How to print a custom string when a value is null, using .NET's string.Join method?

Question

I have the following class:

public class Point
{
    public int X { get; set; }
    public int Y { get; set; }

    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }

    public override string ToString()
    {
        return "[" + X + ", " + Y + "]";
    }
}

I have overridden the ToString method, so that if an attempt to store the value of a Point object in a string is made, it could have the following format: '[X, Y]'.

Let's say I have an array of points and I want to print them, separated by a comma.

Point point1 = new Point(1, 2);
Point point2 = new Point(10, 20);

Point[] points = new Point[] { point1, point2, null };

Console.WriteLine(string.Join<Point>(", ", points));

This will print '[1, 2], [10, 20],' on the screen. The problem is I want somehow to print '[1, 2], [10, 20], null' - meaning, to print 'null', when I have a null value.

I thought of a workaround, but it's really ugly and incorrect in terms of design. I added the following code inside the Point class:

private bool isNull;

public Point(bool isNull = false)
{
    this.isNull = isNull;
}

public override string ToString()
{
    if (!isNull)
    {
        return "[" + X + ", " + Y + "]";
    }

    return "null";
}

So, now if I call the string.Join method, by writing this:

Point[] points = new Point[] { point1, point2, new Point(true) };

I get the desired output '[1, 2], [10, 20], null]', but as I wrote I think this is ugly and incorrect, so can anyone help me with this?

I really need to use the string.Join method with an array of Point objects.

@Mohamed Read the code at the top - Point is a class the OP has written. — Daniel Kelley
– Daniel Kelley, Commented May 27, 2014 at 8:03

Marc Gravell · Accepted Answer · 2014-05-27 08:06:32Z

5

You can't change how Join interprets null, so: change the fact that there is null in the first place; this works:

Console.WriteLine(string.Join(", ", points.Select(Point.Render)));

where Point.Render is:

internal static string Render(Point point)
{
    return point == null ? "null" : point.ToString();
}

However, I wonder if this might be preferable:

public static string Render(IEnumerable<Point> points)
{
    if (points == null) return "";
    var sb = new StringBuilder();
    foreach(var point in points)
    {
        if (sb.Length != 0) sb.Append(", ");
        sb.Append(point == null ? "null" : point.ToString());
    }
    return sb.ToString();
}

with:

Console.WriteLine(Point.Render(points));

Or if you made it an extension method:

Console.WriteLine(points.Render());

answered May 27, 2014 at 8:06

Marc Gravell

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1 Comment

Yulian Over a year ago

I added an extension method, just like the static method you provided (I just added the 'this' keyword in front of the first method argument) and then used the method like this: Console.WriteLine(string.Join(", ", points.Select(point => point.Render()))); It worked exactly as I wanted. Thanks a lot!

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