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Add tests for NH-3050 contributed by Christophe Gijbels
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‎src/NHibernate.Test/NHSpecificTest/NH3050/Fixture.cs

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using System;
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using System.Collections;
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using System.Collections.Generic;
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using System.Reflection;
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using System.Threading;
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using NHibernate.Engine.Query;
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using NHibernate.Linq;
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using NHibernate.Util;
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using NUnit.Framework;
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using System.Linq;
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namespace NHibernate.Test.NHSpecificTest.NH3050
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{
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[TestFixture]
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public class Fixture : BugTestCase
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{
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[Test]
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public void Test()
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{
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// WARNING: This test case makes use of reflection resulting in failures if internals change, but reflection was needed to allow for simulation.
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// This test simulates a heavy load on the QueryPlanCache by making it use a SoftLimitMRUCache instance with a size of 1 instead of the default of 128.
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// Since this cache moves the most recently used query plans to the top, pushing down the less used query plans until they are removed from the cache,
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// the smaller the size of the cache the sooner the query plans will be dropped, making it easier to simulate the problem.
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//
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// What is the exact problem:
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// -> When executing a LINQ query that has a contains with only one element in the collection the same queryExpression string is generated by 2 different types
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// of IQueryExpression, the 'NHibernate.Impl.ExpandedQueryExpression' and the 'NHibernate.Linq.NhLinqExpression' and that key is used to store a query plan
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// in the QueryPlanCache if a query plan is requested and not found in the cache.
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// -> The 'NHibernate.Linq.NhLinqExpression' is typically added during the DefaultQueryProvider.PrepareQuery and the 'NHibernate.Impl.ExpandedQueryExpression'
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// less likely during the execution of the LINQ query
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// -> Unfortunately the PrepareQuery is casting the returned query plan's QueryExpression to a NhLinqExpression, which it assumes will always be the case, but this
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// is not true in a heavy loaded environment where the cache entries are constantly moving when other queries are being executed at the same time.
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// -> If you look at the following method inside the DefaultQueryProvider class, then you'll see that by drilling down in the PrepareQuery and in the ExecuteQuery, that
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// both operations are actually requesting the query plan from the QueryPlanCache at some point
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// public virtual object Execute(Expression expression)
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// {
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// IQuery query;
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// NhLinqExpression nhQuery;
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// NhLinqExpression nhLinqExpression = PrepareQuery(expression, out query, out nhQuery);
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// return ExecuteQuery(nhLinqExpression, query, nhQuery);
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// }
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//
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// When they are requesting the corresponding query plan according to the QueryExpression's key the PrepareQuery assumes it will get back a NhLinqExpression, while it
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// is perfectly possible that the corresponding query plan has a QueryExpression of type ExpandedQueryExpression that has been added during the ExecuteQuery because
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// when a request was made for the query plan during the execution, the load on the cache has put the query plan with a QueryExpression of type NhLinqExpression and with
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// the same key somewhere at the bottom of the MRU cache and it might even have been removed from the cache, resulting in adding a query plan with a QueryExpression value
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// of type ExpandedQueryExpression. When the same LINQ query is executed afterwards, it will go through the PrepareQuery again, assuming that what is returned is a
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// NhLinqExpression, while in reality it is an ExpandedQueryExpression, resulting in a cast exception. This problem might even go away due to the same load, pushing out
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// the cached query plan with a QueryExpression of ExpandedQueryExpression and have a NhLinqExpression added back again during the next Prepare.
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//
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// So this test will simulate the pushing out by clearing the cache as long as the QueryExpression of the query plan is NhLinqExpression, once it is an ExpandedQueryExpression
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// it will stop clearing the cache, and the exception will occur, resulting in a failure of the test.
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// The test will pass once all LINQ expression are executed (1000 max) and no exception occured
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var cache = new SoftLimitMRUCache(1);
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var queryPlanCacheType = typeof (QueryPlanCache);
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// get the planCache field on the QueryPlanCache and overwrite it with the restricted cache
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queryPlanCacheType
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.GetField("planCache", BindingFlags.Instance | BindingFlags.NonPublic)
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.SetValue(sessions.QueryPlanCache, cache);
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// Initiate a LINQ query with a contains with one item in it, of which we know that the underlying IQueryExpression implementations
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// aka NhLinqExpression and the ExpandedQueryExpression generate the same key.
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IEnumerable<int> personIds = new List<int>
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{
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1
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};
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ISession session = null;
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try
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{
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session = OpenSession();
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var allLinqQueriesSucceeded = false;
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// Setup an action delegate that will be executed on a separate thread and that will execute the LINQ query above multiple times.
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// This will constantly interact with the cache (Once in the PrepareQuery method of the DefaultQueryProvider and once in the Execute)
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System.Action queryExecutor = () =>
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{
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var sessionToUse = sessions.OpenSession();
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try
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{
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for (var i = 0; i < 1000; i++)
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{
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(from person in session.Query<Person>()
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where personIds.Contains(person.Id)
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select person).ToList();
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}
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allLinqQueriesSucceeded = true;
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}
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finally
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{
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if (sessionToUse != null && sessionToUse.IsOpen)
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{
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sessionToUse.Close();
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}
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}
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};
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(from person in session.Query<Person>()
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where personIds.Contains(person.Id)
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select person).ToList();
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// the planCache now contains one item with a key of type HQLQueryPlanKey,
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// so we are going to retrieve the generated key so that we can use it afterwards to interact with the cache.
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// The softReferenceCache field value from the SoftLimitMRUCache cache instance contains this key
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var field = cache.GetType().GetField("softReferenceCache", BindingFlags.NonPublic | BindingFlags.Instance);
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var softReferenceCache = (IEnumerable) field.GetValue(cache);
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// Since the cache only contains one item, the first one will be our key
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var queryPlanCacheKey = ((DictionaryEntry) softReferenceCache.First()).Key;
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// Setup an action that will be run on another thread and that will do nothing more than clearing the cache as long
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// as the value stored behind the cachekey is not of type ExpandedQueryExpression, which triggers the error.
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// By running this constantly in concurrency with the thread executing the query, the odds of having the wrong
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// QueryExpression in the cache (wrong as in the PrepareQuery is not expecting it) augments, simulating the workings
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// of the MRU algorithm under load.
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System.Action cacheCleaner = () =>
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{
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while (!allLinqQueriesSucceeded)
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{
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var hqlExpressionQueryPlan = (HQLExpressionQueryPlan) cache[queryPlanCacheKey];
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if (hqlExpressionQueryPlan != null)
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{
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if (hqlExpressionQueryPlan.QueryExpression.GetType().FullName.Contains("NHibernate.Impl.ExpandedQueryExpression"))
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{
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// we'll stop clearing the cache, since the cache now has a different query expression type than expected by the code
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break;
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}
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}
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cache.Clear();
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// we sleep a little, just to make sure the cache is not constantly empty ;-)
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Thread.Sleep(50);
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}
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};
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var queryExecutorAsyncResult = queryExecutor.BeginInvoke(null, null);
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var cacheCleanerAsyncResult = cacheCleaner.BeginInvoke(null, null);
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queryExecutor.EndInvoke(queryExecutorAsyncResult);
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cacheCleaner.EndInvoke(cacheCleanerAsyncResult);
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Assert.IsTrue(allLinqQueriesSucceeded);
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}
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finally
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{
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if (session != null)
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{
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session.Close();
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}
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}
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}
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}
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}

‎src/NHibernate.Test/NHSpecificTest/NH3050/Mappings.hbm.xml

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<?xml version="1.0" encoding="utf-8" ?>
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<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
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namespace="NHibernate.Test.NHSpecificTest.NH3050"
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assembly="NHibernate.Test">
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<class name="Person">
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<id name="Id">
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<generator class="native"/>
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</id>
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</class>
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</hibernate-mapping>

‎src/NHibernate.Test/NHSpecificTest/NH3050/Person.cs

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using System;
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namespace NHibernate.Test.NHSpecificTest.NH3050
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{
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[Serializable]
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public class Person
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{
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public virtual int Id { get; set; }
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}
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}

‎src/NHibernate.Test/NHibernate.Test.csproj

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<Compile Include="NHSpecificTest\BagWithLazyExtraAndFilter\Domain.cs" />
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<Compile Include="NHSpecificTest\BagWithLazyExtraAndFilter\Fixture.cs" />
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<Compile Include="Component\Basic\ComponentWithUniqueConstraintTests.cs" />
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<Compile Include="NHSpecificTest\NH3050\Fixture.cs" />
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<Compile Include="NHSpecificTest\NH3050\Person.cs" />
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<Compile Include="NHSpecificTest\NH2469\Domain.cs" />
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<Compile Include="NHSpecificTest\NH2469\Fixture.cs" />
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<Compile Include="NHSpecificTest\NH2033\Customer.cs" />
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<EmbeddedResource Include="NHSpecificTest\NH1291AnonExample\Mappings.hbm.xml" />
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</ItemGroup>
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<ItemGroup>
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<EmbeddedResource Include="NHSpecificTest\NH3050\Mappings.hbm.xml" />
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<EmbeddedResource Include="NHSpecificTest\NH2469\Mappings.hbm.xml" />
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<EmbeddedResource Include="NHSpecificTest\NH2033\Mappings.hbm.xml" />
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<EmbeddedResource Include="NHSpecificTest\NH2819\Mappings.hbm.xml" />

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