Is the following code an expression statement?

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The C# documentation Introduction - C# language specification | Microsoft Docs[^] gives the following code example demonstrating what is not an expression statement and what is an expression statement:

C#

static void Main() 
    {
        int i;
        i = 123;       // Expression statement
    }

But what if I combine both lines of code into one line of code. Is the bottom line of code an expression statement or is it not an expression statement:

C#

int i = 123;

What I have tried:

No online resource specifies the answer.

Posted 30-Nov-21 15:09pm

OldBounty

Updated 30-Nov-21 17:11pm

v2

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Richard MacCutchan 1-Dec-21 4:02am

The first example is a throwback to the early days of the C programming language, when variables had to be declared at the beginning of a block. With more modern languages and variants (C++, C#, Java etc.) it is generally accepted that declaration and initialisation (the second example) should happen together. In fact most compilers will raise a warning for uninitialised declarations.

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Jon McKee · Accepted Answer · 2021-11-30T17:11:00

To directly answer your question: it is not an expression statement; it is a declaration statement with initialization.

The difference between an expression and a statement is that the execution of a statement doesn't produce a value whereas an expression does. So statements have side-effects but produce no value while expressions can both have side-effects and produce a value. Multiple expressions can make up a statement but there can only be one statement per "line" (line defined by ;).

C#

int i;           //No resulting value but creates an integer i as a side-effect.
i = 123;              //Resulting value is 123; side-effect assigns 123 to i.
Console.WriteLine(i); //Resulting value is void; side-effect prints 123 to console.
++i;                  //Resulting value is 124; side-effect increments i to 124.
i++;                  //Resulting value is 124; side-effect increments i to 125.
Console.WriteLine(i); //Resulting value is void; side-effect prints 125 to console.

"Expression statements" are just expressions that are used for their side-effects only and the result of the expression is ignored; it's an expression used as if it were a statement. So above with i++, we're indicating we only care about the side-effect of incrementing i and not the return value. Another example:

C#

j = i = 123; //Result is 123; side-effects assign 123 to both i and j.

Here the whole j = i = 123 is an expression statement that uses the expression i = 123. But with

C#

int j = i = 123; //No result; side-effects create integer j, assign 123 to i and j.

int j = i = 123 is a declaration statement that uses the expression i = 123.

Hope that helps.