How to create Expression> from property name

Question

I want to create an expression like this:

public class Entity
{        
    public virtual long Id { get; set; }
}

Entity alias = null;
Expression<Func<object>> target = () => alias.Id;//have to be created from "Id"

My question is how to create Expression<Func<object>> target programaticaly having string with property name("Id" in this example).

Have a look at [Expression trees](https://msdn.microsoft.com/en-us/library/mt654263.aspx). — Adil Mammadov, Nov 23 '16 at 12:20
Is an `Expression>` what you want? The `Entity` is coming from your application (not an input to the expression) and its `Id` property is a `long` not an `object`. Would an `Expression>` be more useful? For example: `Expression> target = a => a.Id;` — Andy Lamb, Nov 23 '16 at 12:23
I do want use Expression trees but I only start working with them — Roman Koliada, Nov 23 '16 at 12:23
Possible duplicate of [Retrieving Property name from lambda expression](http://stackoverflow.com/questions/671968/retrieving-property-name-from-lambda-expression) — Gurwinder Singh, Jan 09 '17 at 20:03

Maksim Simkin · Accepted Answer · 2017-01-09T19:01:18.247

4

OK, I hope, i have finally understood, what do you need :)

I suggest to create such extension method:

public static class EntityExtensions
{
    public static Expression<Func<object>> ToExpression(this Entity entity, string property)
    {
        var constant = Expression.Constant(entity);
        var memberExpression = Expression.Property(constant, property);     
        Expression convertExpr = Expression.Convert(memberExpression, typeof(object));
        var expression = Expression.Lambda(convertExpr);

        return (Expression<Func<object>>)expression;
    }
}

and than call it this way:

var entity = new Entity { Id = 4711 };
var expression = entity.ToExpression("Id");
var result = expression.Compile().DynamicInvoke();

You have to give entity as a parameter.

People, who downvote accepted answer, could explain, what the probem it.

edited Jan 09 '17 at 19:01

answered Nov 23 '16 at 12:35

Maksim Simkin

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Thank you, but I really need `Expression>` because I want to pass it to another method which can accept only `Expression>`(it's WithAlias method from NHibernate) – Roman Koliada Nov 23 '16 at 12:42
Sorry, but I need not `Expression>` but `Expression>`. Just like this: `() => alias.Id` – Roman Koliada Nov 24 '16 at 10:13
It this is the desired method, you could replace `this Entity entity` with `this T source` to make this method available on all kind of objects. – Oliver Nov 24 '16 at 11:07
Thanks a lot. That's exactly what I need! – Roman Koliada Nov 24 '16 at 12:22

How to create Expression> from property name

1 Answers1