Question

what if I would have something like this:

if (data == null || (data != null && (data.Count() != 3 || data.IsNotCorrect()))
{
  //error...
}

The data == null and then || (data != null) part is somehow ugly. Is that how to solve this problem in c#?

Edit:
Sorry! Changed data.IsCorrect() to data.IsNotCorrect()

Answer 1

The right side of || gets only evaluated if the left side isn’t true (|| is short-circuiting)

The conditional-OR operator (||) performs a logical-OR of its bool operands. If the first operand evaluates to true, the second operand isn’t evaluated. If the first operand evaluates to false, the second operator determines whether the OR expression as a whole evaluates to true or false.

Quoted from || Operator (C# Reference) (MSDN)

Apply that to your expression:

data == null || (data != null && (data.Count() != 3 || data.IsCorrect())

This means when data != null is reached, data is guaranteed to be different from null, and thus data != null is guaranteed to be true, and you can leave it out.

Thus your expression is equivalent to:

data == null || data.Count() != 3 || data.IsCorrect()