var ints = new int[] {1, 2, 3, 4, 5};
var result = string.Join(",", ints.Select(x => x.ToString()).ToArray());
Console.WriteLine(result); // prints "1,2,3,4,5"

EDIT : As of (at least) .NET 4.5,

var result = string.Join(",", ints.Select(x => x.ToString()).ToArray());

is equivalent to:

var result = string.Join(",", ints);

EDIT :

I see several solutions advertise usage of StringBuilder. Someone complaints that Join method should take an IEnumerable argument.

I'm going to disappoint you :) String.Join requires array for a single reason - performance. Join method needs to know the size of the data to effectively preallocate necessary amount of memory.

Here is a part of internal implementation of String.Join method:

// length computed from length of items in input array and length of separator
string str = FastAllocateString(length);
fixed (char* chRef = &str.m_firstChar) // note than we use direct memory access here
{
    UnSafeCharBuffer buffer = new UnSafeCharBuffer(chRef, length);
    buffer.AppendString(value[startIndex]);
    for (int j = startIndex + 1; j <= num2; j++)
    {
        buffer.AppendString(separator);
        buffer.AppendString(value[j]);
    }
}

I'm too lazy to compare performance of suggested methods. But something tells me that Join will win :)