MongoDB duplicate index does not throw error - mongodb

Question

I am new to MongoDB and trying to make MongoDB throw an error when I insert another document with the same index. According to this answer MongoDB should throw an error.
The steps I did are:
1.) Add an index to Name field. I verified that it is added:
> db.room.getIndexes()
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "test.room"
},
{
"v" : 1,
"key" : {
"Name" : 1
},
"name" : "Name_1",
"ns" : "test.room"
}
]
2.) I tried to add document with the same name and was able to add it:
> db.room.find().pretty()
{
"_id" : 1,
"ModifiedDate" : ISODate("2017-02-12T10:59:35.394Z"),
"CreatedDate" : ISODate("2017-02-12T10:59:35.394Z"),
"Name" : "Sample"
}
{
"_id" : 2,
"ModifiedDate" : ISODate("2017-02-12T10:59:39.474Z"),
"CreatedDate" : ISODate("2017-02-12T10:59:39.474Z"),
"Name" : "Sample"
}
I am using C# MongoDB Driver 2.4.

Answer 1

You have to specify that the index you are creating is unique, otherwise MongoDB will not enforce it. You can do that with the C# driver using the CreateIndexOptions class.
roomCollection.Indexes
.CreateOne(
Builders<Room>.IndexKeys.Ascending(r => r.Name),
new CreateIndexOptions() { Unique = true });
Note that index creation will fail if there are currently duplicate names in the collection.