c# - Why is an ExpandoObject breaking code that otherwise works just fine?

Question

Here's the setup: I have an Open Source project called Massive and I'm slinging around dynamics as a way of creating SQL on the fly, and dynamic result sets on the fly.

To do the database end of things I'm using System.Data.Common and the ProviderFactory stuff. Here's a sample that works just fine (it's static so you can run in a Console):

    static DbCommand CreateCommand(string sql) {
        return DbProviderFactories.GetFactory("System.Data.SqlClient")
                                  .CreateCommand();
    }
    static DbConnection OpenConnection() {
        return DbProviderFactories.GetFactory("System.Data.SqlClient")
                                  .CreateConnection();
    }
    public static dynamic DynamicWeirdness() {
        using (var conn = OpenConnection()) {
            var cmd = CreateCommand("SELECT * FROM Products");
            cmd.Connection = conn;
        }
        Console.WriteLine("It worked!");
        Console.Read();
        return null;
    }

The result of running this code is "It worked!"

Now, if I change the string argument to dynamic - specifically an ExpandoObject (pretend that there's a routine somewhere that crunches the Expando into SQL) - a weird error is thrown. Here's the code:

What worked before now fails with a message that makes no sense. A SqlConnection is a DbConnection - moreover if you mouseover the code in debug, you can see that the types are all SQL types. "conn" is a SqlConnection, "cmd" is a SqlCommand.

This error makes utterly no sense - but more importantly it's cause by the presence of an ExpandoObject that doesn't touch any of the implementation code. The differences between the two routines are: 1 - I've changed the argument in CreateCommand() to accept "dynamic" instead of string 2 - I've created an ExpandoObject and set a property.

It gets weirder.

If simply use a string instead of the ExpandoObject - it all works just fine!

    //THIS WORKS
    static DbCommand CreateCommand(dynamic item) {
        return DbProviderFactories.GetFactory("System.Data.SqlClient").CreateCommand();
    }
    static DbConnection OpenConnection() {
        return DbProviderFactories.GetFactory("System.Data.SqlClient").CreateConnection();
    }
    public static dynamic DynamicWeirdness() {
        dynamic ex = new ExpandoObject();
        ex.TableName = "Products";
        using (var conn = OpenConnection()) {
            //use a string instead of the Expando
            var cmd = CreateCommand("HI THERE");
            cmd.Connection = conn;
        }
        Console.WriteLine("It worked!");
        Console.Read();
        return null;
    }

If I swap out the argument for CreateCommand() to be my ExpandoObject ("ex") - it causes all of the code to be a "dynamic expression" which is evaluated at runtime.

It appears that the runtime evaluation of this code is different than compile-time evaluation... which makes no sense.

**EDIT: I should add here that if I hard-code everything to use SqlConnection and SqlCommand explicitly, it works :) - here's an image of what I mean:

question from:https://stackoverflow.com/questions/7562205/why-is-an-expandoobject-breaking-code-that-otherwise-works-just-fine

Answer 1

When you pass the dynamic to CreateCommand, the compiler is treating its return type as a dynamic that it has to resolve at runtime. Unfortunately, you're hitting some oddities between that resolver and the C# language. Fortunately, it's easy to work around by removing your use of var forcing the compiler to do what you expect:

public static dynamic DynamicWeirdness() {
    dynamic ex = new ExpandoObject ();
    ex.Query = "SELECT * FROM Products";
    using (var conn = OpenConnection()) {
        DbCommand cmd = CreateCommand(ex); // <-- DON'T USE VAR
        cmd.Connection = conn;
    }
    Console.WriteLine("It worked!");
    Console.Read();
    return null;
}

This has been tested on Mono 2.10.5, but I'm sure it works with MS too.