Question mark and colon in statement. What does it mean?

This is the conditional operator expression.

(condition) ? [true path] : [false path];

For example

 string value = someBooleanExpression ? "Alpha" : "Beta";

So if the boolean expression is true, value will hold "Alpha", otherwise, it holds "Beta".

For a common pitfall that people fall into, see this question in the C# tag wiki.

c# asp.net web-services

If the condition in the parenthesis before the ? is true, it returns the value to the left of the :, otherwise the value to the right.

c# asp.net web-services

It's a ternary operator, or the short form for if..else.

condition ? value if true : value if false

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