How to know in C# code which type a variable was declared with

Question

I want to have some function that would return "Base" if a variable of class Base was passed to it, "Derived" if it was declared as Derived, etc. Not depending on runtime type of a value it was assigned to.

Answer 1

See code below for example. The key is to use Generics, extension method was used just for nice syntax.

using System;

static class Program
{
    public static Type GetDeclaredType<T>(this T obj)
    {
        return typeof(T);
    }

    // Demonstrate how GetDeclaredType works
    static void Main(string[] args)
    {
        ICollection iCollection = new List<string>();
        IEnumerable iEnumerable = new List<string>();
        IList<string> iList = new List<string>();
        List<string> list = null;

        Type[] types = new Type[]{
            iCollection.GetDeclaredType(),
            iEnumerable.GetDeclaredType(),
            iList.GetDeclaredType(),
            list.GetDeclaredType()
        };

        foreach (Type t in types)
            Console.WriteLine(t.Name);
    }
}

Result:

ICollection
IEnumerable
IList`1
List`1

EDIT: You may also avoid using extension method here, as it would cause it to appear on every IntelliSense drop-down list. See another example:

using System;
using System.Collections;

static class Program
{
    public static Type GetDeclaredType<T>(T obj)
    {
        return typeof(T);
    }

    static void Main(string[] args)
    {
        ICollection iCollection = new List<string>();
        IEnumerable iEnumerable = new List<string>();

        Type[] types = new Type[]{
                GetDeclaredType(iCollection),
                GetDeclaredType(iEnumerable)
        };

        foreach (Type t in types)
            Console.WriteLine(t.Name);
    }
}

also produces correct results.