why int.TryParse cannot initialise multiple variables

Question

I am using int.TryParse to parse to variables (saved as strings in the database) and am curious why I cannot initialise 2 variables:

int min, 
    max;

using the following conditional statement:

bool lengthCompatible = int.TryParse(string1, out min) &&
                        int.TryParse(string2, out max);

Visual Studio (2015) produces the following code highlighting:

Use of unassigned local variable 'max'

Local variable 'max' might not be initialized before accessing

However, if I use 2 conditional statements:

bool minParse = int.TryParse(sentenceType.MinimumLength, out min);
bool maxParse = int.TryParse(sentenceType.MaximumLength, out max);

I can compile with no errors.

Curiouser and curiouser! Any insight appreciated.

Cheers

The && operator uses short-circuit evaluation. So the second TryParse call might not execute and max will not have a value. Use the & operator instead. — Hans Passant
– Hans Passant, Commented Jun 6, 2017 at 9:26

Jon Skeet · Accepted Answer · 2017-06-06 09:25:29Z

Well you're using &&, which is short-circuiting... if int.TryParse(string1, out min) returns false, the second call to int.TryParse won't be made, so max isn't definitely assigned.

You could write:

if (int.TryParse(string1, out min) &&
    int.TryParse(string2, out max))
{
    // Use min and max here
}

... because then the compiler knows that you only reach the body of the if statement if both calls have been executed.

Alternatively you could use the non-short-circuiting version with & instead of &&:

bool lengthCompatible = int.TryParse(string1, out min) &
                        int.TryParse(string2, out max);

That's slightly unusual though. The advantage of the if version above is that you'll retain the performance benefit of &&, in that you won't bother trying to parse string2 if you don't need to. (It depends on exactly what you're trying to do, of course.)

Matthew Watson · Accepted Answer · 2017-06-06 09:33:29Z

1

Because if the first int.TryParse(string1, out min) returns false, the second int.TryParse(string2, out max) will not be executed due to boolean short-circuiting.

In that case, max will not have been initialised.

You could just initialise max and min to zero:

int min = 0,
    max = 0;

...

bool lengthCompatible = int.TryParse(string1, out min) && int.TryParse(string2, out max);

Or only use the max and min after checking the result of the && as per other answers.

edited Jun 6, 2017 at 9:33

answered Jun 6, 2017 at 9:25

Matthew Watson

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Thorsten Dittmar · Accepted Answer · 2017-06-06 09:31:09Z

0

This is called shortcut boolean evaluation. This makes sure that boolean expressions are only evaluated until the final result is found.

If the first int.TryParse(string1, out min) already, the second one will not be executed, as the overall result will already be false. So the max variable is not guaranteed to be always initialized.

answered Jun 6, 2017 at 9:31

Thorsten Dittmar

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