var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("db221");
database.DropCollection("employees");

var collection = database.GetCollection<Employee>("employees");
collection.InsertOne(new Employee { EmployeeID = 1, ReportsTo = 1 });

ulong longPrm = 2;

var results = collection.AsQueryable()
    .Where(e => e.ReportsTo.Equals(longPrm))
    .ToList();

foreach (var result in results)
{
    Console.WriteLine($"Found {result.EmployeeID}");
}

public class Employee
{
    public int EmployeeID { get; set; } 
    public int? ReportsTo { get; set; }
}

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'System.UInt64' to type 'System.Nullable`1[System.Int32]'.
   at MongoDB.Bson.Serialization.Serializers.SerializerBase`1.MongoDB.Bson.Serialization.IBsonSerializer.Serialize(BsonSerializationContext context, BsonSerializationArgs args, Object value)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Serialize(IBsonSerializer serializer, BsonSerializationContext context, Object value)
   at MongoDB.Driver.Linq.Linq3Implementation.Misc.SerializationHelper.SerializeValue(IBsonSerializer serializer, Object value)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.EqualsMethodToFilterTranslator.Translate(TranslationContext context, Expression expression, Expression expression1, Expression expression2)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.EqualsMethodToFilterTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.MethodTranslators.MethodCallExpressionToFilterTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateUsingQueryOperators(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.Translate(TranslationContext context, Expression expression, Boolean exprOk)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToFilterTranslators.ExpressionToFilterTranslator.TranslateLambda(TranslationContext context, LambdaExpression lambdaExpression, IBsonSerializer parameterSerializer, Boolean asRoot)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.WhereMethodToPipelineTranslator.Translate(TranslationContext context, MethodCallExpression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToPipelineTranslators.ExpressionToPipelineTranslator.Translate(TranslationContext context, Expression expression)
   at MongoDB.Driver.Linq.Linq3Implementation.Translators.ExpressionToExecutableQueryTranslators.ExpressionToExecutableQueryTranslator.Translate[TDocument,TOutput](MongoQueryProvider`1 provider, Expression expression, ExpressionTranslationOptions translationOptions)
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.Execute()
   at MongoDB.Driver.Linq.Linq3Implementation.MongoQuery`2.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at Program.<Main>$(String[] args) in /Users/arthur.vickers/code/OnYourOwnNow/DriverOnly/Program.cs:line 12