C# 7.0 added discards for out arguments, deconstruction and patterns. C# 9.0 extends this to the parameters of lambdas and anonymous methods, so you can signal that you are not interested in a parameter which is required by the signature.

Without this, code analysis tools stumble trying to decide if an unused parameter was intentional or should be removed.

Discard parameters allow you to name any such parameter _ to signal that this omission is intentional.

Code

Now

Action<string, int> logger = (string message, int indent)
    => Log(new string(' ', indent) + message);

if (disableLogging)
    logger = (_, _) => { };

logger("OK", 0);

Before

Action<string, int> logger = (string message, int indent)
    => Log(new string(' ', indent) + message);

if (disableLogging)
    logger = (message, indent) => { };

logger("OK", 0);

Notes

• A lone _ is still an ordinary parameter named _ for backward compatibility; only two or more _ parameters are treated as discards

More information

• Specification
• C# Fundamentals