Why is this an issue?

Conditional expressions which are always true or false can lead to unreachable code.

In the case below, the call of Dispose() never happens.

var a = false;
if (a)
{
    Dispose(); // Never reached
}

Exceptions

This rule will not raise an issue in either of these cases:

• When the condition is a single const bool

  const bool debug = false;
  //...
  if (debug)
  {
    // Print something
  }
  

• When the condition is the literal true or false.

In these cases, it is obvious the code is as intended.

How to fix it

The conditions should be reviewed to decide whether:

• to update the condition or
• to remove the condition.

Code examples

Noncompliant code example

public void Sample(bool b)
{
    bool a = false;
    if (a)                  // Noncompliant: The true branch is never reached
    {
        DoSomething();      // Never reached
    }

    if (!a || b)            // Noncompliant: "!a" is always "true" and the false branch is never reached
    {
        DoSomething();
    }
    else
    {
        DoSomethingElse();  // Never reached
    }

    var c = "xxx";
    var res = c ?? "value"; // Noncompliant: c is always not null, "value" is never used
}

Compliant solution

public void Sample(bool b)
{
    bool a = false;
    if (Foo(a))             // Condition was updated
    {
        DoSomething();
    }

    if (b)                  // Parts of the condition were removed.
    {
        DoSomething();
    }
    else
    {
        DoSomethingElse();
    }

    var c = "xxx";
    var res = c;            // ?? "value" was removed
}

Resources

• MITRE, CWE-570 - Expression is Always False
• MITRE, CWE-571 - Expression is Always True
• Wikipedia - Unreachable code

Documentation

• Microsoft Learn - Conditional logical AND operator &&
• Microsoft Learn - Conditional logical OR operator ||
• Microsoft Learn - ?? and ??= operators - the null-coalescing operators