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, Aug 07 '17 at 20:54
@Servy I write it for generic class,need to handle propertyName to change — John TiXor, Aug 07 '17 at 21:11
You can't create anonymous type at runtime. It's unclear what are you trying to achieve - what will be the type of the result, how it's supposed to be used etc. — Ivan Stoev, Aug 07 '17 at 21:17
You can actually create anonymous types at runtime, but it is rarely worth it. — NetMage, Aug 07 '17 at 21:35
@Ivan Stoev actually I can, I use List.AsQueryable().GroupBy(p=>new{p.myid,p.myid2}).Select(p => p.First()) to replace dinstinct(IEqualityComparer),if I can handle expression "p=>new{p.myid,p.myid2}",then no need to repeat create IEqualityComparer anymore. it's also useful to make dynamic query with combination expression — John TiXor, Aug 07 '17 at 22:39

score 3 · Accepted Answer · edited Aug 08 '17 at 03:31

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);

I tried LINQPad and love it, thanks help alot ^ ^,now I use object type to pass anonymous type to do what I want! — John TiXor, Aug 08 '17 at 12:48

How can I make a Multiple property lambda expression with Linq

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