Inline assignment on IF statement

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); }