Turn ExpandoObject into static type

As many others have shown, the new ExpandoObject introduced in C# 4 can be pretty useful in some scenarios.

However, on the odd occasion when you want to convert an ExpandoObject to a static type you have defined you can be forgiven for feeling a little lost as there are no well documented ways to do that. At least that’s how I felt, until I found out about the ConvertToType methods on the JavaScriptSerializer!


The JavaScriptSerializer, like the DataContractJsonSerializer class, lets you serialize and deserialize objects to and from JSON string. But unlike the DataContractJsonSerializer class, you are not confined to working with types that are marked with the DataContract attribute.

One of the deserialization methods on the JavaScriptSerializer class – DeserializeObject – lets you a JSON string to an object graph, i.e. a Dictionary<string, object> object where the key is the name of the property and the value is the property value. And you can use one of the overloaded ConvertToType methods to convert this dictionary to a static type of your choice, provided that the type has a parameterless constructor (which unfortunately, means you won’t be able to convert an ExpandoObject to an anonymous type).

ExpandoObject as IDictionary<string, object>

Why is this important? It’s because the ExpandoObject implements the IDictionary<string, object> interface and therefore can be used with the ConvertToType methods!

Here’s what you do:

   1: // first you need to instantiate the serializer

   2: var jsSerializer = new JavaScriptSerializer();


   4: // get an expando object and convert it to an instance of MyClass

   5: var expando = GetExpandoObject();

   6: var obj = jsSerializer.ConvertToType<MyClass>(expando);


   8: Console.WriteLine(obj.Id);      // 0db9d9a6-8c7e-4bea-82a8-4d5641c7c0de

   9: Console.WriteLine(obj.Name);    // Yan

  10: Console.WriteLine(obj.Age);     // 29


  12: ...


  14: public ExpandoObject GetExpandoObject()

  15: {

  16:     dynamic expando = new ExpandoObject();

  17:     expando.Id = Guid.NewGuid();

  18:     expando.Name = "Yan";

  19:     expando.Age = 29;


  21:     return expando;

  22: }


  24: public class MyClass

  25: {

  26:     public Guid Id { get; set; }


  28:     public string Name { get; set; }


  30:     public int Age { get; set; }

  31: }

Pretty cool, eh? ;-)

But what if the shape of the expando object doesn’t match your type? E.g. there are properties defined on MyClass but not on the expando object, or vice versa, or both? Well, it’s smart enough to work that out itself and only set the properties which it is able to set:

   1: // same as before


   3: Console.WriteLine(obj.Id);          // 0db9d9a6-8c7e-4bea-82a8-4d5641c7c0de

   4: Console.WriteLine(obj.Name);        // null

   5: Console.WriteLine(obj.Age);         // 29

   6: Console.WriteLine(obj.NickName);    // null


   8: ...


  10: public ExpandoObject GetExpandoObject()

  11: {

  12:     dynamic expando = new ExpandoObject();

  13:     expando.Id = Guid.NewGuid();

  14:     expando.Name = "Yan";

  15:     expando.Age = 29;

  16:     expando.Wife = "Yinan";    // not defined on MyClass


  18:     return expando;

  19: }


  21: public class MyClass

  22: {

  23:     public Guid Id { get; set; }


  25:     public string Name { get; private set; } // non-public setter


  27:     public int Age { get; set; }


  29:     public string NickName { get; set; } // not defined on ExpandoObject

  30: }

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Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

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