what happened when MVVMLight SimpleIoc registered multiple times?

Question

I'd like to know when .Register() is performed multiple times. Based on my experiment, below Console.WriteLine gives same Init value.

So, does .Register() check already registered instance and ignore to re-register?

void Main()
{
    // #1
    SimpleIoc.Default.Register<TestClass>();
    Console.WriteLine(SimpleIoc.Default.GetInstance<TestClass>().Init);

    // #2   
    SimpleIoc.Default.Register<TestClass>();
    Console.WriteLine(SimpleIoc.Default.GetInstance<TestClass>().Init);

    // Result: #1 and #2 give identical Init value.
}

public class TestClass
{
    public string Init { get; set;}
    public TestClass()
    {
        Init = Guid.NewGuid().ToString();
    }
}

@Evk // that would be great :) I was a questioner of this post; https://stackoverflow.com/q/13795596/361100 — Youngjae, Jun 01 '17 at 11:15
Well if look at source code it's clear that it behaves as you describe (registers only once), but I don't think there is some official documentation about that. — Evk, Jun 01 '17 at 11:16
@Evk // well, i tried to find it on mvvmlight codeplex site but due to limited search capability on the site, I couldn't find it. I didn't clone yet — Youngjae, Jun 01 '17 at 11:32

score 1 · Accepted Answer · answered Jun 01 '17 at 11:36

If look at source code of Register method by decompiling it (in case you cannot find \ don't want to bother with actual source code), you will see (with irrelevant parts skipped):

public void Register<TClass>(bool createInstanceImmediately) where TClass : class
{
  Type index = typeof (TClass);
  // skipped irrelevant parts
  if (this._factories.ContainsKey(index) && this._factories[index].ContainsKey(this._defaultKey))
  {
     if (!this._constructorInfos.ContainsKey(index))
       throw new InvalidOperationException(string.Format((IFormatProvider) CultureInfo.InvariantCulture, "Class {0} is already registered.", (object) index));
  } else {
      if (!this._interfaceToClassMap.ContainsKey(index))
        this._interfaceToClassMap.Add(index, (Type) null);
      this._constructorInfos.Add(index, this.GetConstructorInfo(index));
      Func<TClass> factory = new Func<TClass>(this.MakeInstance<TClass>);
      this.DoRegister<TClass>(index, factory, this._defaultKey);
      if (!createInstanceImmediately)
        return;
      this.GetInstance<TClass>();
    }
  }
}

So, if there is already registered factory in _factories dictionary - it checks if there is constructor info registered for this type and if NOT - throws exception about already registered class. I suppose this is a typo and check should be reversed - if there IS already registered constructor (which you register in else block) - there should be exception about already registered class. But as it is implemented now - it just does nothing when you try to register second time.

what happened when MVVMLight SimpleIoc registered multiple times?

1 Answers1