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V3133. Postfix increment/decrement is s…
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V3133. Postfix increment/decrement is senseless because this variable is overwritten.

Feb 07 2017

The analyzer detected a likely error that has to do with using a postfix increment or decrement in an assignment to the same variable.

Consider the following example:

int i = 5;
// Some code
i = i++;

The increment operation here will not affect the expression result and the 'i' variable will be assigned the value 5 after executing this code.

This is explained by the fact that postfix increment and decrement operations are executed after evaluating the right operand of the assignment operator, while the result of the assignment is temporarily cached and is assigned later to the left part of the expression after the increment/decrement operation has executed. Therefore, the result of the increment/decrement is overwritten with the result of the whole expression.

To better understand the mechanics of this behavior, consider the IL code of the example above:

-======- START OF OPERATION "int i = 5" -======-
// Declaring local variable 'i'
// Current stack => []
.locals init ([0] int32 i)

// Passing value 5 to the top of stack 
// Current stack => [5]
IL_0001:  ldc.i4.5

// Assigning value 5 from stack to variable 'i'
// Current stack => []
IL_0002:  stloc.0
-======- END OF OPERATION "int i = 5" -======-

-======- START OF OPERATION "i = i++" -======-
// Passing value of variable 'i' to the top of stack
// Current stack => [5]
IL_0003:  ldloc.0

    -======- START OF OPERATION "i++" -======-
    // Copying top value on stack
    // Current stack => [5, 5]         
    IL_0004:  dup

    // Passing value 1 to the top of stack
    // Current stack => [1, 5, 5]  
    IL_0005:  ldc.i4.1

    // Adding two top values from stack (5 + 1)
    // Result (6) is passed to the top of stack
    // Current stack => [6, 5] 
    IL_0006:  add

    // Assigning value 6 from stack to variable 'i'
    // Current stack => [5]
    IL_0007:  stloc.0
    -======- END OF OPERATION "i++" -======-

// Assigning value 5 from stack to variable 'i'
// Current stack => []
IL_0008:  stloc.0
-======- END OF OPERATION "i = i++" -======-

As for the correct version of this code, it can look differently depending on the intended behavior.

This error may be a typo and the programmer unintentionally wrote variable 'i' twice in the assignment statement. Then the correct version could look as follows:

int i = 5;
// Some code
q = i++;

Another scenario is that the programmer did not know that the postfix increment operator adds one to the value of the variable but returns its initial value. Then the assignment statement is redundant and the fixed code could look like this:

int i = 5;
// Some code
i++;

This example may look more like a synthetic test and you may think nobody really writes code that way, but this error can actually be found in serious projects. Here is an example taken from MSBuild project.

_parsePoint =
  ScanForPropertyExpressionEnd(expression, parsePoint++);

Incrementing the '_parsePoint' variable is pointless because the increment operation will be executed after passing the initial value of this variable to method 'ScanForPropertyExpressionEnd' and will not affect the result of this method in any way. The programmer must have confused postfix and prefix increments. In that case, the correct version of this code could look as follows:

_parsePoint =
  ScanForPropertyExpressionEnd(expression, ++_parsePoint);

This diagnostic is classified as:

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