Retrieve data from mongodb using C# driver

Question

I'm using official mongodb driver for c# in my test project and i've already insert document from c# web application to mongodb. In mongo console, db.blog.find() can display entries I've inserted. but when i tried to retrieve them, .net throw a exception

"System.InvalidOperationException: ReadString can only be called when CurrentBsonType is String, not when CurrentBsonType is ObjectId."

my entity class is very simple

namespace MongoDBTest
{
    public class Blog
    {
        public String _id
        {
            get;
            set;
        }

        public String Title
        {
            get;
            set;
        }
    }
}

and this is my retrieve code

public List<Blog> List()
{
    MongoCollection collection = md.GetCollection<Blog>("blog");
    MongoCursor<Blog> cursor = collection.FindAllAs<Blog>();
    cursor.SetLimit(5);
    return cursor.ToList();
}

can anybody help me out? thanks!

Andrew Orsich · Accepted Answer · 2011-07-21 07:21:36Z

11

I suppose you just need mark your blog Id with BsonId (and insert id yourself) attribute:

public class Blog
{
    [BsonId]
    public String Id {get;set;}

    public String Title{get;set;}
}

And all should be okay. Issue was because you not marked what field will be Mongodb _id and driver generated _id field with type ObjectId. And when driver trying deserialize it back he can't convert ObjectId to String.

Complete example:

MongoCollection collection = md.GetCollection<Blog>("blog");
var blog = new Blog(){Id = ObjectId.GenerateNewId().ToString(), 
                      Title = "First Blog"};
collection .Insert(blog);

MongoCursor<Blog> cursor = collection.FindAllAs<Blog>();
cursor.SetLimit(5);

var list = cursor.ToList();

answered Jul 21, 2011 at 7:21

Andrew Orsich

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2 Comments

Ken Qiu Over a year ago

hi Andrew Orsich, I've inserted document using BsonDocument and mongodb auto generate ObjectId for document and store it, when i use MongoCollection<Blog> and call FindAllAs<Blog> method, ReadString method throws exception. So i change MongoCollection<Blog> to MongoCollection<BsonDocument> and traversal cursor object in foreach statement to convert each document to blog object. Anyway, ur reply help me resolved my problem, thank u :)

CodeMonkey1313 Over a year ago

An easier fix would be to change the datatype of your ID to ObjectId and decorate it with [BsonId]. I ran into the same error message, and making that change fixed it for me.

sujith · Accepted Answer · 2015-07-16 15:05:43Z

3

Retrieving the data from the MongoDB using C# is pretty simple and there are three different ways the data can be rolled out for frontend

List
Cursor

linq

using (var cursor = await col.Find(new BsonDocument()).ToCursorAsync())
{
 while (await cursor.MoveNextAsync())
 {
    foreach (var doc in cursor.Current)
    {
        Console.WriteLine(doc);
    }
  }
}

the above code Shows to retrieve the data using cursors. Referrence

answered Jul 16, 2015 at 15:05

sujith

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Comments

Amit Kumar · Accepted Answer · 2014-02-10 20:05:35Z

1

Take a look at my sample project at github. Off late, I've become really interested in MongoDB. This application shows many useful things that might interest you as well; repository pattern with MongoDB.

answered Feb 10, 2014 at 20:05

Amit Kumar

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Comments

rink.attendant.6 · Accepted Answer · 2014-06-15 07:59:15Z

0

The type and name of the id member should be different as follows

public ObjectId Id { get; set; }

Where ObjectId is in the namespace MongoDB.Bson

edited Jun 15, 2014 at 7:59

rink.attendant.6

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answered Jun 15, 2014 at 7:35

Yossi Lev

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