Get from IQueryable<T> predicate Func<T, bool>

Question

I'm trying apply query from one collection to another. Sample of my test:

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        Expression sourceExpression = Amazings().Where(x => x.Name.Equals("Kasa")).AsQueryable().Expression;

        Expression<Func<Amazing, bool>> predicate = Expression.Lambda<Func<Amazing, bool>>(sourceExpression, Expression.Parameter(typeof(Amazing)));

        var col2 = Amazings().Where(predicate.Compile());
    }

    private IEnumerable<Amazing> Amazings()
    {
        var amazings = new List<Amazing>
            {
                new Amazing
                    {
                        Name = "Kasa",
                    },
                new Amazing
                    {
                        Name = "Ma@p2a"
                    }
            };
        return amazings;
    }

    class Amazing
    {
        public string Name { get; set; }
    }
}

What is wrong? When I run this test in debug I get the exception:

You can not use expressions such as „System.Linq.EnumerableQuery`1[UnitTestProject1.UnitTest1+Amazing]” for the return type „System.Boolean”.

If I run this without parameter I got exception that I don't have parameters.

If I change parameter to boolean type I got exception: you can't use element bool type for delegate parameter type Amazing...

I tested few more options but nothing works.

Also I found that the key may by casting Expression to: MethodCallExpression but it's not or I can't figurate how.

I tried with somothing like this:

MethodCallExpression e = query.Expression as MethodCallExpression;
        MemberExpression memberExpression = (MemberExpression)e.Object;

        Expression<Func<Amazing, bool>> getCallerExpression = Expression<Func<Amazing>>.Lambda<Func<Amazing, bool>>(memberExpression);

Unfortunately memberExpression is null.

I searched the web and the only thing I found it's a tip: ~"don't do this".

How can I achieve my goal?

Answer 1

It isn't clear what you want to do... But as Servy wrote, you put the AsQueryable() in the wrong place. The right place is before the .Where(), and this being a Unit Test, I suggest that the method that returns the data should directly return a IQueryable<T>, to better simulate "normal" queries you can find in code.

Try looking if this is what you want...

public void TestMethod1()
{
    var q1 = Amazings().Where(x => x.Name.Equals("Kasa"));

    Expression predicate = q1.Expression;

    var q2 = Amazings();
    IQueryable<Amazing> q3 = q2.Provider.CreateQuery<Amazing>(predicate);
}

private IQueryable<Amazing> Amazings()
{
    var amazings = new List<Amazing>
    {
        new Amazing
            {
                Name = "Kasa",
            },
        new Amazing
            {
                Name = "Ma@p2a"
            }
    };
    return amazings.AsQueryable();
}

Note that in the most general case, you can't simply move the Expression<Func<Amazing, bool>> condition of a .Where(), because you could have a .Where().Select() or something similar.

Other possible solution, if you know that the last method is a .Where() and you really want to "move" the Expression<Func<Amazing, bool>>:

MethodCallExpression mce = predicate as MethodCallExpression;

if (mce == null)
{
    throw new Exception();
}

UnaryExpression quote = mce.Arguments[1] as UnaryExpression;

if (quote == null || quote.NodeType != ExpressionType.Quote)
{
    throw new Exception();
}

Expression<Func<Amazing, bool>> lambda = quote.Operand as Expression<Func<Amazing, bool>>;

if (lambda == null)
{
    throw new Exception();
}

IQueryable<Amazing> q4 = Amazings().Where(lambda);

Answer 2

Since you're calling Enumerable.Where and not Queryable.Where the lambda you have is never translated into an Expression, it's only ever compiled into a method, and the IQueryable that you have doesn't even have any way of getting to that delegate, because all that it sees is an arbitrary IEnumerable<T>.