Using an existing IQueryable to create a new dynamic IQueryable - entity-framework

Question

I have a query as follows:
var query = from x in context.Employees
where (x.Salary > 0 && x.DeptId == 5) || x.DeptId == 2
order by x.Surname
select x;
The above is the original query and returns let's say 1000 employee entities.
I would now like to use the first query to deconstruct it and recreate a new query that would look like this:
var query = from x in context.Employees
where ((x.Salary > 0 && x.DeptId == 5) || x.DeptId == 2) && (x,i) i % 10 == 0
order by x.Surname
select x.Surname;
This query would return 100 surnames.
The syntax is probably incorrect, but what I need to do is attach an additional where clause and modify the select to a single field.
I've been looking into the ExpressionVisitor but I'm not entirely sure how to create a new query based on an existing query.
Any guidance would be appreciated. Thanks you.

Answer 1

In an expression visitor you would override the method call. Check if the method is Queryable.Where, and if so, the methods second parameter is a quoted expression of type lambda expression. Fish it out and you can screw with it.
static void Main()
{
IQueryable<int> queryable = new List<int>(Enumerable.Range(0, 10)).AsQueryable();
IQueryable<string> queryable2 = queryable
.Where(integer => integer % 2 == 0)
.OrderBy(x => x)
.Select(x => x.ToString());
var expression = Rewrite(queryable2.Expression);
}
private static Expression Rewrite(Expression expression)
{
var visitor = new AddToWhere();
return visitor.Visit(expression);
}
class AddToWhere : ExpressionVisitor
{
protected override Expression VisitMethodCall(MethodCallExpression node)
{
ParameterExpression parameter;
LambdaExpression lambdaExpression;
if (node.Method.DeclaringType != typeof(Queryable) ||
node.Method.Name != "Where" ||
(lambdaExpression = ((UnaryExpression)node.Arguments[1]).Operand as LambdaExpression).Parameters.Count != 1 ||
(parameter = lambdaExpression.Parameters[0]).Type != typeof(int))
{
return base.VisitMethodCall(node);
}
return Expression.Call(
node.Object,
node.Method,
this.Visit(node.Arguments[0]),
Expression.Quote(
Expression.Lambda(
lambdaExpression.Type,
Expression.AndAlso(
lambdaExpression.Body,
Expression.Equal(
Expression.Modulo(
parameter,
Expression.Constant(
4
)
),
Expression.Constant(
0
)
)
),
lambdaExpression.Parameters
)
)
);
}
}
}