How can I make a Multiple property lambda expression with Linq

Question

I use

ParameterExpression parameter = Expression.Parameter(typeof(T), "p");

var myexp=Expression.Lambda<Func<T, TKey>>(Expression.Property(parameter, "myid"), parameter);

to create a lambda expression myexp like this

p=>myid

now I wanna create a mult property like this

p=> new {myid,myid2}

You can just write the lambda you're trying to create and look at it in a debugger to see what expressions it's using, so that you know what you need to create, specifically. — Servy
– Servy, Commented Aug 7, 2017 at 20:54
@Servy I write it for generic class,need to handle propertyName to change — John TiXor
– John TiXor, Commented Aug 7, 2017 at 21:11

T.S. · Accepted Answer · 2017-08-08 03:31:35Z

3

The tricky part of doing this is accessing the anonymous type's type so you can call new for it. I normally use LINQPad to create a sample lambda and dump it to see the format:

Expression<Func<Test,object>> lambdax = p => new { p.myid, p.myid2 };
lambdax.Dump();

Assuming the type of p is Test:

class Test {
    public int myid;
    public int myid2;
}

Then you can create Expressions to recreate the lambdax value:

var exampleTest = new Test();
var example = new { exampleTest.myid, exampleTest.myid2 };
var exampleType = example.GetType();

var rci = exampleType.GetConstructors()[0];
var parm = Expression.Parameter(typeof(Test), "p");
var args = new[] { Expression.PropertyOrField(parm, "myid"), Expression.PropertyOrField(parm, "myid2") };

var body = Expression.New(rci, args, exampleType.GetMembers().Where(m => m.MemberType == MemberTypes.Property));
var lambda = Expression.Lambda(body, parm);

edited Aug 8, 2017 at 3:31

T.S.

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answered Aug 7, 2017 at 21:34

NetMage

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1 Comment

John TiXor Over a year ago

I tried LINQPad and love it, thanks help alot ^ ^,now I use object type to pass anonymous type to do what I want!

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