Thursday, July 28, 2005

Q: How do I write a method that accepts a variable number of parameters?

Q: How do I write a method that accepts a variable number of parameters?

A: Languages like C and C++ have always offered some means for using and creating functions capable to accept a variable number of parameters. The most widely known example of a function which takes a variable number of parameters is printf():

int printf(char *format, …); // the ellipsis means "variable number of params"

Using this function is pretty easy:

printf("Hello, world\n");

printf("The sum of %d and %d is %d\n", a, b, a+b);

However, the creation of such functions in these languages relays on a set of predefined macros and is not particularly elegant or intuitive.

C# offers an elegant solution to this problem through parameter arrays. A parameter array is a single-dimensional array included as the last parameter in the parameter list of a method:

public string Concat(string separator, params string[] strings)


string result = "";

for (int i = 0; i <>


if (i > 0)

result += separator;

result += strings[i];


return result;


Such a function can be called in two different ways:

a) Passing the function an array instance argument:

string[] names = { "Anders", "Eric", "Scott", "Duncan" };

MessageBox.Show(Concat("+", names) + " = great team");

b) Using zero or more type-compatible arguments for the parameter array:

MessageBox.Show(Concat("+", "Anders", "Eric", "Scott", "Duncan") +

" = great team");

In this case, the invocation will create an array from the supplied arguments and use it as the actual argument.

Thanks to the unified .NET type system, object[] can be used as “common denominator” for arguments of different types:

public int SumTheIntegers(params object[] list)


// sum all the integers included in list

int sum = 0;

foreach (object o in list)

if (o.GetType() == typeof(int))

sum += (int) o;

return sum;


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