Null out parameters in C#?

With C#7.0 (since August 2016) you can use the out var construct, and then just ignore the new var in subsequent code.

bool success = DateTime.TryParse(value, out var result);

If you truly do not care about the value of the result, use discards:

bool success = DateTime.TryParse(value, out _);

Nope. I'd wrap it in a method somewhere to keep the noise out of the main flow:

  bool IsValidDate(string value)
  {
     DateTime result;
     return DateTime.TryParse(value, out result); //result is stored, but you only care about the return value of TryParse()
  }