How to resolve ambiguity when argument is null?

问题

Compiling the following code will return The call is ambiguous between the following methods or properties error. How to resolve it since I can't explicitly convert null to any of those classes?

static void Main(string[] args)
{
    Func(null);
}

void Func(Class1 a)
{

}

void Func(Class2 b)
{

}

回答1:

Func((Class1)null);

回答2:

You could also use a variable:

Class1 x = null;
Func(x);

回答3:

Cast null to the type:

Func((Class1)null);

回答4:

Using as for the casting makes it slightly more readable with the same functionality.

Func(null as Class1);

回答5:

The Func() methods accept a reference type as a parameter, which can be null. Since you're calling the method with an explicit null value, the compiler doesn't know whether your null is supposed to be in reference to a Class1 object or a Class2 object.

You have two options:

Cast the null to either the Class1 or Class2 type, as in Func((Class1)null) or Func((Class2)null)

Provide a new overload of the Func() method that accepts no parameters, and call that overload when you don't have an explicit object reference:

void Func()
{
    // call this when no object is available
}

回答6:

You should be able to cast null to either of those, the same as you would a variable Func((Class1)null).

回答7:

Just an alternative solution I prefer

static void Main(string[] args)
{
    Func(Class1.NULL);
}

void Func(Class1 a)
{ }

void Func(Class2 b)
{ }

class Class1
{
    public static readonly Class1 NULL = null;
}

class Class2
{
    public static readonly Class2 NULL = null;
}

来源：https://stackoverflow.com/questions/4044055/how-to-resolve-ambiguity-when-argument-is-null