How to create dynamic selection in IEnumerable <T> at runtime?

Question

Given that I have an IEnumerable<T> , where T is any object, how can I select a specific property from it, given that I know the name of one of the property names at run time as a string?

For example:

 var externalIEnumerable = DataPassedFromConsumingCode(); // `IEnumerable<T>` string knownPropertyName = "Foo"; var fooSelect = externalIEnumerable.Select(...); 

In essence, I'm obviously just doing externalIEnumerable.Select(x=> x.Foo); but I need to execute this Select at runtime when I have no control over its initial creation.

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ANSWER: Based on AlanT's answer, here is what I actually did:

 public Expression<Func<TItem, object>> SelectExpression<TItem>(string fieldName) { var param = Expression.Parameter(typeof(TItem), "item"); var field = Expression.Property(param, fieldName); return Expression.Lambda<Func<TItem, object>>(field, new ParameterExpression[] { param }); } 

I saved this as an expression because a call to Compile caused an IQueryable switch, which meant that the database was deleted unnecessarily. Therefore, to use it, I just do the following:

 string primaryKey = _map.GetPrimaryKeys(typeof(TOriginator)).Single(); var primaryKeyExpression = SelectExpression<TOriginator>(primaryKey); var primaryKeyResults = query.Select(primaryKeyExpression).ToList(); 

Answer 1

This can be done using the expression

eg.

 private class Foo { public string Bar { get; set; } } private IEnumerable<Foo> SomeFoos = new List<Foo>() { new Foo{Bar = "Jan"}, new Foo{Bar = "Feb"}, new Foo{Bar = "Mar"}, new Foo{Bar = "Apr"}, }; [TestMethod] public void GetDynamicProperty() { var expr = SelectExpression<Foo, string>("Bar"); var propValues = SomeFoos.Select(expr); Assert.IsTrue(new[] { "Jan", "Feb", "Mar", "Apr" }.SequenceEqual(propValues)); } public static Func<TItem, TField> SelectExpression<TItem, TField>(string fieldName) { var param = Expression.Parameter(typeof(TItem), "item"); var field = Expression.Property(param, fieldName); return Expression.Lambda<Func<TItem, TField>>(field, new ParameterExpression[] { param }).Compile(); } 

hth
Alan.

Answer 2

The linq dynamic library allows you to define predicates and predictions on the fly and can suit your use case

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

Answer 3

You can dynamically build Expression<Func<T, U>> .