How Does C# Method Overload Resolution Handle Null Arguments?

Null value handling in C# method overload resolution

When overloading multiple methods with different parameters, the method overload resolution system will determine which method to call based on the provided parameters. However, when passing a null value as a parameter, the parsing system follows specific rules.

To understand this process, consider a class named EffectOptions which has the following constructor:

<code class="language-csharp">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) { }</code>

When you pass null value as parameter, for example:

<code class="language-csharp">EffectOptions options = new EffectOptions(null);</code>

The parsing system first excludes all inaccessible constructors. In this case, all constructors are accessible, so we move on to the next step.

Next, it identifies all applicable constructors where each formal parameter has a corresponding parameter that is implicitly convertible to the formal parameter type. Since null values ​​can be implicitly converted to object and object[], there are multiple applicable constructors:

<code class="language-csharp">public EffectOptions(object[] options)
public EffectOptions(IEnumerable<object> options)
public EffectOptions(string name)
public EffectOptions(object owner)</code>

However, if the params object[] constructor works in both its expanded and unexpanded forms, the expanded form is discarded. This leaves us with:

<code class="language-csharp">public EffectOptions(object[] options)
public EffectOptions(IEnumerable<object> options)
public EffectOptions(string name)
public EffectOptions(object owner)</code>

Finally, the system determines the best applicable candidates based on specificity. In this case, object[] and string have the same specificity, resulting in an ambiguity error. The compiler cannot determine which constructor to call.

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