string.Format does not append the given args

Question

I'm writing an extension for string.Format (which I've done before with no problems.)

My extension:

    public static string FormatWith(this string source, params object[] args) {
        source.ThrowIfNull("source");

        return string.Format(source, args);
    }

My test:

    [TestMethod]
    public void FormatWith_ShouldReturnCorrectResult_FromValidArgs() {
        var expected = "testing 123";
        var actual = "test".FormatWith("ing", " ", 123);

        Assert.AreEqual(expected, actual);
    }

In my test, after the call to FormatWith, actual should be "testing 123" but it is just "test".

My test's message:

    Assert.AreEqual failed. Expected:<testing123>. Actual:<test>.

I've tried passing types other than string into the extension but the result is unchanged. And I'm sure source.ThrowIfNull is not throwing an exception.

So what gives? Am I overlooking something? A nice answer would show me a working implementation of FormatWith along with an explanation of why my implementation is not working.

Edit: I'm an idiot and completely forgot about {0}, {1}... I use this on a daily basis too. Will accept first answer when timer is up. Thanks guys.

Answer 1

I'm not a .NET guy, but I can read API docs: http://www.dotnetperls.com/string-format

If I'm not mistaken, your source string needs to explicitly add your arguments to the string.

var actual = "test{0}{1}{2}".FormatWith("ing", " ", 123);

Feel free to correct me .NET folks.

Answer 2

You are looking for String.Concat. String.Format doesn't work like that.Only if you specify arguments like this:

string.Format("{0}, {1} ....",source,args);

Which you can not because you don't know the number of arguments.So you can use String.Concat or String.Join instead.

Answer 3

Rather than modify your usage of the function as some other others suggested, here's an implementation of FormatWith that will work as you intended it:

public static string FormatWith(this string source, params object[] args)
{
    var arguments = new List<object> {source};
    arguments.AddRange(args);

    return String.Concat(arguments);
}