I'm not too familiar with C#, so just a general note: Instead of structures like this:

if (idsToTypes.ContainsKey(id))
{
    return Activator.CreateInstance(idsToTypes[id], args);
}
else
{
    return null;
}

You could write this:

if (idsToTypes.ContainsKey(id))
{
    return Activator.CreateInstance(idsToTypes[id], args);
}
return null;

The else keyword is unnecessary in these cases.

I'm not too familiar with C#, so just two general notes.

1. Instead of structures like this:

    if (idsToTypes.ContainsKey(id))
    {
        return Activator.CreateInstance(idsToTypes[id], args);
    }
    else
    {
        return null;
    }
   

you could write this:

    if (idsToTypes.ContainsKey(id))
    {
        return Activator.CreateInstance(idsToTypes[id], args);
    }
    return null;

The else keyword is unnecessary in these cases.

2. Using guard clauses makes the code flatten and more readable.

    if (!type.IsAssignableFrom(_type)) {
        throw new ArgumentException(String.Format("Registered type, {0}, was not assignable from {1}", _type, type));
    }
    idsToTypes.Add(id, _type);
   

References: Replace Nested Conditional with Guard Clauses in Refactoring: Improving the Design of Existing Code; Flattening Arrow Code

I'm not too familiar with C#, so just a general note: Instead of structures like this:

if (type.IsAssignableFrom(_type))
{
    idsToTypes.Add(id, _type);
}
else
{
    throw new ArgumentException(String.Format("Registered type, {0}, was not assignable from {1}",_type,type));
}

You could write this:

if (type.IsAssignableFrom(_type))
{
    idsToTypes.Add(id, _type);
}
throw new ArgumentException(String.Format("Registered type, {0}, was not assignable from {1}", _type, type));

The else keyword is unnecessary in these cases.

I'm not too familiar with C#, so just a general note: Instead of structures like this:

if (idsToTypes.ContainsKey(id))
{
    return Activator.CreateInstance(idsToTypes[id], args);
}
else
{
    return null;
}

You could write this:

if (idsToTypes.ContainsKey(id))
{
    return Activator.CreateInstance(idsToTypes[id], args);
}
return null;

The else keyword is unnecessary in these cases.