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So I have been searching for a simple example to hopefully a simple problem.

I have a simple object a List of ListTest (List)

public class ListTest{
{
    public string perName { get; set; }
    public string perSex { get; set; }
    public ListTest(string pName, string pSex)
    {
        this.perSex = pSex;
        this.perName = pName;
    }
}

I have loaded it with some data:

        List<ListTest> tryIt = new List<ListTest>();
        tryIt.Add(new ListTest("Karen", "F"));
        tryIt.Add(new ListTest("Kate", "F"));
        tryIt.Add(new ListTest("Glen", "M"));
        tryIt.Add(new ListTest("Tiger", "M"));
        tryIt.Add(new ListTest("Clementine", "F"));
        tryIt.Add(new ListTest("Magnolia", "F"));

Now I want to query it using a Lambda Expression:

        var things = tryIt
                     .Where(sex => (sex.perSex == "F"))
                     .OrderBy(sex => sex.perName);

But I want to do it dynamically, just in case I want to change my where to "perName".

I am able to create the Lambda Expression your Expressions, but I can't figure out how to take it across the goal line and actually assign it to a where clause and execute it.

        IQueryable<ListTest> iq = tryIt.AsQueryable();
        ParameterExpression pe = Expression.Parameter(typeof(ListTest), "Person");
        Expression paEx = Expression.Property(pe, "perSex");
        Expression right = Expression.Constant("F");
        Expression eqEx = Expression.Equal(paEx, right);
        Expression lE = Expression.Lambda<Func<ListTest, bool>>(eqEx, pe);

This should be a simple 4 or 5 line solution, but I can't find an easily decipherable solution example.

Should I use a MethodCallExpression or something down those lines?

Thanks,

Glen Wilkinson
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1 Answers1

1

You can try something like this:

IEnumerable<ListTest> things = tryIt;

if (someBooleanLogic)
    things = things.Where(l => l.perSex == "F");
else
    things = things.Where(l => l.perName == "Tiger");

things = things.OrderBy(sex => sex.perName);

List<ListTest> filteredAndSorted = things.ToList();

Edit:

Or, there's also the popular LINQ Dynamic Query Library, where you can form your query pretty much however you want (dynamically, of course).

shaunmartin
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  • Trying to use the MethodCallExpression in order to keep the query dynamic. I will want to change the parameter expression to different properties of the object. For instance having the query return the collection ordered by perName. – Glen Wilkinson Oct 28 '10 at 00:45
  • Ok. I'm not *entirely* sure I understand what your goal is, but I've added another resource to my answer - the LINQ Dynamic Query Library. You may already be aware of it, but if not, it may be of use to you - it's pretty powerful. – shaunmartin Oct 28 '10 at 01:09