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Code Analyzer for C#

There is a newer version: 10.2.0.105762

Why is this an issue?
Enumerable.Sum() always executes addition in a
checked context, so an OverflowException will be
thrown if the value exceeds MaxValue, even if an unchecked context was specified. Therefore, using this method inside an
unchecked context will only make the code more confusing, since the behavior will still be checked.
This rule raises an issue when an unchecked context is specified for a Sum on integer types.
Exceptions
When the Sum call is inside a try-catch block,
no issues are reported, since the exception is properly handled.
void Add(List<int> list)
{
  unchecked
  {
    try
    {
      int total = list.Sum();
    }
    catch (System.OverflowException e)
    {
      // Exception handling
    }
  }
}

How to fix it
Remove the unchecked operator/statement, and optionally add some exception handling for the OverflowException.
Code examples
Noncompliant code example
void Add(List<int> list)
{
  int total1 = unchecked(list.Sum());  // Noncompliant

  unchecked
  {
    int total2 = list.Sum();  // Noncompliant
  }
}

Compliant solution
void Add(List<int> list)
{
  int total1 = list.Sum();

  try
  {
    int total2 = list.Sum();
  }
  catch (System.OverflowException e)
  {
    // Exception handling
  }
}

Resources
Documentation

   Enumerable.Sum Method 
   Enumerable.Sum implementation 
   checked and
  unchecked statements 
   checked and unchecked operators 
   Exceptions and Exception Handling 
   OverflowException Class 
   Integer overflow