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I have encountered issues with using dynamic variables in C#. This arose while coding NancyFx routing modules, but I have boiled down the issue to the example below. While I received a different exception in the original code, the example code still throws an exception that I believe is erroneous. Does anyone see what is going on here, or have others encountered similar problems?

Note that the following posts may be related: StackOverflowException when accessing member of generic type via dynamic: .NET/C# framework bug? System.Dynamic bug?

The code:

class Program
{
    static void Main(string[] args)
    {
        var dictionary = new Dictionary<string, object>();
        dictionary.Add("number", 12);
        var result = MethodUsesExplicitDeclaration(dictionary);
        var result2 = MethodUsesImplicitDeclaration(dictionary);
    }

    static dynamic MethodUsesExplicitDeclaration(dynamic reallyDictionary)
    {
        // this works, ostensibly because the local variable is explicitly declared
        IDictionary<string, object> dictionary = CastDictionary(reallyDictionary);
        return dictionary.Get<int>("number");
    }

    static dynamic MethodUsesImplicitDeclaration(dynamic reallyDictionary)
    {
        // this throws an exception, and the only difference is 
        // that the variable declaration is implicit
        var dictionary = CastDictionary(reallyDictionary);
        return dictionary.Get<int>("number");
    }

    static IDictionary<string, object> CastDictionary(dynamic arg)
    {
        return arg as IDictionary<string, object>;
    }
}

static class Extensions
{
    public static T Get<T>(this IDictionary<string, object> dictionary, string key)
    {
        var value = dictionary[key];
        if (value is T)
            return (T)value;
        throw new InvalidOperationException();
    }
}

The exception: Microsoft.CSharp.RuntimeBinder.RuntimeBinderException was unhandled HResult=-2146233088 Message='System.Collections.Generic.Dictionary<string,object>' does not contain a definition for 'Get' Source=Anonymously Hosted DynamicMethods Assembly StackTrace: at CallSite.Target(Closure , CallSite , Object , String ) at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1) at DynamicBug.Program.MethodUsesImplicitDeclaration(Object reallyDictionary) in c:\TFS\UnreleasedCode\POC\DynamicBug\DynamicBug\Program.cs:line 28 at DynamicBug.Program.Main(String[] args) in c:\TFS\UnreleasedCode\POC\DynamicBug\DynamicBug\Program.cs:line 16 InnerException:

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  • 1
    Seems like this question is answered here. Commented Jul 15, 2014 at 14:56
  • 1
    I don't believe that stackoverflow.com/questions/258988/… addresses the same question. The example code does not try to use an extension method on a variable declared as dynamic, it uses it on the result of a method which returns an interface type. So in both versions of the method which executes an extension method the type to which the extension method applies is not dynamic. Commented Jul 15, 2014 at 15:02
  • Another way I could have asked the original question is when should a var variable declaration not behave like the type of the value you assign to it? Commented Jul 15, 2014 at 15:05
  • 2
    The first part of Jon Skeet's quoted response states, "Dynamic lookup will not be able to find extension methods." You're getting an error that Get does not exist. Get is an extension method. It seems like it applies just fine. If you change that line to return Extensions.Get<int>(dictionary, "number"); then it executes as expected. Commented Jul 15, 2014 at 15:26
  • Cédric Bignon's answer explains why this occurs, including where in the C# spec this behavior is defined. Commented Jul 23, 2014 at 21:34

1 Answer 1

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The problem is that when you don't explicitly assigned the object to a IDictionary<string,object> declaration, the object will still be a dynamic type (see the type resolution of in the images).

As Eric Lippert points out in https://stackoverflow.com/a/5313149/1039903 extension methods will not be available at the call site, for dynamic types:

To expand on Jon's answer, the reason this doesn't work is because in regular, non-dynamic code extension methods work by doing a full search of all the classes known to the compiler for a static class that has an extension method that match. The search goes in order based on the namespace nesting and available "using" directives in each namespace.

That means that in order to get a dynamic extension method invocation resolved correctly, somehow the DLR has to know at runtime what all the namespace nestings and "using" directives were in your source code. We do not have a mechanism handy for encoding all that information into the call site. We considered inventing such a mechanism, but decided that it was too high cost and produced too much schedule risk to be worth it.

I loaded your code up into a test method to show you what is actually resolved in var dictionary during runtime compared to an explicit declaration.

Explicit declaration:

enter image description here enter image description here

Implicit declaration

enter image description here enter image description here

If you take a look at the Type resolution of dictionary the explicit declaration has type System.Collection.Generic.IDictionary and the implicit declaration has type dynamic{System.Collections.Generic.Dictionary}

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8 Comments

I've never used the dynamic keyword other than playing around and this is great info to know. Is there no way to convert it back to a static type once it is dynamic?
Assign it to the desired type or use the as keyword. (ie var dict = CastDictionary(reallyDictionary) as IDictionary<string,object>)
But, his CastDictionary is already returning an IDictionary.
It still points back to the original dynamic object, and needs to be unwrapped, or explicitly use the extension method.
Like this (Extensions.Get<int>(dictionary,"number"))
|

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