Dynamic Property Assignment Throws RuntimeBinderException

Question

I am getting a RuntimeBinderException with the message

Cannot implicitly convert type 'object' to 'MyNamespace.SomeEnum?'. An explicit conversion exists (are you missing a cast?)

The following code generates that error:

public enum SomeEnum
{
    Val1
}

public class Example
{
    public SomeEnum? EnumMember { get; set; }
}

public static class Program
{
    static void Main()
    {
        dynamic example = new Example();

        // Works without issue
        example.EnumMember = (dynamic)Enum.Parse(typeof(SomeEnum), "Val1");

        // Works without issue
        example.EnumMember = Enum.Parse(example.EnumMember.GetType(), "Val1");

        // Throws the aforementioned RuntimeBinderException
        example.EnumMember = Enum.Parse(typeof(SomeEnum), "Val1");
    }
}

Why do to first two lines work (both return type dynamic), but the third throws an exception (when the return type is object)? I was under the impression that, when assigning to dynamic, the binding is performed using the actual, run-time type of the right-hand-side. Can someone please enlighten me as to why the third line is unable to run as written?

Jon Skeet · Accepted Answer · 2015-09-22 17:31:41Z

4

The compile-time type of the expression on the RHS of the = operator for the first two lines is dynamic. In the first case that's because you've cast to dynamic, and in the second case it's because you're using a dynamic value in one of the arguments.

In the third case, the compile-time type of the expression is object. So you're trying to do the equivalent of:

object x = Enum.Parse(typeof(SomeEnum), "Val1");
example.EnumMember = x;

That doesn't work, because there's no implicit conversion from object to SomeEnum?, which is what the compiler is trying to find at execution time.

Note that the nullability part really isn't relevant here - nor is the fact that it's an enum. It's just that the assignment operator is being bound dynamically, but using the compile-time time of the RHS. Here's a similar but simpler example:

class Test
{
    public int Foo { get; set; }

    static void Main()
    {
        dynamic example = new Test();

        example.Foo = 10; // Fine

        object o = 10;
        example.Foo = o; // Bang
    }
}

If you want the compiler to handle the assignment dynamically using the actual type of the value returned rather than the compile-time type, then using dynamic is exactly what you want to do - either cast to dynamic, or use:

dynamic value = ...;
target.SomeProperty = value;

edited Sep 22, 2015 at 17:31

answered Sep 22, 2015 at 17:22

Jon Skeet

1.5m893 gold badges9.3k silver badges9.3k bronze badges

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

3 Comments

D Stanley Over a year ago

Isn't the compile-time type of the RHS in the second example object, not dynamic? That's where I was stuck - the second and third calls seem the same other than the compile-time resolution of typeof.

Jon Skeet Over a year ago

@DStanley: Nope, because the call is dynamically bound, because the first argument uses example. Try assigning the result to var instead and hover over the declaration and you'll see :)

Jeff G Over a year ago

@DStanley No, because example.EnumMember.GetType() returns dynamic, so the Enum.Parse expression must, by definition, also return type dynamic.

DPac · Accepted Answer · 2015-09-22 17:26:48Z

0

You still need to do an implicit conversion for the third line

example.EnumMember = (SomeEnum) Enum.Parse(typeof(SomeEnum), "Val1");

EDIT

The reason that you still need implicit conversion is because Enum.Parse returns an object. Refer to the documentation below.

https://msdn.microsoft.com/en-us/library/essfb559(v=vs.110).aspx

edited Sep 22, 2015 at 17:26

answered Sep 22, 2015 at 17:20

DPac

5152 silver badges8 bronze badges

2 Comments

D Stanley Over a year ago

OP knows that and is asking "why?"

Jeff G Over a year ago

The problem is that in real code, I don't know the type SomeEnum, so I can't explicitly add that cast. If you know of some other way, without casting to dynamic or SomeEnum to get the assignment to work, I would love to learn it.

Collectives™ on Stack Overflow

Dynamic Property Assignment Throws RuntimeBinderException

2 Answers 2

3 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related