How can I create a dynamic Select on an IEnumerabl

问题:

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 the 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 perform this Select at runtime, when I don't have control over when it's initially created.

--

ANSWER: Based on AlanT's answer, here's 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 kept it as an Expression, because calling Compile caused the IQueryable to be Enumerated, which meant the database was hit unnecessarily. So, 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();

回答1:

It is possible to do this using an Expression

e.g.

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.

回答2:

The dynamic linq library allows you to specify predicates and projections on the fly and may fit your use case-

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

回答3:

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