Construct LambdaExpression for nested property from string

Question

I am trying to create a lambda expression for a nested property at run-time from the name of the propert. Basically I am trying to create the lambda expression specified by:

var expression = CreateExpression<Foo, object>(foo => foo.myBar.name);

private static Expression CreateExpression<TEntity, TReturn>(Expression<Func<TEntity, TReturn>> expression)
{
    return (expression as Expression);
}

With the classes:

class Foo
{
    public Bar myBar { get; set; }
}
class Bar
{
    public string name { get; set; }
}

However all I am given is the type of Foo and the string "myBar.name"

If it were a normal property such as just needing the value "myBar" then I could use the following:

private static LambdaExpression GetPropertyAccessLambda(Type type, string propertyName)
{
    ParameterExpression odataItParameter = Expression.Parameter(type, "$it");
    MemberExpression propertyAccess = Expression.Property(odataItParameter, propertyName);
    return Expression.Lambda(propertyAccess, odataItParameter);
}

However this code does not work for nested properties and I'm not sure how to create the LambdaExpression to do the work of foo.myBar.name.

I think it's going to be something like this:

GetExpression(Expression.Call(GetExpression(Foo, "myBar"), "name"))

But I can't seem to work out how to get it all working, or if there's a better way to do this at run-time.

Marc Gravell · Accepted Answer · 2013-04-25 07:17:15Z

145

Do you mean:

static LambdaExpression CreateExpression(Type type, string propertyName) {
    var param = Expression.Parameter(type, "x");
    Expression body = param;
    foreach (var member in propertyName.Split('.')) {
        body = Expression.PropertyOrField(body, member);
    }
    return Expression.Lambda(body, param);
}

For example:

class Foo {
    public Bar myBar { get; set; }
}
class Bar {
    public string name { get; set; }
}
static void Main() {
    var expression = CreateExpression(typeof(Foo), "myBar.name");
    // x => x.myBar.name
}

?

answered Apr 25, 2013 at 7:17

Marc Gravell

1.1m273 gold badges2.6k silver badges3k bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Seph Over a year ago

Yep that's exactly what I was looking for, my mistake was thinking that I needed to invoke the body expression (using Call) before getting the nested property.

The Senator Over a year ago

Fantastic answer from Marc Gravell which solved it for me. As a bonus, resharper did this to your code: var param = Expression.Parameter(type, "x"); Expression body = propertyName.Split('.').Aggregate<string, Expression>(param, Expression.PropertyOrField); return Expression.Lambda(body, param);

Kate Over a year ago

It is realy cool. Marc, are there any way to use the same behaviour with nested list? I mean class Foo {public List<Bar> myBar { get; set; }}

HamedFathi Over a year ago

@marc-gravell Any change to support indexers too (for arrays & lists)? like myBar.name[1].address[2]

Marc Gravell Over a year ago

@HamedFathi yes, but trickier; you need to use Expression.Call with the method-info of the relevant indexer accessor; see Expression.Call here: sharplab.io/… - note that you don't need the LdMemberToken - but you'd need to find the accessor via reflection

Zoidbergseasharp · Accepted Answer · 2022-03-16 16:49:53Z

2

For this method, propertyPath is "Car", "Colour" for Driver.Car.Colour.


public static MemberExpression NestedProperty(Expression propertyHolder, params string[] propertyPath)
{
    MemberExpression memberExpression = Expression.Property(propertyHolder, propertyPath[0]);

    foreach (var member in propertyPath.Skip(1))
    {
        memberExpression = Expression.Property(memberExpression, member);
    }

    return memberExpression;
}

answered Mar 16, 2022 at 16:49

Zoidbergseasharp

4,6681 gold badge19 silver badges22 bronze badges

Comments

Moff452 · Accepted Answer · 2020-10-09 07:52:28Z

-1

To construct a lambda expression but with an inline solution you can do:

var param = Expression.Parameter(typeOf(FooBar), "x");

// you "concat" your expression here :
var propertyExpression = Expression.PropertyOrField(param, "myBar");
propertyExpression = Expression.PropertyOrField(propertyExpression, "name");
// expected result : "x.myBar.name" as a body expression

var expression = Expression.Lambda(propertyExpression, param);
// x => x.myBar.name

edited Oct 9, 2020 at 7:52

answered Oct 9, 2020 at 7:23

Moff452

1312 silver badges8 bronze badges

Collectives™ on Stack Overflow

Construct LambdaExpression for nested property from string

3 Answers 3

5 Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

5 Comments

Comments

Comments

Linked

Related