Casting, reflection or?

public interface IType
{
    public string Type { get; set; }
}

public class TypeA : IType
{
    public string Type { get; set; }
    public string FancyA { get; set; }
}

public class TypeB : IType
{
    public string Type { get; set; }
    public string FancyB { get; set; }
}

void Get(json)
{
    var x = GetJson;
    IType returnObject = null;

    switch(x.GetType)
    {
        "TypeA":
            returnObject = JsonSerializer.Deserialize<TypeA>(x);
            break;
        "TypeB":
            returnObject = JsonSerializer.Deserialize<TypeB>(x);
            break;
    }
    return returnObject;
}

void DoParse()
{

    var callbacks = Dictionary<string, Action<IType>> { ("TypeA", DoStuffTypeA) };

    while(true)
    {
        var content = Retrieve();
        var parsed = Get(content);

        var callback = callbacks[parsed.Type]
        callback.Invoke(parsed)
    }
}

// Works
void DoStuffTypeA(IType content)
{
    //...    
}


// Doesn't work
void DoStuffTypeA(TypeA content)
{
    //...
}