First, in the .NET framework there is the ExpandoObject class , which allows the expansion of an object at runtime.

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ExpandoObject has several uses, one of them is in Database Access, storing the results of a query. The ability to change the object proves to be very useful in this task.

Dapper, in its own definition, is a simple framework for manipulating objects in .NET, which aims to provide better performance.

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Now about FastExpandoObject , I did not find it in the original Dapper definition (link above).

I found it in a GitHub repository:

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where it is declared as:

private class FastExpando : System.Dynamic.DynamicObject, IDictionary<string, object>

In the SqlMapper.cs of this repository, there is a reference to an old page:

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And this page has an old Dapper package, but it also has no FastExpando definition:

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In conclusion, as the original question that left you with doubt is 2012, I imagine FastExpandoObject is no longer used. And judging by its name, it was meant to be faster than the ExpandoObject.

Searching on ExpandoObject, I found another class called BetterExpando, which in its definition is "better than expando".

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And there probably should be other classes ...

ExpandoObject are dynamic objects, or be it objects that you can add or remove properties at runtime.

dynamic meuCache;
meuCache.QualquerCoisa = "Qualquer coisa mesmo";
meuCache.OutraCoisa = new { FaladoSerio = true };
meuCache.MaisUmaCoisa = Enum.Empty<string>();

Today, the most popular .NET Framework dynamic object is the ViewBag , from ASP.NET MVC .

In the case of Dapper , it uses dynamic objects to deserialize information that will come from a source where its format is unknown.

We can see your Dapper application in your class GridReader method .Read(bool)

  

class GridReader : The grid reader provides interfaces for reading multiple result sets from a Dapper query

     

IEnumerable < dynamic > GridReader.Read (bool) : Read the next grid of results, returned as a dynamic object

Returning lists is always better than returning tables .