Passing array to function that takes either params object[] or IEnumerable<T>

问题

I want to pass an array of custom objects to a function like String.Join which has the following signatures:

• public static string Join(string separator, params Object[] values)
• public static string Join(string separator, IEnumerable<T> values)

If I call the function like this:

var arr = new MyClass[]{ new MyClass(), new MyClass() };
string text = string.Join("\n", arr);

I get a compiler error:

The call is ambiguous between the following methods or properties: 'string.Join(string, params object[])' and 'string.Join(string, System.Collections.Generic.IEnumerable)'

I can resolve the ambiguity by using the IEnumerable<T> function:

var arr = new MyClass[]{ new MyClass(), new MyClass() };
string text = string.Join<MyClass>("\n", arr);

But can I call the params object[] function? In a performance critical scenario, it would be preferable to access the array directly rather than through an enumerator.

I am using C# 4.0, if that makes any difference.

回答1:

If you pass an object[] as the second parameter, the compiler should choose the object[] overload since it exactly matches. In the case where you have a differently-typed array (MyClass[] in this case) just cast the array to object[]:

string.Join("\n", (object[])arr);

You are not actually changing the types of any objects or performing any conversion at runtime, you're only giving the compiler a hint regarding which overload to use.

And regarding your comment about performance, don't forget to benchmark both options if performance is that critical. Don't assume one is faster than the other. (And always profile your entire application -- it's likely that any bottlenecks are going to be elsewhere.)

回答2:

If you change the type of your arr variable to object[] you will call the other overload:

object[] arr = new MyClass[] { new MyClass(), new MyClass() };
string text = string.Join("\n", arr);

You can also explicitly cast it to object[]: string.Join("\n", (object[])arr);

回答3:

You can call the other overload like this (that's what param are used for) -

string text = string.Join("\n", new MyClass(), new MyClass());

回答4:

The simplest change to your code would be to go from:

    var arr = new MyClass[]{ new MyClass(), new MyClass() };
    string text = string.Join("\n", arr);

To:

    var arr = new object[]{ new MyClass(), new MyClass() };
    string text = string.Join("\n", arr);

As said before, casting also works:

    var arr = new MyClass[]{ new MyClass(), new MyClass() };
    string text = string.Join("\n", (object[])arr);

To learn more about this subject, research C# overload resolution.

Overload resolution is an interesting subject in its own right, but I have yet to find it to be the bottleneck for a performance problem.

回答5:

If your are using IEnumerable you can use the <object> generic overload of the ToArray() method:

var allFoos = foo.GetAllFoos().ToArray<object>(); 
string s = string.Join(", ", allFoos); 

Looks less bloated and more readable to me.

来源：https://stackoverflow.com/questions/15933632/passing-array-to-function-that-takes-either-params-object-or-ienumerablet