在C＃中创建动态扩展方法？

Question

Is it possible to workaround this error:

public static class LayoutExtensions
{
    /// <summary>
    /// Verifies if an object is DynamicNull or just has a null value.
    /// </summary>
    public static bool IsDynamicNull(this dynamic obj)
    {
        return (obj == null || obj is DynamicNull);
    }

Compile time

Error: The first parameter of an extension method 
       cannot be of type 'dynamic'  

Answer 1

No. See https://stackoverflow.com/a/5311527/613130

When you use a dynamic object, you can't call an extension method through the "extension method syntax". To make it clear:

int[] arr = new int[5];
int first1 = arr.First(); // extension method syntax, OK
int first2 = Enumerable.First(arr); // plain syntax, OK

Both of these are ok, but with dynamic

dynamic arr = new int[5];
int first1 = arr.First(); // BOOM!
int first2 = Enumerable.First(arr); // plain syntax, OK

This is logical if you know how dynamic objects work. A dynamic variable/field/... is just an object variable/field/... (plus an attribute) that the C# compiler knows that should be treated as dynamic . And what does "treating as dynamic" means? It means that generated code, instead of using directly the variable, uses reflection to search for required methods/properties/... inside the type of the object (so in this case, inside the int[] type). Clearly reflection can't go around all the loaded assemblies to look for extension methods that could be anywhere.

Answer 2

All classes derived by object class. Maybe try this code

public static bool IsDynamicNull(this object obj)
{
    return (obj == null || obj is DynamicNull);
}