Fix null check optimization for IQueryable/DbSet types

Co-authored-by: cincuranet <[email protected]>

Author: Copilot

src/EFCore/Query/Internal/NullCheckRemovingExpressionVisitor.cs

@@ -24,7 +24,9 @@ protected override Expression VisitBinary(BinaryExpression binaryExpression)
     {
         var visitedExpression = base.VisitBinary(binaryExpression);
 
-        return TryOptimizeConditionalEquality(visitedExpression) ?? visitedExpression;
+        return TryOptimizeQueryableNullCheck(visitedExpression)
+            ?? TryOptimizeConditionalEquality(visitedExpression)
+            ?? visitedExpression;
     }
 
     /// <summary>
@@ -75,6 +77,34 @@ protected override Expression VisitConditional(ConditionalExpression conditional
         return base.VisitConditional(conditionalExpression);
     }
 
+    private static Expression? TryOptimizeQueryableNullCheck(Expression expression)
+    {
+        // Optimize IQueryable/DbSet null checks
+        // IQueryable != null => true
+        // IQueryable == null => false
+        if (expression is BinaryExpression
+            {
+                NodeType: ExpressionType.Equal or ExpressionType.NotEqual
+            } binaryExpression)
+        {
+            var isLeftNull = IsNullConstant(binaryExpression.Left);
+            var isRightNull = IsNullConstant(binaryExpression.Right);
+
+            if (isLeftNull != isRightNull)
+            {
+                var nonNullExpression = isLeftNull ? binaryExpression.Right : binaryExpression.Left;
+
+                if (IsQueryableType(nonNullExpression.Type))
+                {
+                    var result = binaryExpression.NodeType == ExpressionType.NotEqual;
+                    return Expression.Constant(result, typeof(bool));
+                }
+            }
+        }
+
+        return null;
+    }
+
     private static Expression? TryOptimizeConditionalEquality(Expression expression)
     {
         // Simplify (a ? b : null) == null => !a || b == null
@@ -161,4 +191,17 @@ protected override Expression VisitUnary(UnaryExpression unaryExpression)
 
     private static bool IsNullConstant(Expression expression)
         => expression is ConstantExpression { Value: null };
+
+    private static bool IsQueryableType(Type type)
+    {
+        if (type.IsGenericType)
+        {
+            var genericTypeDefinition = type.GetGenericTypeDefinition();
+            return genericTypeDefinition == typeof(IQueryable<>)
+                || genericTypeDefinition == typeof(IOrderedQueryable<>)
+                || genericTypeDefinition == typeof(DbSet<>);
+        }
+
+        return type == typeof(IQueryable);
+    }
 }

test/EFCore.Specification.Tests/Query/NorthwindWhereQueryTestBase.cs

@@ -1418,6 +1418,18 @@ public virtual Task Where_Queryable_AsEnumerable_Contains_negated(bool async)
             elementSorter: e => e.CustomerID,
             elementAsserter: (e, a) => AssertCollection(e.Subquery, a.Subquery));
 
+    [ConditionalTheory, MemberData(nameof(IsAsyncData))]
+    public virtual Task Where_Queryable_null_check_with_Contains(bool async)
+    {
+        return AssertQuery(
+            async,
+            ss =>
+            {
+                var ids = ss.Set<Customer>().Select(c => c.CustomerID);
+                return ss.Set<Customer>().Where(c => ids != null && ids.Contains(c.CustomerID));
+            });
+    }
+
     [ConditionalTheory, MemberData(nameof(IsAsyncData))]
     public virtual Task Where_Queryable_ToList_Count_member(bool async)
         => AssertQuery(