Using a variable as an out argument at point of declaration

Question

When reading a comment to an answer I saw the following construct to declare and initialize a variable:

int variable = int.TryParse(stringValue, out variable) ? variable : 0;

Is this allowed, correct and well defined in C#? What happens under the hood? Is the following what happens?

1. Is variable first initialized to zero?
2. then passed to int.TryParse (which assigns a value)?
3. then optionally read (if int.TryParse return true)?
4. and then, once again assigned/initialized?

Answer 1

Yes you are right for execution. You can also look into MSIL generated here

C# Code

 string stringValue = "5";
 int variable = int.TryParse(stringValue, out variable) ? variable : 0;

MSIL generated

1.  IL_0000:  nop    
2.  IL_0001:  ldstr      "5" // load string
3.  IL_0006:  stloc.0
4.  IL_0007:  ldloc.0
5.  IL_0008:  ldloca.s   variable
6.  IL_000a:  call       bool [mscorlib]System.Int32::TryParse(string, int32&)
7.  IL_000f:  brtrue.s   IL_0014
8.  IL_0011:  ldc.i4.0
9.  IL_0012:  br.s       IL_0015
10. IL_0014:  ldloc.1
11. IL_0015:  stloc.1
12. IL_0016:  ret

Which clarifies what it does behind the scene.

Statement 5 is allocating the variable onto stack. Statement 6 is calling the method. Statement 7,8,9 are actually exeuting the bool expression.

Answer 2

This is a trick that happens to work because it is simply a rewriting of an ordinary if-statement. This code is equivalent to this:

int variable;
if (int.TryParse(stringVariable, out variable))
    variable = variable;
else
    variable = 0;

The sequence is as follows:

int.TryParse is called, variable is not initialized before this but it doesn't have to either. An out parameter does not require a definite assigned variable. As part of the method execution, the variable will be given a value, and int.TryParse will return true or false.

If the method returns true then the result of the expression will be variable and thus we will execute basically variable = variable.

If the method returns false then the result of the expression will instead be 0, and variable will now be given the value 0 regardless of what it was given as part of int.TryParse. In this case, however, this will not change the variable because int.TryParse has already given the variable a default value when it returns false which also happens to be 0.

This is basically a way to get everything onto one line.

Personally I would've written this code like this:

int variable;
int.TryParse(stringValue, out variable);