[Regression] Exception on query comparing nullable int with ?? fallback on EF Core 9

[ChristophHornung]

File a bug

When comparing a nullable integer that has a ?? fallback an exception occurs at query time.

System.InvalidOperationException: 'The binary operator Equal is not defined for the types 'System.Int32' and 'System.Nullable`1[System.Int32]'.'

Include your code

In the get started sample add the following

int? test = 1;
var blog = db.Blogs
	.Where(b => b.BlogId == (test ?? 0)).ToList();

Notice that there is no error with the PackageReference to Microsoft.EntityFrameworkCore.Sqlite:8.0
now switch to 9.0, rebuild => Exception

Include stack traces

System.InvalidOperationException
  HResult=0x80131509
  Message=The binary operator Equal is not defined for the types 'System.Int32' and 'System.Nullable`1[System.Int32]'.
  Source=System.Linq.Expressions
  StackTrace:
   at System.Linq.Expressions.Expression.GetEqualityComparisonOperator(ExpressionType binaryType, String opName, Expression left, Expression right, Boolean liftToNull)
   at System.Linq.Expressions.Expression.Equal(Expression left, Expression right, Boolean liftToNull, MethodInfo method)
   at System.Linq.Expressions.Expression.MakeBinary(ExpressionType binaryType, Expression left, Expression right, Boolean liftToNull, MethodInfo method, LambdaExpression conversion)
   at System.Linq.Expressions.BinaryExpression.Update(Expression left, LambdaExpression conversion, Expression right)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.VisitBinary(BinaryExpression binary)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit(Expression expression, State& state)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.VisitLambda[T](Expression`1 lambda)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit(Expression expression, State& state)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.VisitUnary(UnaryExpression unary)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit(Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit[T](ReadOnlyCollection`1 expressions, Func`2 elementVisitor, StateType& aggregateStateType, State[]& expressionStates, Boolean poolExpressionStates)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit(ReadOnlyCollection`1 expressions, StateType& aggregateStateType, State[]& expressionStates, Boolean poolExpressionStates)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.VisitMethodCall(MethodCallExpression methodCall)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.Visit(Expression expression, State& state)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.ExtractParameters(Expression expression, IParameterValues parameterValues, Boolean parameterize, Boolean clearParameterizedValues, Boolean precompiledQuery, IReadOnlySet`1& nonNullableReferenceTypeParameters)
   at Microsoft.EntityFrameworkCore.Query.Internal.ExpressionTreeFuncletizer.ExtractParameters(Expression expression, IParameterValues parameterValues, Boolean parameterize, Boolean clearParameterizedValues)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExtractParameters(Expression query, IParameterValues parameterValues, IDiagnosticsLogger`1 logger, Boolean compiledQuery, Boolean generateContextAccessors)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.ExecuteCore[TResult](Expression query, Boolean async, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.Execute[TResult](Expression query)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider.Execute[TResult](Expression expression)
   at Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryable`1.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 T:\test\Program.cs:line 17

Include provider and version information

EF Core version: 9.0
Database provider: multiple (teste on SqlLite and Sql)
Target framework: net9 and net8
Operating system: Windows
IDE: On all

[ajcvickers]

Confirmed regression from EF8 to 9.

[Steve887]

I hit this too updating to EfCore 9.0.0. Is there a workaround or will the fix be in 9.0.1?

[ChristophHornung]

@Steve887 Depending on your query it seems you can un-confuse the Expression checking. E.g. this works:

.Where(b => b.BlogId == (int)(test ?? 0))

[roji]

The problem here is the funcletizer's simplification of the binary expression test ?? 0. Since test evalutes to non-null, the coalesce is optimized away, replaced by test. However, the type of the BinaryExpression is a non-nullable int (post-coalesce), but test is a nullable int (pre-coalesce). A Convert node needs to be added for this case (we have a few other similar cases where we evaluate and replace nodes, these need to be handled too).

Runnable repro

await using var context = new BlogContext();
await context.Database.EnsureDeletedAsync();
await context.Database.EnsureCreatedAsync();

int? test = 1;
var blog = context.Blogs.Where(b => b.Id == (test ?? 0)).ToList();

public class BlogContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        => optionsBuilder
            .UseSqlServer("Server=localhost;Database=test;User=SA;Password=Abcd5678;Connect Timeout=60;ConnectRetryCount=0;Encrypt=false")
            .LogTo(Console.WriteLine, LogLevel.Information)
            .EnableSensitiveDataLogging();
}

public class Blog
{
    public int Id { get; set; }
    public string Name { get; set; }
}