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

Question

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?

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

Answer 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.)

Answer 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);