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Code Analyzer for C#

There is a newer version: 10.2.0.105762

Why is this an issue?
Locking on a class field synchronizes not on the
field itself, but on the object assigned to it. Thus, there are some good practices to follow to avoid problems related to thread synchronization.

   Locking on a non-readonly field makes it possible for the field’s value to change while a thread is in the code block, locked on the old value. This allows another thread to lock on the new value and access the same block concurrently. private Color color = new Color("red");
private void DoSomething()
{
  // Synchronizing access via "color"
  lock (color) // Noncompliant: lock is actually on object instance "red" referred to by the "color" field
  {
    //...
    color = new Color("green"); // other threads now allowed into this block
    // ...
  }
}
 
   Locking on a new instance of an object undermines synchronization because two different threads running the same method in parallel will lock on different instances of the same object, allowing them to access the synchronized block at the same time. private void DoSomething()
{
  lock (new object()) // Noncompliant: every thread locks on a different new instance
  {
    // ...
  }
}
 
   Locking on a string literal is also dangerous since, depending on whether the string is interned or not, different threads may or may not synchronize on the same object instance. private readonly string colorString = "red";
private void DoSomething()
{
  lock (colorString)  // Noncompliant: strings can be interned
  {
    // ...
  }
}
 

How to fix it
Code examples
Noncompliant code example
private Color color = new Color("red");
private void DoSomething()
{
  // Synchronizing access via "color"
  lock (color) // Noncompliant: lock is actually on object instance "red" referred to by the "color" field
  {
    //...
    color = new Color("green"); // other threads now allowed into this block
    // ...
  }
}

Compliant solution
private Color color = new Color("red");
private readonly object lockObj = new object();

private void DoSomething()
{
  lock (lockObj)
  {
    //...
    color = new Color("green");
    // ...
  }
}

Resources

   Lock Statement - lock statement - ensure
  exclusive access to a shared resource 
   String.Intern - String.Intern(String) Method 
   CWE - CWE-412 - Unrestricted Externally Accessible Lock 
   CWE - CWE-413 - Improper Resource Locking 
   Threads and threading