How does the method overload resolution system decide which method to call when a null value is passed?

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For the exact rules, see the overload resolution spec. But briefly, it goes like this.

First, make a list of all the accessible constructors.

public EffectOptions ( params object [ ] options )
public EffectOptions ( IEnumerable<object> options ) 
public EffectOptions ( string name )
public EffectOptions ( object owner ) 
public EffectOptions ( int count ) 
public EffectOptions ( Point point )

Next, eliminate all the inapplicable constructors. An applicable constructor is one where every formal parameter has a corresponding argument, and the argument is implicitly convertible to the formal parameter type. Assuming that Point is a value type, we eliminate the “int” and “Point” versions. That leaves

public EffectOptions ( params object[] options )
public EffectOptions ( IEnumerable<object> options ) 
public EffectOptions ( string name )
public EffectOptions ( object owner )

Now, we have to consider whether the one with “params” is applicable in its expanded or unexpanded form. In this case it is applicable in both forms. When that happens, we discard the expanded form. So that leaves

public EffectOptions ( object[] options )
public EffectOptions ( IEnumerable<object> options ) 
public EffectOptions ( string name )
public EffectOptions ( object owner )

Now we must determine the best of the applicable candidates. The bestness rules are complicated, but the short version is that more specific is better than less specific. Giraffe is more specific than Mammal, Mammal is more specific than Animal, Animal is more specific than object.

The object version is less specific than all of them, so it can be eliminated. The IEnumerable<object> version is less specific than the object[] version (do you see why?) so it can be eliminated too. That leaves

public EffectOptions ( object[] options )
public EffectOptions ( string name )

And now we are stuck. object[] is neither more nor less specific than string. Therefore this gives an ambiguity error.

That is just a brief sketch; the real tiebreaking algorithm is much more complicated. But those are the basics.

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