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copied!<p>The official C# driver will write a "_t" discriminator value whenever the actual type of an object is different than the nominal type. So for example:</p> <pre><code>MyRootClass obj = new MyDerivedClass(); collection.Insert(obj); </code></pre> <p>The Insert statement could also have been written:</p> <pre><code>collection.Insert<MyRootClass>(obj); </code></pre> <p>but it's easier to let the compiler infer the type parameter.</p> <p>Since the actual type of obj is different than the nominal type the "_t" discriminator will be written.</p> <p>When reading back the object you will have to ensure that MyDerivedClass has been properly registered:</p> <pre><code>BsonClassMap.RegisterClassMap<MyDerivedClass>(); </code></pre> <p>or the serializer won't recognize the discriminator (this may seem like a restriction, but it's only logical that the serializer can only work with types it knows about).</p> <p>You mentioned that you don't know the classes at compile time, so the above registration code must be invoked dynamically. One way to do it is:</p> <pre><code>Type myDerivedClass; // your plugged-in class var registerClassMapDefinition = typeof(BsonClassMap).GetMethod("RegisterClassMap", new Type[0]); var registerClassMapInfo = registerClassMapDefinition.MakeGenericMethod(myDerivedClass); registerClassMapInfo.Invoke(null, new object[0]); </code></pre> <p>Technically, the serialization is not using reflection; it is metadata driven. Reflection is used once to construct the class map, but after that the class map is used directly without reflection, and the overhead is rather low.</p>

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