When should one use dynamic keyword in c# 4.0?

Dynamic should be used only when not using it is painful. Like in MS Office libraries. In all other cases it should be avoided as compile type checking is beneficial. Following are the good situation of using dynamic.

1. Calling javascript method from Silverlight.
2. COM interop.
3. Maybe reading Xml, Json without creating custom classes.

How about this? Something I've been looking for and was wondering why it was so hard to do without 'dynamic'.

interface ISomeData {}
class SomeActualData : ISomeData {}
class SomeOtherData : ISomeData {}

interface ISomeInterface
{
    void DoSomething(ISomeData data);
}

class SomeImplementation : ISomeInterface
{
    public void DoSomething(ISomeData data)
    {
        dynamic specificData = data;
        HandleThis( specificData );
    }
    private void HandleThis(SomeActualData data)
    { /* ... */ }
    private void HandleThis(SomeOtherData data)
    { /* ... */ }

}

You just have to maybe catch for the Runtime exception and handle how you want if you do not have an overloaded method that takes the concrete type.

Equivalent of not using dynamic will be:

    public void DoSomething(ISomeData data)
    {
        if(data is SomeActualData)
          HandleThis( (SomeActualData) data);
        else if(data is SomeOtherData)
          HandleThis( (SomeOtherData) data);
        ...
        else
         throw new SomeRuntimeException();
    }