Inline assignment on IF statement

Question

Is it possible to "short-circuit" assignment and validation into an IF statement..

Something line this..

if ((Match m = Regex.Match(data, pattern)).Success)
{
    Console.WriteLine(m.Groups["myField"].Value;
}

Answer 1

In general: assignment yes, declaration no:

Match m;
if ((m = Regex.Match(data, pattern)).Success)
{
    Console.WriteLine(m.Groups["myField"].Value);
}

Declaration expressions have been suggested but have not been implemented yet.

However, depending on your C# version, there are workarounds for your particular use case.

---

C# 7: Since C# 7, arguments to out parameters in a method call can be declared inline:

string s = "123";
if (int.TryParse(s, out int i))
    Console.WriteLine($"{s} has been parsed as {i}.");
else
    Console.WriteLine($"Unable to parse {s}.");

so you could work around your problem by designing a custom RegexTryMatch method:

public static bool RegexTryMatch(string input, string pattern, out Match match)
{
    match = regex.Match(input, pattern);
    return match.Success;
}

an use this pattern when calling it:

if (RegexTryMatch(data, pattern, out Match m))
{
    Console.WriteLine(m.Groups["myField"].Value);
}

Adding such a method to the .NET class library is currently being discussed.

---

C# 9: The pattern matching features of C# 9 allow for a solution which is very similar to what you proposed:

if (Regex.Match(data, pattern) is { Success: true } m)
{
    Console.WriteLine(m.Groups["myField"].Value);
}

or, alternatively:

if (Regex.Match(data, pattern) is {} m && m.Success)
{
    Console.WriteLine(m.Groups["myField"].Value);
}