SelectMany之后无法通过ExpressionVisitor找到Where表达式

Question

I have a ExpressionVisitor derived class with following overridden VisitMethodCall() method.

class CommonExpressionVisitor : ExpressionVisitor
{
    protected override Expression VisitMethodCall(MethodCallExpression node)
    {
        if (node.Method.DeclaringType == typeof(Queryable))
        {
            switch (node.Method.Name)
            {
                case nameof(Queryable.Where):
                    Visit(node.Arguments[0]);
                    return node;

                case nameof(Queryable.Select):
                    Visit(node.Arguments[0]);
                    return node;

                case nameof(Queryable.SelectMany):
                    Visit(node.Arguments[0]);
                    Visit(node.Arguments[1]);
                    Visit(node.Arguments[2]);
                    return node;
            }
        }
        return base.VisitMethodCall(node);
    }
}

The visitor can easily find Where expression when there is no SelectMany call in the query. Such as:

var queryable = from item in collection
                where item.X >= 0 && item.Y >= 0
                select item;

However, if the query contains SelectMany , the entire Where expression is gone and the visitor is unable to reach it anymore.

var queryable = from item in collection
                from sub in item.SubItems
                where sub.A >= 0 && sub.B >= 0
                select sub;

How do I fix my ExpressionVisitor in order to find Where expression?

Answer 1

Going to answer my own question. There are two phases in the expression. In first phase the IQueryable.Provider property is called to generate anonymous objects that consists of two variables item and sub . In second phase, same property is called again where the expression now contains the anonymous type and where clause. So the where clause does not disappear, it is just in second phase.