“Object reference not set to an instance of an object” - but nothing is null?

Question

Yeah, you probably think; "God, another one?".

Yes, another one.

"Object reference not set to an instance of an object."

I've been working with EF6 lately and after developing for some time, I found that a little bit more optimization was needed. Alot has been reworked without problems, but it seems I can't figure out this one.

In my application I've been using this piece of pseudo code to get items from the database.

DbContext context = new DbContext();

public IEnumerable<string> GetExistingNames(IEnumerable<string> names)
{
    foreach(string name in names)
    {
        string existingName = context.Names.Where(n => n.Name == name).FirstOrDefault();
        if(existingName == null) continue;
        yield return existingName;
    }
}

Note that the DbContext is only there for clarification. It gets disposed when it's needed.

This approach "works" but it would mean that if I had, say, 20 names to look up, I would hit the database for about 20 times. Ouch!

Therefore I started looking for a way to implement a single query. I've found a way, but it's not really working as it should. This is my current approach;

public IEnumerable<string> GetExistingNames(ICollection<string> names)
{
    IQueryable<Names> query = context.Names.Where(n => names.Contains(n.Name));
    if(query == null) yield break;
    foreach(var name in query)
    {
        yield return name.Name;
    }
}

This should, to my knowledge, translate in SELECT ... FROM Names WHERE ... IN (...) . However, my application crashes at foreach(var name in query) as soon as it hits name , throwing the feared NullReferenceException . It does, however, pass if(query == null) , meaning the query is not null. At this point, I was confused. How can it not be null, but still throw this error?

I was not sure if the query gets executed if I try to access it with this approach. Therefore, I tried to create a list from the query using ToList() , but this throws the same exception upon creating the list.

It seems like everytime I make a call to query , it gives me a NullReferenceException . However, it still passes if(query == null) . So, my question is;

Why is it passing the test, but is it not accessible? Did I misinterpret IQueryable<> ? And if I did misinterpret it, how should it be done properly?

EDIT

I have debugged before posting. I know for sure that;

• names is not null.
• context is not null.

Code calling the function:

//A wrapper for the DbContext. This is only used for some methods
//which require the DbContext
DbContextWrapper wrapper = new DbContextWrapper();

public void ProcessNames(List<string> inputNames)
{
    //...

    foreach(string existingName in wrapper.GetExistingNames(inputNames))
    {
        //Do something with the names
    }

    //...
}

EDIT 2

After some more debugging, I found that the query being created is somewhat different. It is supposed to be;

SELECT `Extent1`.`Name` 
FROM `Names` AS `Extent1` 
WHERE (`Extent1`.`Name` IN ( @gp1,@gp2))

However, I get this;

System.Data.Entity.Infrastructure.DbQuery<MyDbContext.Names>

As the actual query.

The stack trace;

at MySql.Data.Entity.SqlGenerator.Visit(DbPropertyExpression expression)
at MySql.Data.Entity.SqlGenerator.Visit(DbInExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbInExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitBinaryExpression(DbExpression left, DbExpression right, String op)
at MySql.Data.Entity.SqlGenerator.Visit(DbAndExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbAndExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.Visit(DbFilterExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbFilterExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SqlGenerator.VisitInputExpression(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.VisitInputExpressionEnsureSelect(DbExpression e, String name, TypeUsage type)
at MySql.Data.Entity.SelectGenerator.Visit(DbProjectExpression expression)
at System.Data.Entity.Core.Common.CommandTrees.DbProjectExpression.Accept[TResultType](DbExpressionVisitor`1 visitor)
at MySql.Data.Entity.SelectGenerator.GenerateSQL(DbCommandTree tree)
at MySql.Data.MySqlClient.MySqlProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree)
at System.Data.Entity.Core.Common.DbProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.EntityClient.Internal.EntityCommandDefinition..ctor(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver, BridgeDataReaderFactory bridgeDataReaderFactory, ColumnMapFactory columnMapFactory)
at System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateCommandDefinition(DbProviderFactory storeProviderFactory, DbCommandTree commandTree, DbInterceptionContext interceptionContext, IDbDependencyResolver resolver)
at System.Data.Entity.Core.EntityClient.Internal.EntityProviderServices.CreateDbCommandDefinition(DbProviderManifest providerManifest, DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.Common.DbProviderServices.CreateCommandDefinition(DbCommandTree commandTree, DbInterceptionContext interceptionContext)
at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.CreateCommandDefinition(ObjectContext context, DbQueryCommandTree tree)
at System.Data.Entity.Core.Objects.Internal.ObjectQueryExecutionPlanFactory.Prepare(ObjectContext context, DbQueryCommandTree tree, Type elementType, MergeOption mergeOption, Boolean streaming, Span span, IEnumerable`1 compiledQueryParameters, AliasGenerator aliasGenerator)
at System.Data.Entity.Core.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__6()
at System.Data.Entity.Core.Objects.ObjectContext.ExecuteInTransaction[T](Func`1 func, IDbExecutionStrategy executionStrategy, Boolean startLocalTransaction, Boolean releaseConnectionOnSuccess)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<>c__DisplayClass7.<GetResults>b__5()
at System.Data.Entity.Infrastructure.DefaultExecutionStrategy.Execute[TResult](Func`1 operation)
at System.Data.Entity.Core.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
at System.Data.Entity.Core.Objects.ObjectQuery`1.<System.Collections.Generic.IEnumerable<T>.GetEnumerator>b__0()
at System.Data.Entity.Internal.LazyEnumerator`1.MoveNext()
at MyNameSpace.DbContextWrapper.<GetExistingNames>d__1b.MoveNext() in c:~omitted~\DbContextWrapper.cs:line 70
at MyNameSpace.NameProcessor.ProcessNames(List<string> inputNames) in c:~omitted~\NameProcessor.cs:line 60

Answer 1

After you posted stacktrace I spotted that you using MySQL and so my guess is that you hit this bug: Exception when using IEnumera.Contains(model.property) in Where predicate

So solution would be to ensure you have versions of MySQL Connector/NET 6.7.6 / 6.8.4 / 6.9.5 and newer. Or try to use Any method instead of Contains .

Ps This bug report came from this post by Alnedru: Int[].Contains doesn't work in EF6

Answer 2

Your null check on query will never fail, because it's returning the IQueryable object (that is, the query being built). You've instantiated it and started building a query, so it will always pass.

To be clear - IQueryable is roughly equivalent to a string containing the ADO.Net Select statement. It is not, inherently, the actual data.

This doesn't explain why it's throwing a null exception, but it does explain why the null check passes, and the foreach could still fail.

EDIT: When attempting to duplicate this, I found that I get an exception when using the below code:

public IEnumerable<string> GetExistingNames(ICollection<string> names)
{
    IQueryable<Names> query = Names.Where(n => names.Contains(n.Name));
    if (query == null) yield break;
    foreach (var name in query)
    {
        yield return name.Name;
    }
}

It wasn't a NullReferenceException , but a NotSupportedException , as ICollections Contains has no supported translation to SQL. Switching the parameter to List caused the problem to disappear:

public IEnumerable<string> GetExistingNames(List<string> names)

Or you can convert it to a list on the fly:

IQueryable<Names> query = Names.Where(n => names.ToList().Contains(n.Name));

Answer 3

Why don't you just add an extension method to alleviate the stress this brings you. try this piece of code

namespace HelperExtensionMethods
{
    public static class ExtensionMethods
    {
        public static string UpdateNullString(this string testNullstring)
        {
            if (TestNullstring == null)
                return "";
            return Testullstring;
        }
    }
}

and then call it like so

using HelperExtesionMethods
DbContext context = new DbContext();

public IEnumerable<string> GetExistingNames(ICollection<string> names)
{
    IQueryable<Names> query = context.Names.Where(n => names.UpdateNullString().Contains(n.Name.UpdateNullString()));
    if(query == null) yield break;
    foreach(var name in query)
    {
        yield return name.Name;
    }
}