How to cast a generic type without knowing the type

c#, generics

Solution

As far as I know, the exact thing you need is not possible in C#. Basically, you need for a method to have different types or the returned variable, based on the value of a regular, non-generic, parameter, i.e. if the parameter is `typeof(Enum1)`, than the result variable is `EnumMapper<Enum1>` and if the parameter is `typeof(Enum2)`, than the result variable is `EnumMapper<Enum2>`.

You can do this with generic parameters, however, since generics is all about compile-time information, and you only have the value at runtime, they cannot be used in this case.

What you can do (and what I have done) is to use dynamic code to work around this problem, being careful to go into statically-typed land as soon as possible (dynamic is really contagious, some say like a smile, some say like a virus):

public dynamic GetMapperObject(Type enumType)
{
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType);
  return mapper;
}

with calling code like:

var mapper = GetMapperObject(enumType);
//dynamic call, bind the resut to a statically typed variable
bool result = mapper.HasEnumValue("SomeStringValue") 

(old answer, just adds another level of indirection to the problem :) )

You could wrap all that in a generic method, something like this:

public EnumMapper<T> GetMapperObject<T>()
{
  Type enumType = typeof(T);
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType);
  return mapper as EnumMapper<T>;
}

and invoke it with

var mapper = GetMapperObject<EnumMapperTestEnum>();

However, if you only have a single value for the enum, you can use type inference, something like:

//everything is the same, just different signature
public EnumMapper<T> GetMapperByExample<T>(T item)
{
  Type enumType = typeof(T);
  Type enumMapperType = typeof(EnumMapper<>)
                           .GetGenericTypeDefinition()
                           .MakeGenericType(enumType);
  var mapper = Activator.CreateInstance(enumMapperType); // OK
  return mapper as EnumMapper<T>;
}

and call it with type inference

var mapper = GetMapperByExample(EnumMapperTestEnum.SomeValue); 

Problem

Original source