Use reflection to make dynamic LINQ statements in C#

Question

If I have a LINQ statement like

x = Table.SingleOrDefault(o => o.id == 1).o.name;

how can I replace "id" and "name" with passed in variables using reflection? I keep getting object reference not set to instance of an object errors when I try. My attempts are things like

x = (string)Table.SingleOrDefault(o => (int?)o.GetType().GetProperty(idString)
.GetValue(o, null) == 1).GetType().GetField(nameString).GetValue(x);

Any help would be great. Thanks.

Sorry, I could not understand your problem or even your code :(. Could you remake the question body? — gustavodidomenico
– gustavodidomenico, Commented May 22, 2014 at 18:53
What kind of LINQ is it? LINQ to Objects, LINQ to Entities, LINQ to SQL? — MarcinJuraszek
– MarcinJuraszek, Commented May 22, 2014 at 18:55
Try to specify BindingFlags.Instance | BindingFlags.Public as binding flags — vc 74
– vc 74, Commented May 22, 2014 at 19:00

MarcinJuraszek · Accepted Answer · 2014-05-22 19:00:16Z

47

You should use Expression Trees instead of reflection. It will perform better, and you'll be able to use it with both LINQ to Objects and LINQ to SQL/Entities.

var source = new List<Test> { new Test { Id = 1, Name = "FirsT" }, new Test { Id = 2, Name = "Second" } };
var idName = "Id";
var idValue = 1;

var param = Expression.Parameter(typeof(Test));
var condition =
    Expression.Lambda<Func<Test, bool>>(
        Expression.Equal(
            Expression.Property(param, idName),
            Expression.Constant(idValue, typeof(int))
        ),
        param
    ).Compile(); // for LINQ to SQl/Entities skip Compile() call

var item = source.SingleOrDefault(condition);

then, you can get Name property using reflection (you'll do it just once, so it's fine to do it using reflection.

var nameName = "Name";
var name = item == null ? null : (string) typeof(Test).GetProperty(nameName).GetValue(item);

Test class is defined as

public class Test
{
    public int Id { get; set; }
    public string Name { get; set; }
}

answered May 22, 2014 at 19:00

MarcinJuraszek

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2 Comments

erosebe Over a year ago

This works great. Just a tiny tweak to the code and it does what I wanted. Thank you.

Debopam Chanda Over a year ago

@MarcinJuraszek This is EXACTLY what I was looking for. Thanks.

Brian Mains · Accepted Answer · 2014-05-22 18:53:48Z

1

You can't use reflection, but you can use dynamic Linq as described here. If you are using Entity Framework for this, you should also be able to use Entity SQL, which is basically a hard-coded SQL string.

answered May 22, 2014 at 18:53

Brian Mains

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1 Comment

gustavodidomenico Over a year ago

Maybe he could rewrite the lambda using expressions. But I am not sure of that he needs.

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