The conditional operator ?: looks confusing enough that some programmers assume it must be more efficient than a comparable but longer if-else statement. This example uses the following code to compare the speeds of the conditional operator and an if-else statement.

// Trials using if.
x = 1;
start_time = DateTime.Now;
for (long i = 0; i < num_trials; i++)
{
    if (x % 2 == 0) x = x + 1;
    else x = x + 3;
}
elapsed = DateTime.Now - start_time;
lblIfTime.Text = elapsed.TotalSeconds.ToString("0.00") + " seconds";
Refresh();

// Trials using ?:.
x = 1;
start_time = DateTime.Now;
for (long i = 0; i < num_trials; i++)
{
    x = (x % 2 == 0) ? x + 1 : x + 3;
}
elapsed = DateTime.Now - start_time;
lblConditionalTime.Text = elapsed.TotalSeconds.ToString("0.00") + " seconds";
Refresh();

The code records the current time in the start_time variable and then enters a loop. The loop determines whether the variable x is odd or even and adds either 1 or 3 to it.

When the loop finishes, the program subtracts start_time from the current time and displays the number of seconds that have elapsed.

The program then repeats the same steps using the conditional operator instead of an if-else statement.

You can see from the figure that the difference between the two approaches is negligible. In this trial, after 100 million tests the difference in time was only 30 milliseconds. Variations in the operating system are more significant so if you run the tests several times you will get different results and sometimes the if-else statement will be faster.

The moral is you should use whichever of these statements seems easier to read and understand and not worry about performance. For a very short statement, you may find ?: more compact than if-else. Often ?: is confusing so I usually use if-else instead.

(Note that it's a good thing that the performance of the two statements is the same. It means the C# code is being converted into comparable code before execution. It would be bad if Visual Studio compiled one of these two equivalent statements into something that was less efficient than the other. But it's also useful to check.)