItem
    {
        [BsonElement("Stock Number")]
        public string stockNumber { get; set; }

        [BsonElement("Qty In Stock")]
        public int qtyInStock { get; set; }

    }

I can successfully get a Bson document but when I try using a class, I am unable to map it to a variable/object, trying the above code gives me the following error.

Severity    Code    Description Project File    Line    Suppression State
Error   CS1503  Argument 2: cannot convert from 'MongoDB.Driver.FilterDefinition<Program.StockItem>' to 'System.Linq.Expressions.Expression<System.Func<MongoDB.Bson.BsonDocument, bool>>'  Storesincsharp  C:\Users\zaks\source\repos\Storesincsharp\Storesincsharp\Program.cs 46  Active

Changing the document filter to Bson works okay, but trying to return it as a class object gives me this error instead.

Severity    Code    Description Project File    Line    Suppression State
Error   CS0029  Cannot implicitly convert type 'MongoDB.Bson.BsonDocument' to 'Program.StockItem'   Storesincsharp  C:\Users\zaks\source\repos\Storesincsharp\Storesincsharp\Program.cs 48  Active

Could some one point me in the right direction?

you should create typed collection. Instead: var collection = db.GetCollection<BsonDocument>("storeslist");, try this: var collection = db.GetCollection<StockItem>("storeslist"); — dododo
– dododo, Commented Sep 18, 2020 at 17:56

Tim · Accepted Answer · 2020-09-22 23:45:21Z

2

You will want to use a typed collection. In addition, to avoid typo's you can also specify your filter names via linq rather than with string names. It also helps when you refactor your classes. Example:

    public static StockItem GetStockItem(string StockNumber)
    {
        var client = new MongoClient();
        var db = client.GetDatabase("storesdb");
        var collection = db.GetCollection<StockItem>("storeslist");  // Typed collection.
        var filter = Builders<StockItem>.Filter.Eq(x=> x.stockNumber, StockNumber); // Referenced property name.
        StockItem returnValue = collection.Find(filter).FirstOrDefault();
        return returnValue;
    }

answered Sep 22, 2020 at 23:45

Tim

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