Coalesce ?? operator in C# 2.0

Dmytro Shteflyuk — Mon, 14 Jan 2008 19:18:27 +0000

Operator ??, that was introduced in the .NET 2.0, takes first place in my top used C# idioms list a long time, but unfortunately it is rarely used in projects I’ve participated. Therefore these snippets could be found in production code very often:

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public string Caption
{
get { return ViewState["Caption"] != null ? (string) ViewState["Caption"] : ""; }
set { ViewState["Caption"] = value; }
}

Or even:

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public string VisibleStatistic
{
get
{
string retval = (string) ViewState["VisibleStatistic"];
return retval == null ? "" : retval;
}
set
{
ViewState["VisibleStatistic"] = value;
}
}

public string SelectedCategoryName
{
get
{
object retval = ViewState["SelectedCategoryName"];
if (retval != null)
return (string) retval;
return String.Empty;
}
set
{
ViewState["SelectedCategoryName"] = value;
}
}

Almost 50% of this could be easily removed, and you will get beautiful and clear code, just right after you will understand what the hell is operator ??. Here is information from MSDN:

The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.

Easy and clean explanation. Let’s try to rewrite all these examples using this operator:

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public string Caption
{
get { return (string) ViewState["Caption"] ?? String.Empty; }
set { ViewState["Caption"] = value; }
}

public string VisibleStatistic
{
get { return (string) ViewState["VisibleStatistic"] ?? String.Empty; }
set { ViewState["VisibleStatistic"] = value; }
}

public string SelectedCategoryName
{
get { return (string) ViewState["SelectedCategoryName"] ?? String.Empty; }
set { ViewState["SelectedCategoryName"] = value; }
}

In case of value-types this approach will not work: you will get NullReferenceException. But do not worry — in this case we can use Nullable-types. Before:

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public int FirstWidth
{
get { return ViewState["FirstWidth"] != null ? (int) ViewState["FirstWidth"] : 0; }
set { ViewState["FirstWidth"] = value; }
}

After:

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public int FirstWidth
{
get { return (int?) ViewState["FirstWidth"] ?? 0; }
set { ViewState["FirstWidth"] = value; }
}

When you are using ?? operator, expression will be evaluated in the left to right order, so you can use something like this:

1	string name = FirstName ?? LastName ?? "Anonymous";

Of course, it looks more readably than the:

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string name;
if (FirstName != null)
name = FirstName;
else if (LastName != null)
name = LastName;
else
name = "Anonymous";

And a fortiori than:

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string name = FirstName != null
? FirstName
: (LastName != null ? LastName : "Anonymous");

Add it to your armoury!

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Coalesce ?? operator in C# 2.0